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The brown absorption continuum is the major absorption featureresponsible for brown coloration. TheC defect absorption continuum is the major absorption feature determining color. The C defectabsorption continuum differs from the brown absorption continuum by much lower absorption inthe yellow and red spectral ranges. Curve 3 represents type Ia diamonds of cape yellow color. Themain absorption center of these diamonds is the N3 N2 center band absorbing in the blue andgreen spectral ranges. Curve 4 represents non-colored type Ia diamonds.
Traces of the N3 N2 center band in the spectral range from to nm make most of these diamonds near-colorless [based on the data from De Weerdt and Collins ]. Curve 5 represents colorlesstype IIa diamonds2. These features may result in yellow, green andpink modifying colors. According to the DTC color scale, the intensity of brown color of diamond ischaracterized by six grades from C1 the least intense through C6 the darkestbrown.
The brown color is determined visually using master stones of corre-sponding colors. It can be also evaluated from the measurements of the absorptionintensity of the Brown absorption continuum Fig. The brown color scale isnot uniform in terms of absorption intensity.
The absorption strength of the BrownAbsorption Continuum increases only slightly for the grade change from C3 to C4,whereas the absorption increase is considerable when the grade changes from C1to C3 and from C5 to C6. Another grade scale of color of brown diamonds uses four categories: An approximate correspondence between the DTC scale and the Brscale is: The Br scale is moreuniform as compared with the DTC scale. The Br scale is more convenient for thecharacterization of rough brown diamonds selected as starting material for HPHTtreatment.
Brown diamonds can be grouped into two major categories, which reflect twodifferent types of color distribution: These diamonds exhibit strong tracesof plastic deformation on the surface like etched pits and grooves. Plasticallydeformed diamonds, as the most populated category of brown diamonds, can betermed as regular brown diamonds.
Diamonds of this category are the mostcommon starting material for commercial HPHT treatment. Although the brown graining is localized in the deformed areas, the deforma-tion itself is not the primary reason of brown coloration. Because of this, not allplastically deformed diamonds are brown. Artificially induced plastic deformationduring non-hydrostatic HPHT treatment at a temperature of 1, C and0 1 2 3 4 5 6 Thus, theplastically deformed areas are just the places where the optical centers responsiblefor the brown coloration are generated.
These defects can be vacancy clusters,defects responsible for the Amber Center and dislocations Fritsch et al. Although all these defects cancontribute to the brown graining, the vacancy clusters seem to be the mosteffective ones Bangert et al. Production of vacancies by plastic deformation and their accumulation intovacancy clusters may begin at a temperature of 1, C, when diamond loses itsrigidity and dislocations start to form.
Thus, it is assumed that the generation ofbrown color in natural diamonds may occur in earth at rather low temperatures andthe defect structure of such diamonds may reveal characteristic features of lowtemperature heating. For instance, it can be a trace concentration of single non-aggregated vacancies, which survived the natural heating and which can bedetected as a weak GR1 center see Chap.
Consequently, traces of theGR1 center in natural diamonds with brown coloration is an indicator of theirpristine state. In plastically deformed brown type Ia diamonds, no correlation between browncolor and nitrogen content has been found Chapman However, there issome correlation between internal strain in type Ia diamonds and their browncoloration, the brown color being associated with lower strain fields Van Royen; Chapman Brown color can be artificially induced in diamonds, when they are heated athigh pressure at non-hydrostatic conditions resulting in internal strain Howell However, the strain alone does not result in brown color.
Instead, the browncoloration in the deformed diamonds is distributed rather uniformly throughout thebulk and it does not follow the slip bands or any other crystallographic features. It might be that the concentration of the defects responsible for the brownFig. Along with the plastically deformed brown diamonds colored by the defor-mation-induced graining, there are brown diamonds, color of which is caused bymicro-inclusions and defects generated by non-deformational mechanisms. Thesediamonds can be termed as irregular brown diamonds. The defects contributing tobrown color of irregular brown diamonds have been identified as C defects in typeIb diamond, micro-inclusions of non-diamond phases, imbedded CO2 molecules,hydrogen-related defects, some unknown intrinsic defects in type IIa and IaBdiamonds Ewels et al.
Prominent non-graining brown diamonds are CO2-rich ones Hai-nschwang et al. These diamonds can be recognized by shapeless brownareas Fig. The nature of the optical centers responsible for brown color ofCO2-rich diamonds has not been established yet. However, it is known that thesecenters are much more stable than the vacancy clusters in plastically deformedbrown diamonds.
Due to this high stability, the brown color of CO2-rich diamondsstands HPHT treatment even when performed at very high temperatures. The N3 and N2 absorptions cause the color of so-calledcape-yellow diamonds, which are the most common naturally-colored gemdiamonds of pleasant colors Johnson and Moe Fig. The cape-yellowdiamonds are of type Ia with nitrogen content of at least ppm. A considerableportion of nitrogen in these diamonds is in the form of B defects.
The vast majorityof light yellow natural diamonds are colored this way. Some cape-yellow dia-monds have high content of hydrogen, which provides an additional weakabsorption in the yellow to green spectral range. Because of this absorption, thecolor of the hydrogen-rich yellow diamonds may have an unwanted gray modifier.
The N3 and N2 centers are very temperature stable and therefore cape-yellowcolor component of natural diamonds cannot be removed or reduced by HPHTtreatment. Instead, in some cape-yellow diamonds, HPHT treatment may enhanceabsorption of the N3 and N2 centers resulting in a deeper yellow color. Thus, HPHT-treated cape-yellow diamonds are also something to encounterwith in the gem diamond market.
