ruby sphere anneal

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High Pressure Research: An

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Ruby-spheres as pressure

gauge for optically transparent

high pressure cellsJ. C. Chervin

a, B. Canny

a& M. Mancinelli

b

aLaboratoire de Physique des Milieux Condenss,

UMR 7602, Universit Pierre et Marie Curie, B77, 4,Place Jlissieii, Paris, 75252, Franceb

R.S.A. Le RUBIS SA, RN 85-BP 16, Jarrie, 38560,France

Version of record first published: 19 Aug 2006.

To cite this article: J. C. Chervin, B. Canny & M. Mancinelli (2001): Ruby-spheres aspressure gauge for optically transparent high pressure cells, High Pressure Research:An International Journal, 21:6, 305-314

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High Prxtmwe Researrh. 2001, Vol. 21, pp. 305-314Reprints available directly from the publisherPhotocopying permitted by license only

2001 OPA (Overseas Publishers A ssociation) N.VPublished by license under

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RUBY-SPHERES AS PRESSURE GAUGE FOROPTICALLY TRANSPARENT HIGH PRESSURECELLSJ. C. CHERVIN", B. CANNY* and M. MANCINELLIb

aLaboratoire de Physique des Milieux CondensPs, UMR 7602, Universitk Pierre etMarie Curie, B77, 4, Place Jlissieii, 75 252 Paris, France;bR.S.A. Le RUBIS SA, -RN 85 - BP 16, 38560 Jarrie, France

(Received January 12. 2002; Revised February 17, 2002; In j n a l form February IS, 2002)

Ruby is widely used as an in siru pressure gauge for optically transparent pressure cells up to themegabar range. Usually ruby chips cut from bulk crystals are used which are ill-characterized andinconvenient to handle and t o identify visually. Here we present a systematic stu dy on corundumsamples doped with C 2+ ions with concentration from 60 to 23500 ppm to determine the optimalconditions for the use as an accurate pressure marker. Th e influence of the excitation wavelengthon the lum inescence spectra was investigated. These stud ies led to the synthesis of small (1-50micrometer) ruby spheres with 3000 ppm chromium concentration. After annealing and a heattreatment to avoid internal strains we find reproducible values of the position and the width ofthe fluorescence lines. These ruby spheres are not only well suited for a reliable and accuratepressure determination in experiments using diamond anvil cells, but can also be used as anin sihr micro-thermometer in high pressure-low temperature studies.Keywords: DAC; high pressure; ruby gauge

INTRODUCTIONOptically transparent pressure cells, and particularly diamond anvil cells allowa variety of measurements under high static pressures to beyond 1 Mbar(100GPa) [1,2]. In this technique, the pressure in the experimental volumeis generally measured using a gauge based on the secondary scale of ruby(A1203:Cr3+). The pressure dependence of the photo luminescent emissionwas determined previously as a function of temperature [3] and pressure[4]. A calibration of the ruby scale at low temperature was performed up to1.2 GPa [5]. However, the ruby scale calibration depends to a certain extent* Corresponding author. Fax: 33-1-4427 4469; E-mail: [email protected]

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also on the growth conditions of the samples [6], the chromium concentration[7] and also the nature of the light excitation [S]. Some discrepancies occurwhen recording spectra of crystals from different sources, and it appearsnecessary to obtain convenient and well-characterized high-quality sampleswith homogenous and reproducible luminescence characteristics. This is thepurpose of this short report.The Ruby LuminescenceAluminum oxide, so-called corundum or sapphire, doped with thetrivalent chromium, is well known by mineralogist as ruby. Depending onthe C f + concentration, pink, standard and dark red rubies are obtained.The optical properties of ruby were widely studied in the 1950s, as it wasused for the first solid laser material. Corundum is rhombohedra1 (space groupD3,,) with two formula units per unit cell. The A13+ ion is at the C3-axis closeto the center of a distorted octahedron and the six oxygen ions at its vortices,placed in two parallel planes [9]. When Cr3+ is replaced by A13+ which has asmaller ionic radius, additional trigonal or tetragonal distortions occur. Follow-ing Sugano and Tanabe [lo], the fimdamental level 4F of the free Cr3+ ion,whose symmetry becomes 4A2 in the crystalline field, is split by 0.02nm intwo sub-levels, the zero field splitting [l 11, which can be observed at verylow (


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RUBY-SPHERES AS PRESSURE GAUGE 307

