-Indian Journal of Pure & Applied PhysicsVol. 44, June 2006, pp. 440-445

Melting point variation with pressure and material synthesis by a laser heateddiamond anvil cell

S Meenakshi, V Vijayakumar & B K Godwal*High Pressure Physics Division, Bhabha Atomic Research Centre, Mumbai 400085

Received 30 August 2005; accepted 10 April 2006

The details of a high temperature-high pressure (HT-HP) facility based on the coupling of the laser heating by diamondanvil cell are described. The measured melting curve of platinum up to 12 GPa pressures is compared with existing piston-cylinder data and with theoretical calculations. The reasonable agreement reveals the utility of this technique for the reliablemeasurements of high temperature-high pressure phase diagrams and melting curves for materials. The suitability ofhexamethylenetetramine (HMT) as a precursor for the synthesis of C)N4 at HT-HP, has been investigated. It is conjecturedthat CN, (C)N4) or C:H is formed at HT-HP, but is not quenchable.

Keywords:Diamond anvil cell, Melting point, Material synthesis, High pressure, High temperatureIPC Code: GOIN 25/04, BOlJ 3/06

1 IntroductionHigh temperature-high pressure (HT-HP)

behaviour of materials is of significant interest inseveral areas of science and technology especially inmaterial science and geophysics'. The pivotal role ofthe diamond anvil cell (DAC) in material research athigh pressures and high temperatures derives from theunique set of physical properties of diamonds viz.;hardness/strength, transparency across a broad rangeof electromagnetic spectrum, high thermalconductivity, etc. The two techniques capable ofgenerating very high pressure and temperatureconditions are the heated (laser or resistive) diamondanvil cells and shock compression. With electricalresistance coils external to the diamond anvils,1100 K and 100 GPa have been reached for in-situ X-ray diffraction measurements". The most difficultproblem in heating the sample externally is thesoftening of the stress-bearing components, i.e., thegasket, diamonds, backing plates and the cell body, athigh temperatures and the graphitization of thediamonds. Direct electric heating' of metallic samples(iron wire) located at the center of the DAC andthermally insulated from it has also been attempted,but is of limited applications. Laser heatingconstitutes an alternative method for heating thesamples compressed in the DAC. Laser heatingtechniques in conjunction with the DAC exploits the

*E-mail: bkgodwal@magnum.barc.ernet.in

transparency and high thermal conductivity ofdiamonds to provide a unique opportunity toinvestigate HT-HP physical and chemical propertiesof materials I ,4,5.. The technique of HT -HP generationcan be used to obtain P- V- T equation of state (EOS)and phase stability (structural transition and melting).It can also be used for the synthesis of novel materialsand to bridge the gap between shock wave and statichigh pressure region". Recently, there is a lot ofinterest in in-situ Raman spectroscopy at HT-HP forprobing the phase transitions and bonding relations'.The main advantage of laser heated DAC over shockexperiments is that it provides a controlled means ofachieving P- T conditions which can be kept constantfor long periods of time to allow' variety of visual,

. )spectroscopic and X-ray measurements. Thequantitative results obtained with laser heated DACrely on the accuracy of temperature measurement ofheated samples. The accuracy of temperaturedetermination is reduced because of the largetemperature gradients in both radial and axialdirections'<. Nevertheless, it remains an evolving toolfor the investigation of HT-HP behaviour ofmaterials.

A laser heated diamond anvil cell facility has beenbuilt at HPPD, BARC with the following aim:(i) Investigation of melting behaviour of materials at

HT-HP.(ii) Synthesis of materials at HT-HP with interesting

and useful material properties.

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tran.infntemjbearTernconsradiiin tlfromchoirrespc(nonlaserthe' I:focusand \Nd:Yused.is iderequiilaser:the arothersmall:otherstablebeamsufficienhan,with gcondu.gasketabsorpin aximateri:materiacombinyield aboth Imateriagas sorelevanlthermal

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MEENAKSHI et al.: MELTING POINT VARIATION WITH PRESSURE AND MATERIAL SYNTHESIS 441

Here, after a brief review of the current status,details of the set-up and the results of melting pointvariation of Pt with pressure and HT-HP stabilitystudies on hexamethylenetetramine (HMT) undernitrogen atmosphere are given.