The absorption in the Vis spectral range due to C defects causes orangey-yellowor canary-yellow color, which can be easily distinguished from the cape-yellow16 2 Diamonds Used for HPHT Treatmentcolor. The absorption spectra of cape-yellow and canary-yellow diamonds are alsovery different compare Fig. The C defects are very effective inabsorption. Just a trace concentration of C defects may affect color grade of naturaldiamond. A concentration of C defects of 0. A few ppm C defects is sufficient to cause a fancy yellowcolor Collins ; Kitawaki Deep yellow color, characteristic of rarenatural type Ib diamonds and most synthetic diamonds, is caused by C defectconcentration of 20 ppm and above Claus Very deep yellow color ofsynthetic diamonds is due to high content of C defects in the concentration rangeof ppm Collins Although theincrease in the intensity of yellow color follows the C defect concentration, it isdifficult to accurately link defect concentration to color grade for different dia-monds due to the effect of size, shape and cut Fisher Very rarely, natural diamonds with strong absorptions in H3 and H4 centers see below Chap.
Some natural yellow diamonds with rather pro-nounced orange modifier owe their color to a broad absorption band with maxi-mum intensity at a wavelength of nm accompanied by an absorptioncontinuum steadily increasing towards shorter wavelengths Collins Fig. C defects have moderate temperature stability. HPHT treatment, when Enhanced absorptionstarting in the green spectral range and rapidly increasing towards blue and UV spectral range isthe cause of the yellow coloration. C defects absorb light in the yellow and green spectralranges stronger than N2 centers in cape-yellow diamonds.
Due to this difference, type Ibdiamonds have an orange modifier in their color when compared with the cape-yellow diamonds2. However, HPHT treatmentperformed at high temperature 2, C and above , reversibly produces Cdefects in type Ia diamonds and may strengthen the yellow color component. When viewed in polarized light, natural IIb stones, like other type II diamonds,have characteristic tatami-structure due to low-nitrogen content. Some natural bluediamonds owe their color to high concentration of hydrogen, which absorb in redand yellow-green spectral ranges. Rarely, high-hydrogen diamonds reveal a strongbroad-band absorption starting at nm and spreading towards IR spectral range.
This absorption may add a greenish tint to the blue and violet color Darley andKing Fig. Blue and violet colors cannot be produced by HPHT treatment. However,HPHT treatment can considerably improve the boron-induced blue color of bluish-gray or gray type IIb diamonds via reduction or removal of the gray component. The H3 absorption band, when recorded at room temperature,resembles the nm band.
Yet the measurement at liquid nitrogen temperature reveals finestructure of the H3 band, what makes it distinguishable from the nm band. In this diamond,the H3 center also exhibits noticeable green transmission effect which makes the diamond tolook greener. The nm band exhibitsno fine spectral structure. Orange tint of the color of this diamond is enhanced by theluminescence band at nm, which is the emission replica of the nm absorption band. Thiseffect is analogous to the green transmission effect of the H3 center18 2 Diamonds Used for HPHT TreatmentThe bluish color can be also stimulated by natural irradiation, which producesthe GR1 center.
Electron-irradiated pale-blue diamonds called ice-blue dia-monds are quite popular in the gem market Kitawaki This type of coloration of natural rough diamonds wasdescribed in Orlov The main reason of the radiation-induced green color isthe absorption of GR1 center Fig. Light green coloration may be observedthrough the whole diamond body body distribution of vacancies producing theGR1 center indicating the action of deeply penetrating beta- and gamma-radiation Yelisseyev et al.
Alternatively, green coloration can be concentrated closeto the surface as a result of dominating irradiation with alpha-particles, whichpenetrate into diamond to a depth of a few microns only. Natural untreated green diamonds have very light color because of weak GR1absorption. These diamonds often exhibit green to brown irradiation spots radio-halos, Fig.
This feature makes the naturally irradiated greendiamonds distinguishable from the green diamonds processed by the radiationtreatment. However, it is important to understand that the radiohalos are not theultimate proof of the natural body color of green diamonds. Instead, the diamondswith natural radiohalos can be irradiated with high energy electrons with the aimof improvement of their body color and then the radiohalos can be convenientlyused as a solid proof that these diamonds are naturally irradiated. Moreover,radiohalos can be produced on the diamond surface artificially applying irradiation The violet modifier ofthe color is caused by thetransparency windows at awavelength of nm.
Theabsorption feature at nm,which also adds to the blue-violet coloration, is probablydue to hydrogen2. He or C ions through a stencil mask e. The color of the artificial radiohalos can be madequite identical to those of natural ones Fig. Some natural diamonds exhibit green color when viewed under daylightillumination. This light-induced green color is the result of intense luminescenceof H3 optical center excited by blue and UV components of the daylight.
Suchdiamonds are known as green transmitters. In perfect low-nitrogen diamondswith little content of A defects, the H3 defects are very effective in greenluminescence under daylight excitation. Thus, even being present in small con-centrations, the H3 defects may considerably contribute to green color via thecharacteristic green emission in the spectral range nm Fig.
Thegreen color of green transmitters considerably weakens or completely disap-pears when diamond is viewed in incandescent light. The green color of this diamond is distributedthrough the whole diamond body. Low intensity of the GR1 center absorption coefficient is about0. These spots have been irradiated with different ions, atdifferent energies, with different doses and annealed at different temperatures.