(Cr3+/A13+ ratio) determined in the initial mixture varied from 60ppm to23500ppm. However, analyses by atomic absorption and by a microprobe(model micro-Camebax SX50) showed that the real concentration of chro-mium in the samples was -50% lower. Crystals were cut in slabs of100 itm thickness and used in the following experiments.Luminescence spectra were recorded on a triple monochromator T-800Coderg equipped with a photon counting detector. The resolution of thissetup was 0.01 nm when slits of 40 pm were used. A krypton laser Coherent3000K provided the excitation source which was adjusted to 1 mW at agiven wavelength. Each sample was successively excited by the blue(419.1 nm), green (530.8 nm) and red (647.1 nm) krypton lines. Measurementswere made at room temperature and also at variable temperature, from 4.2 K to300 K.Figure 1 shows the variation of the intensity of the R -lines as a function ofthe chromium concentration on a semi-logarithmic plot for two exciting wave-lengths, 419.1 nm and 5 30.8nm, at 300K. Measurements with the red laserline at 647.1 nm did not give significant information, the luminescencebeing very weak in that case. A concentration of 5000ppm Cr3+ gave amaximal intensity in both cases.The frequency and the FW HM of the R lines were also studied at room tem-perature as a function of different excitation light power. It was found that thevariation of the R, and R2 ine frequencies is not significant when the powerremains lower than 100mW. For larger powers, a shift of 0.1 nm of the w holespectrum can be observed, no doubt due to local heating of the sample. Simi-larly the FWHM of the R, line remains at 0.55 nm for low power levels butincreases to 0.65 nm for powers larger than 10 mW. Figure 2 presents the evo-lution of the FWHM of R , and R2 t the same conditions as those in Figure 1.For R,, the width (FWHM) varies from 0.50f0.02nm for low chromiumconcentrations and up to 0.84 f .02 nm for the highest concentration.At 3000ppm, the FWHM is around 0.55 and 0.58nm, which is close to theintrinsic line width at this temperature. This value of the C r3 + concentrationappears to be an acceptable compromise between line width and intensityfor the use of ruby as pressure marker.Ruby SpheresThe R.S.A. Le Rubis Company synthesized ruby spheres with an effectiveCr3+ concentration in the range 3000-4000 ppm. The diameter of the spheresvaries from 1 pm to 50pin (see Fig. 3). Compared to ruby sam ples in form ofchips, such ruby spheres have the considerable advantage that they can beeasily identified and distinguished from other samples in the experimental



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RUBY-SPHERES AS PRESSURE GAUGE

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RUBY-SPHERES AS PRESSURE GAUGE 311

volume. Such as grown ruby spheres presented very inhomogeneousluminescent spectra. They are m ainly in the corundum y phase which is opa-que. It was obvious that the samples should be annealed to transform them tothe transparent a phase and to eliminate the internal strains. The samples areplaced in a crucible and heated in a standard furnace in air. The temperaturewas increased at a constant rate to 1500 C within 3 days, maintained underthese conditions for one week, and then decreased to 300K at a rate of70 K per day. After this heat treatment, arbitrarily chosen samples exhibiteda FWHM of 0.55 f 0.02nm with good reproducibility. After temperaturecycles with a maximum temperature lower than 1500 C, or after rapid coolingfrom this temperature, we found a large amount of non-transparent spheres aswell as a substantially larger dispersion on the observed line widths.

Ruby as a Micro-ThermometerOptical transparent pressure cells are often able to operate at variable tempera-ture, in particular membrane diamond anvil cells (MDAC). The design devel-oped in our laboratory [17], for example, can be easily placed in a vacuumchamber and used experiments from 4 to 800K. The pressure is monitoredfrom outside. In such a set-up, the ruby spheres may also be used as amicro-thermometer.The variation of the ruby lum inescence under variable temperature is wellknown, particularly at low tem perature [7]. For our ruby spheres , a polynomialfi t to the R1 ine position was made in the range 108K-300 K; below -108 K,the wavelength of the R1 ine remains essentially constant (see Fig. 4). Thebest fit gave the following relation:

A(nm) =693.4 - O.O22(T - 108)+1.310-5(T -where A is the R1 wavelength in nanometer at am bient pressure as a function oftemperature in Kelvin.For temperatures below - 00 K, Weinstein showed that the respectivepopulation of the emitting levels allows to determine the temperaturethrough the relation [181:where 1, and I2 are respectively the intens ities of the R, and R2 ines. For theruby spheres with 3000 ppm Cr3 + concentration, the coefficients were deter-



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RUBY-SPHERES AS PRESSURE GAUGE 31 3

mined to be A =41.86 K and q =0.625 which gives a precision of -1 K in therange 10-100K. [19].For temperatures above 100K, both pressure and temperature can be deter-mined in sitii by the two calibrants technique. The luminescent compoundSrB4 07 doped with S m 2+ [20] is a favorite candidate for use as a secondcalibrant. Since the Sm 2+luminescent lines are quasi insensitive to tempera-ture, it is possible to determine the pressure and the temperature simulta-neously when a ruby and a SrB4 07:Sm 2+ sample are placed in the high-pressure chamber. This technique was previously proposed by F. Datchi

et al . [21]. For tem peratures above 750K, it was shown that no reliable datacan be obtained, due to the broadening of the luminescent lines, the decreaseof the intensity and the increase of the background [22].