2 PrincipleDAC with its rmrnature size coupled with the

transparency of diamonds to visible as well asinfrared radiation enables safe generation of hightemperature and high pressure by focusing a laserbeam on the material under investigation.Temperature can be conveniently determined byconsidering the laser heated hot spot as a black body

\ radiator' and measuring the' thermal radiation profilein the visible spectral region that is well separatedfrom the heating laser. Several factors decide thechoice of the laser viz., its power level, stability withrespect to power and beam pointing, laser wavelength(non -overlapping with thermal radiation), extent oflaser absorption in the material, and characteristics ofthe laser that decide the focal spot size and depth offocusing (viz., beam divergence, mode/beam profileand wavelength). Commonly, CW CO2, Nd:YAG orNd:YLF lasers, that meet the above requirements, areused. CO2 laser (10.6 11m)with larger focal spot sizeis ideal for heating transparent samples. However, itrequires separate optics for visual observation andlaser focusing. In addition, it is more likely to damagethe anvils because of larger focal spot size. On theother hand, Nd: YAG and Nd: YLF lasers, due tosmaller focal spot, are suitable for heating metals andother opaque materials. The Nd:YLF laser is morestable than Nd: YAG laser with respect to power andbeam pointing. A total laser power of 100 W issufficient, though, sometimes it may be necessary toenhance the laser absorption by mixing the sampleswith good absorbers like Pt black. The high thermalconductivity of diamond, proximity of the metallicgasket, convergence of the laser beam and largerabsorption of the laser at the material surface resultsin axial and radial temperature gradients in thematerial at HT-HP. This is reduced by insulating thematerial from the diamond, employing suitable modecombinations (TEMol and TEMoo) of the laser thatyield a flat focal spot and heating of material fromboth the axial directions. Various transparentmaterials like MgO, Ah03, nitrogen, NaCl and raregas solids are used for insulating purpose. Therelevant physical properties like melting point andthermal conductivity of the material are pressure

dependent. Thus, the pressure gradient which theinsulating material may support and its reaction withthe material under investigation are important factorsto be considered. Nitrogen being very easy to load is aconvenient choice, but often reacts with the material;however as a spin off, several metal nitrides havebeen synthesized. Inert gases are the best choice buteven they become reactive at HT-HP. Another factorthat decides thermal gradient is gasket thickness as itdecides the sample thickness and insulation thickness.This is increased by employing Re gaskets and byimpregnating gasket with diamond or Al203 powderduring indentation/'!''. The double sided laser heatingtechnique!' wherein the heating laser is split into twobeams that pass through the opposing diamond anvilsto heat the high-pressure sample simultaneously fromboth sides reducing the temperature gradientssubstantially and providing a flat power distribution.

3 Temperature Stability and Accuracy of Me-asurements

Temperature stability that is achieved depends onthe surface properties of the material (laserabsorption, chemical stability) and the laser stability.To achieve steady temperature, feedback of theemitted thermal radiation can be employed to stabilizethe laser power. This feedback from the thermalradiation can be used to control the laser power via q-switching 10, employing polarizer, liquid crystalvariable attenuator", or compensated wedges 13. Fastresponse liquid crystal attenuator is the best choice.

The accuracy of temperature measurement byradiometry at ambient pressure is not high in a DACprobably due to the thermal and pressure' gradientsthat exist. Thus, lot of effort is being made to measurethe temperature gradient by employing fiber optics 10,

binning control'v" of the CCD by selecting a smallerspot with pin holes, by using Abel transform of thespectral intensity distribution of the hot spot", etc.For synthesis of new materials or for strain relieving,temperature control is not as important as in themeasurement of melting point and phase boundary. Amethod analogous to differential thermal analysis(DT A) can be employed 16 for identifying melting andphase boundary. Other methods of detection ofmelting are decay of X-ray diffraction pattern,freezing of temperature versus laser power curve,change in reflectivity of the hot spot and visualobservation 17. The most stringent requirement ontemperature (profile) control is when in-situ X-ray

442 INDIAN J PURE & APPL PHYS, VOL 44, JUNE 2006

diffraction measurements are to be carried out withsynchrotron beamlO,18,19,

gives a Gaussian beam power distribution with lowdivergence and high stability. The opticalarrangement is shown schematically in Fig. 2. Thelaser is focused employing an infinity corrected longworking distance objective (Mitutoyo make). The lenscombination L, and L2 serves to reduce the beam sizeand to adjust the beam divergence. The beam steerer(BS1) and a hot mirror (HM) mounted on an x-y-8-<j)