Depending on theparameters of the irradiation and annealing, the color of the irradiated areas can be green, orange,brown and black Nasdala et al. Color of these diamondsresembles the color of synthetic diamonds grown at elevated temperatures in thepresence of Ni impurity Vins Thus, one may assume that the natural Ni-rich diamonds of greenish color also grew in earth at elevated temperaturesexceeding 1, C.
Therefore, these stones may exhibit some features charac-teristic of HPHT-treated diamonds processed at low temperatures of1,, C. Green color of natural diamonds induced both by H3 and Ni-related centers canbe considerably modified, enhanced or reduced by HPHT treatment. The mostcharacteristic example of this change is the HPHT-induced green transmitters. Whereas the green transmission effect is a rare feature of pristine natural dia-monds, it is a very common result of HPHT treatment performed at temperaturesbelow 2, C.
HPHT-induced green transmission effect is particularly pro-nounced in diamonds with low content of A defects. Pink coloration of natural diamonds does not depend on thepresence of nitrogen impurity, and these diamonds can be of types I and II Kinget al. Pink coloration, like brown graining, is also restricted to thin bands parallel tooctahedral slip planes pink graining. However, pink color is distributed ratherhomogenously within these bands. Nitrogen-containing pink-purple diamonds are Thedominating green color ofthis diamond results from theH3 center luminescence seenin the absorption spectrum asa reverse structured bandin the spectral range nm.
The finestructure of this bandcorresponds to the vibronicfeatures of the H3 center inluminescence2. This observation implies thatpink-purple diamonds had been brought from the earth interior to the surface atearly stages of their formation and the processes of the nitrogen aggregation inthese diamonds had not been completed.
Therefore, the detection of C defects in these diamonds cannotbe considered as a proof of HPHT treatment. The primary reason of pink color of plastically deformed natural diamonds is abroad absorption band with maximum at about nmso-called Pink Band Fig. Pink color may be also highlighted by the presence of an absorptionband with maximum at nm Fig.
A diamond, the absorption spectrumof which has these two bands of comparable strength, has particularly pleasantpink color. The and nm bands can occur in both type I and type IIadiamonds. However, their strength is usually weaker in nitrogen-free diamonds. Naturally occurring NV centers and nm centers, see below may alsocontribute to red color of untreated natural diamonds. Although it is an extremelyrare event that a natural diamond has the nm center strong enough to affect itscolor Fritsch ; Scarratt , such diamonds are documented in literature.
Visible absorption spectra of these diamonds may contain nm NV- center ,H3 and H4 absorptions Wang et al. In this diamond, the Pink Band superimposes with the brown absorptioncontinuum of a moderate strengtha typical absorption spectrum of natural pink diamonds. Insert shows photo of a diamond of this type [re-plotted from Chapman et al. Two major transparency windows atwavelengths from to nm blue color and above nm red color produce pink colorof this diamond22 2 Diamonds Used for HPHT Treatment2.
Since perfectly uniform absorption in a wide spectral rangeoccurs rarely, gray color of natural diamonds is usually accompanied by faint tints ofyellow, green, blue or pink. It is believed that the two major sources of gray color aregraphitic micro-inclusions and hydrogen Vins and Kononov The graphiticinclusions work as light scattering centers causing Rayleigh scattering, rather thanlight absorbing centers.
Because of this scattering, gray diamonds may have trans-lucent appearance. The size of the graphitic micro-inclusions varies and may reach afew micrometers. The big graphitic inclusions can be seen in microscope andidentified by their characteristic hexagonal shape.
The most probable explanation ofthe uniform formation of graphite nano-crystals through the body of gray diamonds isthat these diamonds grew at the temperaturepressure parameters close to the dia-mond-graphite phase transition. A less probable mechanism could be the directincorporation of graphitic inclusions during the diamond growth. The hydrogen-related defects work like regular absorbing optical color centers. All natural diamonds contain considerable amount of hydrogen, some of which ispresent in form of optically active defects. In nature, diamonds always grow in thepresence of hydrocarbons mainly methane and the products of its dissociation ,carbon oxides, nitrogen hydrides, hydrogen and other gasses Digonsky and Di-gonsky It is also believed that the crystallization of diamond from graphiteoccurs directly during dissociation of hydrocarbons into carbon and hydrogenirrespective of the growth medium: During the crystallization of carbon, hydrogen may form CH radicals on diamondsurface and on the lateral edges of the graphite crystals.
It is known that duringheating with increasing temperature, hydrocarbons polymerize and experiencecomplex transformations until final graphite phase is formed. During thesetransformations, hydrogen content reduces. Upon completion of the conversion ofhydrocarbons into graphite, hydrogen can localize only on the lateral edges of thegraphite crystals.
Hence the amount of the remaining hydrogen is determined bythe concentration of the graphite crystals and their size: Gray diamonds grew at the conditions of high oversaturation, when the diamondgrowth medium rapidly cooled down during its travel to the earth surface. Analysis ofthe impurity-defect structure of gray diamonds suggests that these diamondsexperienced rapid annealing, the last period of which was at the conditions close tothe graphite-diamond phase equilibrium.
Low growth temperature of gray diamondsresulted in the preferential formation of low-temperature nitrogen defects: Thus, natural nitrogen-containing diamonds of gray color are usuallyof type IaA. The intensity of A center in spectra of these diamonds can be as highas 65 cm Yet some type IaA gray diamonds may exhibit the presence of tracesof C defects. In addition to A- and C defect absorptions, all gray diamonds,2. In type IIb diamonds, gray color may be accompanied by additional browncoloration.