CONCLUSIONRuby samples should be carefully chosen for accurate and reliable pressuremeasurements. We have investigated some of the luminescence properties ofruby (frequency, width and intensity of the R-lines) as a function of C 2 + con-centration. A concentration of - 000ppm chromium seems to be optimal foruse in high-pressure experiments. Small ruby spheres of 1 to 50 pm with thisC?+ content where synthesized which (after annealing) were shown to beconvenient pressure markers for in situ pressure measurements in DACcells. The temperature dependence of the R-lines allows these ruby spheresalso to be used as an in situ thermometer.AcknowledgenientsThis work was initiated by J. M. Besson and supported in part by the FrenchBureau National de MCtrologie under contract nr.85 246 00 39. We are grate-ful to Dr. s. Klotz for helpful discussions and an anonym ous referee for hisus eh l comments.References

[ I ] Mao, H . K., Bell, P. M., Shaner, J. W. and Steinberg, D. J. (1978). Specific volumemeasurementsof Cu, Mo, Pd, and Ag calibration of the R, luorescence pressure gauge from0.06 to I Mbar. J . Appl. Pliys., 49, 3276.[2] Mao, H. K., Xu, J. A . and Bell, P. M., J. (1986). Calibration of the ruby gauge to 800kbarunder quasi-hydrostatic conditions. Geophys. Res. E, 91, 4673.[3] McCumber, D. E. and Sturge, M. D. ( 1963 ). Linewidth and temperature shift of the R lines inruby.1 Appl. fhys., 34(6), 1682.



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[4] Bamett, J. D., Block, S. and Piermarini, G. J. (1973), An optical fluorescence system forquantitative pressure m easurement in the diam ond anvil cell. Rev. Sci. Instrzmi., 44, 1.[5] Noak, R. A. and Holzapfel, W. B. (1978). Calibration of the ruby pressure scale at lowtemperatures. Proc. 6th AIRAPT Conference, Plenum Press, New York, p. 748.[6] Nakamura, Y., Fijishiro, 1. and Taniguchi, K. (1991). Hysteresis o f ruby fluorescent line bypressure and a nnea ling effect. High Pressw e Research, 6 , 301.[7] Powell, R. C., DiBartolo, B., Birang, B. and Naiman, C . S. ( I 966). Temperature dependence o fthe w idths and positions of the R and N lines in heavily doped ruby. 1 Appl. Phys., 37,4973.[8] Maiman, T. H., Hoskins, R. H., D'Haenens, 1. J., Asawa, C. K. and Evtuhov, V (1961).Stimulated optical emission in fluorescent solids. 11. Spectroscopy and stimulated emission inruby. Phys. Rev., 123, 1151.[9] Mc Clure, D. S. ( 1 962). Optical spectra of transition metal-ions in coru ndum ,1 Chein. Phys.,36. 2757.[lo] Sugano, S. and Tanabe, Y. (1958). Absorption spectra of Cr 3+ in A12 03.1 Phys. SOC. apan,13, 880.[ I 11 Mac Farlane, R. M., (1965). O n the gro und state splitting o f ruby. 1 Chetn.Phys., 41 ,442 .[I21 Nelson, D. F. and Sturge, M. D. (1965). Relation between absorption and emission in the

[13] Curie, D. ( 1 968). Champ Cristallin et Lutnitiescence,Gauthier-Villars, Paris.[I41 Canny, B., Chervin, J. C., Curie, D., Venkatapen, V and Jing Q ing, L. (1988). Raman effectand phonon replicas in Ruby and Alexandrite crystals. In: Boulon, G., Jorgensen, C. K. an dReisfeld, R. (Eds.), French-IsraeliWorkshopoti Solid State Lasers, Vol. 74, 12-14 December.[15] Williams, Q. an d Jeanloz, R. (1985). Pressure shift of Cr3+ -ion-p air emission lines in ruby.Phys. Rev. B, 31,1449.[16] Le Rubis SA, R. S. A. RN 85- BP 16,3 856 0 Janie, France.[I71 Chervin, J. C., Canny, B., Besson, J. M. and Pruzan, Ph. (1995). A diamond anvil cell forinfrared microspectroscopy. Rev. Sci. Aistrzmt.,66, 2595.(181 Weinstein, B. A. (1986). Ruby thermometer for cryobaric diamond-anvil cell. Rev. Sci.Instriuii.,57, 910.[19] Chervin, J. C., Canny, B., Gauthier, M. and Pruzan, Ph. (1993). Micro-Raman at lowtemperature and very high pressure. Rev. Sci. ~isb-trin.,4, 203.

[20] Leger, J. M., Chateau, C . and Lacam, A. (1990). SrB,07: Sm2+ pressure optical sensor:Investigations in the megabar ra nge. 1Appl. Phys., 68,2351.[2 I ] Datchi, F., Le Toullec, R. and P. Loubeyre. ( I 997). Improved calibration of the SrB307: SmZfoptical pressure gauge: Advantages at very high pressures and high temperatures. 1 Appl.P/i.vs., 81, 3333.[22] Yamaoka, S., Shimomura, 0. and Fukunaga, 0. (1980). Simultaneous measurements oftemperature and pressure by the ruby fluorescence line. Proc. Japan Aca d., 56B(3), 103.

region of the R lines of Ruby. Pbys. Rev. A , 137, 11 17.



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