4 High-Temperature High-Pressure (HT-HP) Fac-ility

The HT-HP facility shown in Fig. 1, employs aNd:YAG laser (30 W, TEMooand 200 W, multimode,Korus Laser, Korea). The TEMoo mode employed

Fig. I-High-temperature high-pressure laser heating facility at BARe

L1

Spectrometer

BS2

L1

Nd:YAG Laser

Fig. 2--Optical arrangement for the high-temperature high-pressure set-up

stageTheradiatmmcooleInstasfocuscoincoptic,micreusingNd:Yfocalthe Dsteererespediver)bringsamealongaidedthe 0

spots;coincmoveadjustfiltercolleconly.contrrresolu10 III

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ICA,T)

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with lowoptical

g. 2. The-cted long. The lensbeam size

r

un steerer.Il x-y-8-<j>

MEENAKSHI et al.: MELTING POINT VARIATION WITH PRESSURE AND MATERIAL SYNTHESIS 443

stage direct the Nd: YAG beam to a pre-selected spot.The microscope objective collects the thermal"radiation from this focal spot and feeds it into a 250mm spectrograph (Oriel 77200) equipped with acooled charge coupled device (CCD) detector (Oriel,Instaspec). It is very important to make sure that thefocus of the thermal radiation collection opticscoincides with that of the heating laser. For thisoptical alignment, the optical path from themicroscope objective to the spectrometer is alignedusing a 1 mW He-Ne laser. The DAC and theNd:YAG laser spots are then brought to the samefocal point of the microscope objective by mountingthe DAC on a x-y-z stage, and employing the beamsteerer and the hot mirror in the Nd: YAG laser pathrespectively. Adjusting the Nd:Y AG laser beamdivergence with Ll and L2 lenses is also needed tobring the YAG laser and He-Ne laser focus to thesame point. It is also ensured that both the lasers arealong the axis of the objective. These adjustments areaided by a video camera into which the signal fromthe objective (both from He-Ne and Nd:YAG laserspots) can be diverted. When both the laser spotscoincide and expand symmetrically when DAC ismoved along the beam direction, the requiredadjustment is attained. An aperture (A) and a spatialfilter are included in the collection part to ensurecollection of thermal radiation from the focal spotonly. Currently, we do not employ any feedbackcontrol for the Nd: YAG laser power. The spatialresolution of the signal collection optics is better than10 urn as evaluated by measuring the signals fromfine ruby powder. Reducing the aperture size furtherimproves the resolution.

Temperature is determined by treating the laserheated hot spot as a black body radiator'r". Theintensity of light radiated from a black body attemperature T is given as a function of wavelength Aby Planck's profile as:

I(A,1) = (2nc2h)fA'5 [exp(hclkA1) _1]"L ... (1)

where e, h, k: and c are the emissivity, Planck'sconstant, Boltzman' s constant and velocity of light,respectively. The emissivity (e) of the sample isdependent on a number of parameters such as materialstate, surface roughness, wavelength, temperature,pressure, etc. However, it does not change much withwavelength within the visible region. In addition,since e is a multiplicative constant, it also encom-passes any factor that modulates intensity such as

contributing sample volume and exposure time. In thepresent measurements the sample and wavelengthdependence of emissivity have been ignored. Themeasured radiation profile needs to be corrected forthe dispersion in the collection optics and electronicamplification. For this, the count of each channel hasto be multiplied by a factor for obtaining the countexpected from a black body radiator. The calibrationfactor for each CCD channel is obtained by findingout the multiplicative factor required to change themeasured profile at a standard temperature (meltingpoint of Fe, W, Ta and Au or a calibrated lamp) to aPlanck profile. This calibration takes care of factorslike, electronic amplification of the signal, dispersionby the various optical components and diamond.However, the pressure, temperature and wavelengthdependence of emissivity and complications arisingfrom temperature and pressure gradients resulting incollection of signal from points other than theconstant temperature part of the hot spot are ignored.In a typical measurement, initially, backgroundspectra due to the noise in the CCD and the spectrafrom the calibrant are collected. From the backgroundsubtracted calibration spectra, the scale factors foreach channel, which converts the observed spectra toPlanck's profile at that particular temperature aredetermined and stored. For determining thetemperature, the background is subtracted from thecollected spectra and least square fitted to Planck'sprofile [Eq. (1)] after multiplying each channel countwith the corresponding scale factor. Irrespective ofduration of collection of the spectra, all the processingis carried out with spectra normalized to 10 s.Furthermore, the collected spectra are smoothened byfitting to a polynomial before the above processing.To facilitate this, a software, which can carry outcalibration or the sample temperature determination,optionally, as per the above procedure is employed.Typical data collection time was 120 s.