Combined effect of the absorption continuum due to boron and thebrown absorption continuum due to vacancy clusters may essentially result in graycolor. Majority of natural type IIb diamonds have grayish tint because of thiseffect. The HPHT-induced color changes are very different for dia-monds of different categories. The first category comprises type IIa brown dia-monds. These diamonds, and especially those of high clarity and light brown color,are suitable for the production of the most perfect high color stones. Brown typeIIa diamonds are also used for production of light pink stones, the pink color ofwhich is identical to that of untreated natural pink diamonds.
The diamonds of thiscategory are the most valuable starting material for HPHT treatment. The second category comprises type IaB brown diamonds. The primary aim ofHPHT treatment of these diamonds is the production of near colorless stones and,in rare cases, low grade colorless stones. Since HPHT treatment always producesin nitrogen-containing diamonds at least traces of the optical centers active in thevisible spectral range, brown type IaB diamonds cannot be converted into colorlessstones of high color grade.
Another aim of treatment of brown as well as near-colorless and low color grade type IaB diamonds is production of pink stones. Inthis case, the diamonds are subjected to multi-process treatment resulting in theformation of NV- center of moderate intensity. Hence theresulting color of the treated diamonds is not affected by the C defect absorptioncontinuum. The most beautiful pink color is achieved when two major absorptioncenters N3 and NV- have comparable intensities. Visible transmission spectrumof these diamonds has two windows at wavelengths around nm blue andabove nm red.
The combination of these two colors makes diamonds pink. The third and the most populated category is high clarity type IaAB browndiamonds. Inclusion-free brown diamonds of type Ia are starting material formaking beautiful high clarity diamonds of fancy yellow and green-yellow colors. Appearance of some HPHT-treated type Ia diamonds can be very attractive by farexceeding that of any natural untreated stone of this hue.
Another aim of treatmentof brown type IaAB diamonds is the production of red stones Imperial Reddiamonds. For this purpose, multi-process treatment is applied. The treated reddiamonds owe their color to the very strong NV- center, the absorption of whichforms pronounced transmission window in the red spectral range. High-nitrogendiamonds treated in this way may also reveal strong C defect absorption24 2 Diamonds Used for HPHT Treatmentcontinuum, which adds to diamond color an orange modifier.
In order to achieve apleasant red color, the diamonds should not contain much nitrogen. High con-centration of nitrogen results in too intense optical centers and too deep red color,which could be even modified with unwanted brown tint. Low clarity type Ia diamonds of any unattractive colors mostly brown andgray are the diamonds of the fourth category. The aim of HPHT treatment of thesediamonds is the color improvement and obtaining saturated fancy colors, whichcould hide the included interior. The four diamond categories mentioned above comprise the diamonds, whichare most frequently used for HPHT treatment.
However, any natural diamond,regardless its type, color and clarity, can be treated by HPHT annealing with theaim of increasing its commercial value. Even colorless diamonds of the highestcolor and clarity can be subjected to HPHT treatment with the aim to add them rarefancy colors. Thus, its mainparameters are the annealing temperature, the chemical composition of the med-ium atmosphere in which annealing is performed, the pressure the diamondsexperience in the medium during annealing, and the duration of the annealing.
During HPHT treatment, pressure and temperature are set so that the treateddiamonds are brought into the thermodynamic range, where the atomic diffusion isfast enough to provide considerable atomic transformations of optically activedefects in a reasonably short time ranging from few seconds to few hours Anthony et al. For most defects, these transformations occur more effec-tively, when diamond is in plastic state. The pressuretemperature range of thethreshold, beyond which diamond loses its rigidness and becomes ductile, is in therange of 57 GPa and , C respectively [e.
DeVriesshowed that at pressures of 46 GPa diamond could be plastically deformed attemperatures starting from C DeVries It isseen that HPHT treatment is always performed in the area of plasticity of diamond. Since at normal pressure diamond is a metastable carbon phase, it is importantto compare the pressure-temperature parameters of HPHT treatment with those ofthe graphite-diamond phase transition. Yet, in order to reduce pressure and to simplify the process, HPHTtreatment is frequently performed within the range of stability of graphite.
In thesecases, the treatment time is kept rather short a few minutes in order to avoidextensive graphitization. When working in the range of stability of graphite, it isalso important to use perfect starting material with the least content of non-diamond inclusions and voidites. The structural imperfections cause internal stressand weaken the diamond lattice thus promoting the conversion of diamond intographite. A small deviation from the diamond-graphite phase equilibrium line does notplay significant role in the transformation of the nitrogen defects and does notI.
However, it may be a crucial for kinetics of growth and dissolving of the Platelets. Increase intemperature exponentially accelerates the atomic diffusion and initiates the defecttransformation processes with high activation energies. For each C in tem-perature increase, the mobility of nitrogen in diamond increases almost by an orderof magnitude Evans et al.
Althoughone tries to perform HPHT treatment at maximum possible temperatures, it isdifficult and expensive to run a controlled HPHT process at temperatures above2, C. The diamond-graphiteequilibrium line Simon-Berman line is shown by black solid line and the pressuretemperaturethreshold of the diamond plastic yield is shown by dashed curve Kennedy and Kennedy ;Anthony et al.
Parameters of annealing at low pressure in vacuum or at ambient pressure are shown with squares and line. The colored areas show the pressuretemperature ranges ofpossible natural HPHT annealing light green , commercial HPHT treatment of differentcompanies red ovals , natural growth and natural high temperature annealing green , naturalplastic deformation brown , and natural low temperature annealing yellow. For instance, low temperature HPHT annealing maybe performed in combination with electron irradiation.
The temperature range used for heat treatment of diamonds may be split into thefollowing intervals, each being characterized by major defect transformationsinfluencing the diamond color. Very low temperatures spread from to 1, C. Annealing of diamond atvery low temperatures does not require stabilizing pressure and it can be per-formed in vacuum or inert gas atmosphere for a long time. These temperatures aretoo low for the direct transformations of the major nitrogen defects. However, attemperatures close to 1, C, the vacancy clusters may start to dissociate.
Thereleased vacancies may form the H3 defects in type Ia brown diamonds and makethem green. Very low temperatures are not used for HPHT treatment alone. Though, thesetemperatures are used to achieve the color changes in irradiated diamonds. Theirradiation-induced vacancies become mobile at temperatures over C, theyform complexes with all major nitrogen defects and stimulate the nitrogenaggregation.
These processes may cause considerable color changes in diamond. Annealing at very low temperatures is a common step of multi-process treatments,e. The range of very low temperatures comprises the temperatures of growth andpost-growth annealing of diamonds in nature.
At these temperatures, the motion ofinterstitials and vacancies is activated, the aggregation of vacancies occurs andnatural diamonds acquire brown color. Very low temperatures are used for thetreatments where the formation of vacancy-related defects is required: Low temperatures cover the range from 1, to 1, C. These temperaturesare high enough to cause rapid graphitization of some natural diamond. Hence thesafe annealing at low temperatures must be performed under stabilizing pressure.
Low temperature annealing, like very low temperature annealing, is rarely used forsole HPHT treatment for it does not change rapidly the color of most naturaldiamonds. However, HPHT annealing at low temperatures may cause considerablecolor changes, when it is applied to irradiated diamonds. Besides, low temperatureHPHT treatment performed for a few hours may be sufficient for complete removalof brown hew in light brown diamonds. Temperaturepressure parameters of lowtemperature HPHT treatment are very close to those of the natural annealing inearth.
Hence its recognition poses the most severe problems. Moderate annealing temperatures cover the range from 1, to 2, C. Atthese temperatures, most defects, responsible for brown color of natural regularbrown diamonds, anneal out and major nitrogen-vacancy defects are formed. Theaggregation and dissociation processes of major nitrogen defects become notice-able at these temperatures too. HPHT annealing at moderate temperatures is usedas an inexpensive and reliable treatment of light brown diamonds. Annealing atmoderate temperatures is frequently used to achieve pink color in type IIa browndiamonds.
At these temperatures, brown color of most brown naturaldiamonds is removed in several minutes. At high temperatures, the processes ofaggregation and dissociation of nitrogen defects occur most effectively. Yet, hightemperatures generate an appreciable concentration of C defects in type Ia dia-monds and this peculiarity makes high temperature HPHT treatment fairly easilyrecognizable.
The temperatures over 2, C are very high temperatures. CommercialHPHT treatment is rarely performed at these temperatures. At very high temper-atures most nitrogen defects aggregate. The rate of aggregation depends onpressure and the initial nitrogen content. At the pressures above the graphite-diamond phase transition, the aggregation dominates and nitrogen-containingdiamonds are converted into type IaB.
At pressures below the graphite-diamondphase transition, the dissociation processes may dominate and the treateddiamonds have type Ia B [ A. When discussing temperature as a parameter of HPHT treatment, it is importantto keep in mind that accurate measurement of temperature in high pressure cellstill remains an unmet challenge.
Temperature can be fairly well measured directlyusing thermocouples up to 1, C. Higher temperatures are measured onlyindirectly using calibrations against electrical power used for heating of highpressure cell. These calibrations strongly depend on many technical parameters ofthe high pressure apparatuses and they can differ considerably. At very hightemperatures above 2, C, the error of the temperature measurement mayexceed C. This error is one of the main reasons of discrepancy between theresults reported by different authors.
High pressure helps to reach the area of plasticity. Plastic flow mayconsiderably stimulate the defect transformations and generation of the defects,which otherwise cannot be formed at high temperature alone. Yet, pressure doesnot influence the defect transformation as much as temperature: Our experiments on high temperature annealingperformed at low pressure revealed that natural diamonds can withstand a fewminutes heating in vacuum at a temperature of 2, C.
Although severe outergraphitization may occur during such a heating, the interior of inclusion-freediamonds remains perfect. In hydrogen atmosphere, diamond can withstand short-time annealing at temperatures as high as 2, C. At such a high temperature,the annealing time is kept a few seconds only Fleischer and Williams Although the temperature range of the low pressure annealing may be the same asthat used for HPHT treatment, its result is different.
Indeed, at low pressure, even30 3 Parameters of HPHT Treatmentat a temperature of 2, C, diamond still remains in the rigid state, whereas it isalready in the range of plasticity at a temperature as low as 1, C, if the heatingis performed at a pressure of 5 GPa Fig. In addition to rendering diamond plastic, the external pressure also increases thephase stability of diamond.
At normal conditions, diamond lattice is in a com-pressed state sp3 CC covalent bond is shorter in diamond lattice than it is inequilibrium state. Because of this stress, diamond is metastable. This internalcompression can be considered as the internal pressure, the magnitude of which iscomparable with the diamond-graphite phase equilibrium line on the pressuretemperature diagram Fig. The presence of impurity atoms with atomic radii greater than that of carbon e.