5 Experimental Details and ResultsSeveral melting experiments were carried out on

polished standard materials like W, Ta, Pt etc atambient conditions. Melting was identified visually

Table I-Experimental melting temperature of Wand Tacompared with their published values

Element Published21 melting Experimental meltingtemperature (K) temperature (K)

Tungsten (W)Tantalum (Ta)

36833269

3774 ± 913120 ± 149

444 INDIAN J PURE & APPL PHYS, VOL 44, JUNE 2006

during laser heating by observing the sample with thehelp of the video camera. The melting temperaturesfor Wand Ta at ambient conditions as shown in Table1 were found to be in reasonable agreement with thepublished values". Deviation from the ideal Planckdistribution due to the temperature gradients may bethe reasons for the deviation of the measured values.

6 HT -HP Melting Behaviour of PlatinumMeasurements of melting curve under pressure

provide extremely useful data for inter-comparisonwith shockwave data and computational predictions.The effect of high pressure on the meltingtemperature of elemental platinum was studied usingthe set-up as a prototype experiment for other studiesof melting at high pressures. The study on platinumwas also motivated by its inert nature and due to itsimportance as EOS standard in high pressureexperiments. The melting experiments for platinumwere carried out in a Merrill-Bassett type DAC withtype Ia diamonds of culet - 400 urn. A hardened SSgasket with 200 urn diameter hole formed thepressure chamber. Prior to the experiment, the gasket(initial thickness - 250 urn) was pre-indented betweenthe diamond anvils to a final thickness of - 50 urn.First the gasket was pre-compacted with fine aluminapowder till it appeared completely transparent. Asmall dip was made at the center of the compactedalumina where a platinum foil (-20 urn x 20 urn) wasloaded. The foil was covered again on the top byruby. Thus, the foil was completely surrounded by apowdered ruby or alumina powder. The sample wasloaded without any other medium to avoid unwantedchemical reactions. After sample loading, the DACwas clamped and the sample was laser heated up to its

2500

g~ 2000:::l~Q)a.E 1500

(3!.Ol.~~ 1000~

Platinum Melting• Calculated values [231• Experiment of Strong et al [221• Present experiment

Pressure ( GPa)

Fig. 3--Melting temperature of platinum as a function of pressure

In bothmelting temperature at various pressures up to about 12 as foundGPa. The pressure was measured at the point nearest to he fact tlthe laser heated sample by ruby fluorescence onditionstechnique. Fig. 3 shows the melting temperature of praphite isplatinum as a function of pressure. The data is in from thereasonable agreement (taking into consideration the therwise,temperature gradients) with the piston cylinder data" itu Ramalas well as computed values based on dislocation onfirm thmediated melting / Lindermann criterion". tatus of m

as beenonditions7 High Temperature-High Pressure Synthesis

Nitratedlhydrogenated diamond like carbon(CNJC:H) compounds, especially the C3N4 isomorphicto Si3N4, are novel compounds attracting a lot ofattention". Kinetic barrier for the formation of sp'bonded carbon phases may be overcome by employingprecursors with Sp3 bonding or by applying highpre§sure and temperature. Direct reaction of carbon andnitrogen" or decomposition of Sp2 or Sp3 bondedprecursors like diphenylamine, paracyanogen (C1N,),bexamethylenetetrarnine'" (HMT, C6H12N4) at HT-HPdo not result in C3N4.This may be due to the formationof free nitrogen molecule, deficiency of nitrogen in thematerial to form C3N4, or C3N4 not being stable atambient conditions. We have attempted to synthesizeC3N4by employing HMT as a precursor material in theDAC with and without excess nitrogen. HMT alongwith Pd was loaded in the DAC and pressurized tovarious pressures up to 10 GPa and then heated up tothe melting point of Pd. In another set' of experiments,Pd was melted along with HMT and liquid nitrogenabove 15 GPa. The recovered material wascharacterized by collecting micro Raman spectra fromthe sample in the spectral region 400 to 2000 cm'.Fig. 4 shows a typical Raman spectrum collected.