The reduction of the internal pressure is greater in diamonds with nitrogen com-plexes and therefore the nitrogen aggregation is a favorable process of the nitrogendefect transformations. Application of external pressure brings diamond lattice toequilibrium and promotes the nitrogen aggregation. For this reason, the externalpressure suppresses the dissociation of nitrogen complexes. Correspondingly, thepressure reduction during HPHT treatment shifts the aggregation-dissociationprocesses towards dissociation.
Allow this favorite library to be seen by others Keep this favorite library private. At the pressures above the graphite-diamond phase transition, the aggregation dominates and nitrogen-containingdiamonds are converted into type IaB. The four diamond categories mentioned above comprise the diamonds, whichare most frequently used for HPHT treatment. These temperatures aretoo low for the direct transformations of the major nitrogen defects. Recognizing the increasing importance of materials science in future devicetechnologies, the book titles in this series reflect the state-of-the-art in under-standing and controlling the structure and properties of all important classes ofmaterials. As a result, the as-grown nitrogen-containing diamonds, both natural andsynthetic, are of type Ib.
An example of the influence of pressure on the nitrogen defect transformationsin diamond is the dissociation of nitrogen complexes into isolated atoms. It wasfound that heating at 2, C at a pressure of 8. The influence of pressure on the aggregation of carbon interstitials is similar tothat of nitrogen: It is veryslow at temperatures and pressures in range of diamond stability and it increasesdrastically when diamond is annealed under lower pressure in the range of stabilityof graphite.
Opposite effect is expected for small radius impurity atoms and vacancies. Forthese defects, external pressure should promote the dissociation. Indeed, the dis-sociation of vacancy clusters in diamonds goes faster under higher pressure.
Hence, the reduction of brown color is more effective for HPHT treatments per-formed at elevated pressure. For instance, annealing of a brown diamond attemperatures in the range from 1, to 2, C at normal pressure reduces itscolor only moderately. In contrast, HPHT annealing of a diamond of the sameinitial color at the same temperatures may remove brown color almost completely.
An important factor of pressure is its uniformity or hydrostaticity. DuringHPHT treatment, the pressure applied to diamond should be as hydrostatic aspossible. Non-hydrostatic pressure may break diamond, or it may cause plasticdeformation, which in turn may induce unwanted defects affecting the final color3. The minimization of the pressure non-uniformity is achieved byappropriate design of the high pressure cell, where diamond is annealed. However,even in the ideal cell, the non-uniform stress inside the diamond during annealingmay be considerable.
The reason is the unpredictable distribution of the initialinternal stress, which is present in any natural diamond and which is impossible tocontrol. In order to reduce the influence of the initial internal stress, the diamondsare pre-shaped before HPHT treatment. The aim of the pre-shaping is the removalof the areas of possible non-uniformity: It mayvary from a few seconds pulse HPHT treatment to a few minutes as it is in mostcases of commercial treatment and even several hours.
The physical principles ofthe short-time HPHT treatment is similar to those of the pulse annealing used inelectronic industry for processing of ion-implanted semiconductors. The pulseHPHT treatment is an inexpensive procedure and therefore favorable.
A charac-teristic feature of the pulse heating is a very non-equilibrium defect structureleading to a distinctively unnatural combination of defects. Because of this, theshort-time HPHT treatments can be relatively easily recognized by comparing theconcentrations of the defects with different temperature stability and kinetics rate. For instance, pulse HPHT treatment is too short to allow nitrogen to diffuse andaggregate.
Thus, the resulting set of the nitrogen defects may reveal increasedcontent of the simplest forms like NV defects. Single vacancies are very mobile defects and, once theactivation temperature is achieved e. In contrast, the diffusion of impuritiesin diamond lattice is a much slower process and the transformation of the impu-rity-related defects may take few hours at temperatures of HPHT treatment.
Forinstance, the dissociation of A defects and the increase in the concentration of Cdefects during HPHT annealing increases almost linearly with time for short-timetreatment and it may take up to 1 h until the equilibrium concentration of the Cdefects is achieved Claus At lower temperatures, like the ones naturaldiamonds experience in earth, the establishment of equilibrium may take billionsyears. The duration of HPHT annealing required to achieve the new equilibriumconcentrations of defects also depends on the initial defect content.
De Weerdt andCollins showed that in diamonds with initial concentration of A defects 7 and15 ppm, 3 and 10 min HPHT treatments at a temperature of 2, C increase theamount of the C defect concentration to 0. This result suggests that 10 min annealing is sufficient to reach thedefect equilibrium in diamond with low impurity content and it requires muchlonger time when the impurity concentration in diamond is high. A further example of the influence of the annealing time on the defect com-position in HPHT-treated diamonds is given in De Weerdt and Collins Ithas been shown that short-time HPHT treatment of type Ia brown diamonds makesthem green, whereas the prolonged one makes the diamonds yellow.
This colorchange suggests that the formation of H3 defects in brown diamonds occurs muchfaster than the dissociation of A defects into C defects. Hence, at the initial stagesof HPHT treatment, the H3 center is the dominating color center and its highabsorption intensity makes diamond green. During long-time HPHT annealing, thevacancy clusters anneal out and the source of vacancies depletes. As a result, theformation of new H3 defects stops and the H3 defect concentration starts todecrease due to the dissociation of the H3 defects into C defects.
If the annealingtime is long enough, the C defect concentration becomes high enough to providedominating yellow color.
Short-time HPHT treatment may also result in a high concentration of defectswith low-temperature stability and slow annealing kinetics. The NV defects are anexample. Duration of HPHT treatment is also an important parameter in term of graph-itization. Commercial HPHT treatment is frequently performed at temperatures inthe range of stability of graphite.
Thus, the treatment time has to be kept shortenough in order to prevent excessive graphitization. The time of safe annealing3. For instance, HPHT treatmentat a temperature of 1, C can last for many hours without a trace of graphi-tization. If the temperature is increased to 2, C, diamond may be completelygraphitized in a minute. General picture of evolution of concentration of the basic defects determiningthe final color of initially brown type Ia diamonds after short and prolonged HPHTtreatment is shown in Fig.
Short-time HPHT annealing at low temperaturesdoes not change initial brown color considerably. Yet, prolonged low temperaturetreatment makes diamonds brownish-green. When brown type Ia diamonds areHPHT annealed at moderate temperatures, in a short time they become brownishgreen. For longer treatment times, the diamonds acquire yellowish-green color. Ifthe annealing temperature is raised to 2, C, short-time annealing makes dia-monds yellow-green.
Prolonged annealing at high temperatures makes initiallybrown diamonds greenish-yellow and yellow. At lowannealing temperatures, slow dissociation of the vacancy clusters and slow increase inconcentration of H3 defects occurs. Formation of C- and NV defects is negligible. At moderatetemperatures, the dissociation of vacancy clusters and the reduction of brown color go muchfaster. A considerable concentration of H3 defects appears. When the vacancy clusters anneal out,the concentration of H3 defects stabilizes and the concentration of NV defects reduces. Theconcentration of C defects steadily grows with time.
At high temperatures, complete dissolving ofvacancy clusters occurs in a short time. High concentration of H3 defects and enhancedconcentration of NV defects are created during the destruction of the vacancy clusters. This fact is the background for the diamond treaters trying to justifyHPHT treatment as a natural process and, as such, undistinguishable from thenatural annealing. Yet, the studies of HPHT-treated diamonds show that theresulting defect structure produced by HPHT treatment does differ from that ofuntreated natural diamonds.
Understanding these differences is the key for therecognition of HPHT-treated diamonds. The defect structure of natural diamonds differs from that of the HPHT-treatedcounterparts because of the difference in the parameters of HPHT treatment andnatural annealing.
Although this difference is quantitative rather than qualitative,its impact on the resulting defect structure of diamond is considerable. HPHTtreatment is always performed for time incomparably shorter than that of thenatural annealing. HPHT treatment is almost always performed at temperaturesconsiderably higher than those, diamonds experience in nature. CommercialHPHT treatments are frequently performed at pressures corresponding to the phasestability of graphitean unlikely condition for diamonds in nature.
Thus, HPHT-treated and natural untreated diamonds have different defect compositions, whichreflect very different equilibrium defect concentrations characteristic for low andhigh temperatures. For instance, HPHT-treated diamonds always have unpropor-tionally high content of temperature stable defects. Natural diamonds have been exposed to geological temperature and pressure fora long time up to 3. Majority of natural diamonds were formed insubcontinental lithosphere at depths of km, where temperature andpressure could vary from to 1, C and 47 GPa respectively.
For instance,type I diamonds from the kimberlite pipe Mir, grew at temperatures of1,, C under pressure of 46 GPa Vins and Kononov After the period of growth, most natural diamonds passed through the hotperiods of post-growth annealing, during which temperature could reach 1, C Kiflawi and Bruley ; Howell Some diamonds from South Africa andBrazil show evidence of the residence in the lower mantle at depths up to km,what suggests a pressure over 8 GPa and a temperature over 1, C Meyer andSeal ; Kirkley et al.
This natural HPHT annealing occurred for very long time and resulted in theaggregation of nitrogen impurity and conversion of nitrogen containing diamondsinto type Ia. At temperatures of 1, C, the aggregation of nitrogen approachescompletion when B- and N3 defects dominate and C defects are almost unde-tectable Kiflawi and Bruley , Collins et al.
Yet, since nitrogenaggregation at geological temperatures is an extremely slow process, the inter-mediate nitrogen aggregates like A- and H3 defects are readily present too. It is remarkable that natural diamonds of pure type IaA always contain minorcontent of C defects, the concentration of which is usually in a certain ratio withthe concentration of A defects. This concentration ratio is rather constant fordiamonds mined from one and the same pipe.
The most probable C defect concentrations are 0. This definite amount of C defects is the result of the annealing atthe particular temperaturepressure conditions, which were specific for a givenpipe. This fact suggests that in natural diamonds the nitrogen defects are inequilibrium concentrations characteristic of temperatures below 1, C andthese diamonds have never experienced short-time heating at high temperature.
High temperature considerablyincreases the equilibrium concentration of dispersed nitrogen. Hence, afterHPHT treatment, vast majority of type Ia diamonds reveal reverse decompositionof the nitrogen aggregates into C defects and formation of the defect composition,which is highly non-equilibrium for low temperatures.
Generation of C defects, which are donors in diamond lattice, charges nega-tively many other defects. As a result, HPHT-treated type Ia diamonds revealunnatural presence in optical spectra strong optical centers due to negativelycharged defects. Although the defect composition is different for treated and untreated dia-monds, it cannot be considered as the ultimate proof of treatment or otherwise itsabsence.
Indeed, some natural diamonds may exhibit very non-equilibrium defectcompositions typical of HPHT-treated diamonds. For instance, these are the dia-monds, which experienced natural heating at the conditions of the stability ofgraphite. These diamonds have unproportionally low concentration of Platelets,although they clearly exhibit the nitrogen defect composition characteristic of thelast stage of the nitrogen aggregation. Of course, the destruction of Platelets is notnecessarily stipulated by high temperatures.