Fig, 4--Micro Raman spectrum of the recovered sample after HT-HP treatment

4UO 2000800 1200 1600Raman Shift (em -1)

ConclusiThe del

iven. Melelting po

latinum a:onjecture:ompoundesults pos:s not quen

cknowleThe autl

ith Dr S I

eference:Mao H:PhysicsHemleyDC), 19(Eds) CItaliana .Fei Y &BoehlerResearciSyono 'Geophy:BoehlerVijayakiTechnoliVarandaDelhi), LGodwal

'es up to about 12e point nearest toby fluorescence~ temperature of

The data is in:onsideration then cylinder data22

I on dislocation)n

23.

,Synthesislike carbon

C3N4 isomorphic.acting a lot oformation of Sp3ne by employingr applying highon of carbon andor Sp3 bonded

yanogen (CINI),[12N4) at HT-HPto the formationIfnitrogen in thebeing stable at

ed to synthesize)r material in theen. HMT alongj pressurized toien heated up toof experiments,liquid nitrogenmaterial was

Ianspectra from) to 2000 ern".1collected.

~~:!I

~oo 2000

j sample after HT-

Cohen M L, Science, 248 (1990) 462.7 Santoro M, Lin J F, Mao H K & Hemley R J, J Chem Phys,

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10 Watanuki T, Shimomura 0, Yagi T, Kondo T & Isshiki M,Rev Sci lnstrum, 72 (2001) 1289.

11 Shen G, Mao H K & Hemley R J, in Advanced Materials '96- New Trends in High Pressure Research (Eds) Akaishi M,Arima M, Irifune T, et al., (National Institute for Research inInorganic Materials, Japan), 1996, p. 149.

12 Heinz D L & Sweeney J S, Rev Sci Instrum, 62 (1991) 1568.13 Yoo C S, Akella J & Moriarty J A, Phys Rev B, 48 (1993)

15529.14 Yagi T, Kondo T, Watanuki T, Shimomura 0 & Kikegawa

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Trends in High Pressure Research (Eds) Akaishi M, ArimaM, Irifune T et al., (National Institute for Research inInorganic Materials, Japan), 1996, p. 143.

16 Sweeney J S & Heinz D L, Geophys Res Lett, 20, (1993)855.

17 Advanced Materials '96 - New Trends in High PressureResearch (Eds) Akaishi M, Arima M, Irifune T et al,(National Institute for Research in Inorganic Materials,Japan), 1996.

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MEENAKSHI et al.: MELTING POINT VARIATION WITH PRESSURE AND MATERIAL SYNTHESIS 445

In both sets of experiments, the recovered materialwas found to be amorphous graphite. Thus, in spite ofthe fact that the material was exposed to HT-HPconditions where diamond is the stable phase,graphite is formed. This could have resulted onlyfrom the decomposition of CNx or C:H formed,otherwise, diamond should have been recovered. In-situ Raman measurements at HT-HP will be able toconfirm this conjucture and shall provide the in-situstatus of material evolution. It may be noted that C3N4has been synthesized under fast quenching"conditions from HMT.

8 ConclusionThe details of the HT-HP set-up at BARC are

given. Melting temperatures of materials with knownmelting points were reproduced and melting curve ofplatinum as a function of pressure was generated. It isconjectured that the HT-HP treatment of the organiccompound HMT with and without excess nitrogenresults possibly in the formation of C:H or CNx, but itis not quenchable.

AcknowledgementThe authors acknowledge many fruitful discussions

with Dr S K Sikka during the course of the work.

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Physics & Chemistry of the Earth's Deep Interior (Ed)Hemley R J, (Mineralogical Society of America, WashingtonDC), 1998, p. 1; Boehler R, in High Pressure Phenomena(Eds) Chiarotti G L, Bernasconi M & Ulivi L, (SocietaItaliana di Fisica), 2002, p. 55 .

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