In plastically deformed diamonds, thePlatelets can be destroyed by moving dislocations at relatively low temperatures. However, in diamonds with no sign of plastic deformation, one has to assume thatit is the temperature what caused the collapse of Platelets Lang et al. Reductionof brown coloration does not require high temperatures and therefore is supposedto be a common natural process. It has been found that bicolor Argyle diamonds,which comprise colorless zones and zones of brown color, have considerably lessnitrogen in the brown zones than in the colorless ones 25 times less.
Thecolorless zones tend to be predominantly of type IaB, whereas the brown zones arepredominantly of type IaA. There is also a tendency that the colorless zones havesmaller amount of Platelets than the brown zones. A conclusion has been madethat nitrogen stiffens diamond lattice and does not allow it to be easily deformed. Itmight be also that the colorless zones are those which experienced a high tem-perature natural HPHT annealing, what resulted in the removal of brown color andreduction of concentration of A defects and Platelets.
To do so, the impurity-defect structure of diamond must bemodified so that the unwanted color centers are removed and the desirable ones areinduced. Yet there are two major processes, whichcontrol the formation of the impurity-defect structure of diamond during HPHTtreatment. They are the aggregation of simple nitrogen defects into multi-atomcomplexes and the dissociation of complex nitrogen aggregates into more simplecomplexes and isolated nitrogen atoms.
Understanding these processes is the keyfor both an HPHT technologist trying to achieve the best color for a particulardiamond and a gemologist working on recognition of HPHT-treated diamonds. The HPHT-induced nitrogen defect transformations involve many differentdefects. It is a complex process, which strongly depends on the parameters oftreatment and the initial impurity-defects content and the structural perfection ofdiamond.
Due to this complexity the results of HPHT treatment may considerablyvary even for identically treated diamonds. Diamond crystal lattice, even perfect one, is not in equilibrium. Itis internally compressed and, because of this internal compressive stress, diamondis a metastable phase at low external pressure. In order to bring diamond toequilibrium, an external pressure equalizing the diamonds internal pressure mustbe applied.
At room temperature it is about 2 GPa Fig. With temperature, theinternal pressure grows and makes diamond increasingly metastable. Presence ofnitrogen expands the diamond crystal lattice and reduces the internal stress. For instance, thepresence of A defects at a concentration of 1, ppm increases the lattice con-stant of diamond by 0. Hence the aggregation of nitrogen is a thermodynamically favorable pro-cess and it occurs at any temperature.
Like for any thermodynamic process, the nitrogen aggregation cannot go to fullcompletion. An equilibrium amount of dispersed nitrogen is always present. Theequilibrium concentration of dispersed nitrogen increases with temperature, yet atany temperature, it remains much smaller than that of the nitrogen aggregates. Therefore, for most natural diamonds, the main tendency in the transformation ofthe nitrogen defects during HPHT treatment is aggregation Kiflawi andLawson During growth, diamonds capture nitrogen impurity in form of individualatoms.
As a result, the as-grown nitrogen-containing diamonds, both natural andsynthetic, are of type Ib. When as-grown diamonds are further exposed to hightemperature annealing, the isolated nitrogen atoms migrate and form energeticallymore favorable multi-atom defects Evans and Qi ; Goss et al.
The nitrogen aggregation is a multistep process involving migration ofisolated nitrogen atoms C defects , pairs of nitrogen atoms A defects andmultiatom complexes like B defects. The diffusivity of nitrogen decreases with thelevel of aggregation. The C defects are the most mobile, whereas the B defects arethe least mobile Koga et al. Formation of two-atom aggregates is the first step of the aggregation process.
Itinvolves the aggregation of C defects and their derivatives e. Although the nitrogen aggre-gation occurs at any temperature, in diamonds with moderate nitrogen content, themeasurable change in the concentration of dispersed and aggregated nitrogenoccurs only at temperatures over 1, C. Heating at temperatures below1, C does not result in obvious nitrogen aggregation even in type Ib naturaldiamond heated for a long time. No change of yellow color of these diamondsoccurs too Howell Yet in high-nitrogen type Ib diamonds with nitrogenconcentration of ppm, the aggregation of dispersed nitrogen atoms into Adefects becomes noticeable at a temperature of 1, C.
In diamonds with evenhigher nitrogen content, well above ppm, the aggregation of the dispersednitrogen can be detected at a temperature as low as 1, C, the process beingespecially noticeable at low pressures Kiflawi et al. In high-nitrogendiamonds, the aggregation of C defects into A defects can almost rich its equi-librium after a few hour annealing at temperatures 1,, C Shiryaev et al. The activation energy of the aggregation of C defects into A defects in naturaldiamonds ranges from 4. In syntheticdiamonds, which may contain considerable concentration of transition metalatoms, the activation energy of the C- to A-defect aggregation may vary from 2.
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Berlin ; New York: Springer series in materials science En ligne , v. English View all editions and formats Summary:. Find a copy online Links to this item SpringerLink. Allow this favorite library to be seen by others Keep this favorite library private.
Springer Series in Materials Science Diamonds Forever first book providing a comprehensive review of the properties of HPHT-treated diamonds, based on. Editorial Reviews. From the Back Cover. High-temperature and high-pressure treatment of HPHT-Treated Diamonds: Diamonds Forever: (Springer Series in Materials Science) Edition, Kindle Edition. by.
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