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[Hitoshi Yusa](https://orcid.org/0000-0001-6980-9279), [Masashi Miyakawa](https://orcid.org/0000-0002-0838-8156)

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[Temperature and pressure dependence of the fluorescence spectrum of high-pressure synthetic corundum-type Ga2O3:Cr3+, structural stability, and crystal growth under high pressure](https://mdr.nims.go.jp/datasets/765b5f8d-5bd5-44f0-a6f3-440cea3c25cc)

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Temperature and pressure dependence of the fluorescence spectrum of high-pressure synthetic corundum-type Ga2O3:Cr3+, structural stability, and crystal growth under high pressureViewOnlineExportCitationRESEARCH ARTICLE |  JANUARY 15 2025Temperature and pressure dependence of the fluorescencespectrum of high-pressure synthetic corundum-typeGa2O3:Cr3+, structural stability, and crystal growth under highpressureHitoshi Yusa   ; Masashi Miyakawa J. Appl. Phys. 137, 035902 (2025)https://doi.org/10.1063/5.0246260Articles You May Be Interested InRefractive index and phase transformation of sapphire under shock pressures up to 210 GPaJ. Appl. Phys. (March 2017)Effect of impurity on high pressure behavior of nano indium titanateAIP Conference Proceedings (June 2015)High-pressure lattice dynamical study of bulk and nanocrystalline In2O3J. Appl. Phys. (December 2012) 16 January 2025 01:57:00https://pubs.aip.org/aip/jap/article/137/3/035902/3331478/Temperature-and-pressure-dependence-of-thehttps://pubs.aip.org/aip/jap/article/137/3/035902/3331478/Temperature-and-pressure-dependence-of-the?pdfCoverIconEvent=citejavascript:;https://orcid.org/0000-0001-6980-9279javascript:;https://orcid.org/0000-0002-0838-8156https://crossmark.crossref.org/dialog/?doi=10.1063/5.0246260&domain=pdf&date_stamp=2025-01-15https://doi.org/10.1063/5.0246260https://pubs.aip.org/aip/jap/article/121/11/115903/976851/Refractive-index-and-phase-transformation-ofhttps://pubs.aip.org/aip/acp/article/1665/1/030024/776920/Effect-of-impurity-on-high-pressure-behavior-ofhttps://pubs.aip.org/aip/jap/article/112/12/123511/354086/High-pressure-lattice-dynamical-study-of-bulk-andhttps://e-11492.adzerk.net/r?e=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&s=ELH5p8ch7gZob9Y1zql6E3sSSN4Temperature and pressure dependence of thefluorescence spectrum of high-pressure syntheticcorundum-type Ga2O3:Cr3+, structural stability,and crystal growth under high pressureCite as: J. Appl. Phys. 137, 035902 (2025); doi: 10.1063/5.0246260View Online Export Citation CrossMarkSubmitted: 31 October 2024 · Accepted: 28 December 2024 ·Published Online: 15 January 2025Hitoshi Yusaa) and Masashi MiyakawaAFFILIATIONSResearch Center for Materials Nanoarchitectonics, National Institute for Materials Science (NIMS), 1-1 Namiki,Tsukuba, Ibaraki 305-0044, Japana)Author to whom correspondence should be addressed: yusa.hitoshi@nims.go.jpABSTRACTThe pressure dependence of the R1 and R2 peaks of the fluorescence spectra of high-pressure-synthesized corundum-type Ga2O3:Cr3+ wasmeasured up to 50 GPa using a diamond anvil cell pressurized at room temperature in an argon medium. The temperature dependence ofthe R1 and R2 peaks was measured at low temperatures under ambient pressure from 303 to 83 K. From the measurement results, thepressure scale and low-temperature scale were determined using R1 and R2. X-ray diffraction experiments at high pressure, which wereperformed to confirm the effective range of the pressure scale, showed that the corundum structure undergoes a phase transition to aRh2O3(II)-type structure at 54–65 GPa; this pressure scale is, thus, valid up to approximately 50 GPa. We also investigated the crystal growthtime to optimize the crystal size of Ga2O3:Cr3+ for diamond anvil cell experiments.© 2025 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International (CC BY-NC-ND) license (https://creativecommons.org/licenses/by-nc-nd/4.0/). https://doi.org/10.1063/5.0246260I. INTRODUCTIONThe ruby fluorescence method is widely used for pressuremeasurement in diamond anvil cell (DAC) high-pressure devices.1,2It is based on the pressure dependence of fluorescence peaks (R1and R2) derived from Cr3+ in corundum crystals, and its pressurescale has been determined and updated to higher pressures byvarious reports.1,3–8 Instead of Al2O3 corundum, we have recentlysynthesized corundum-type Ga2O3 doped with Cr3+ at high pres-sure and have shown that it excites ruby-like fluorescence R1 andR2.9 We reported that not only is the fluorescence intensity strongerthan that of ruby, but also light at longer wavelengths than that ofruby fluorescence can be used as the excitation light source. Thepressure scale was determined up to 11 GPa at room temperatureby measuring its pressure dependence under hydrostatic pressure.10In terms of ionic radius, Ga3+ (0.620 Å) is slightly larger thanAl3+ (0.535 Å) but closer to Cr3+ (0.615 Å), making it suitable forallowing Cr3+ as a dopant. Interestingly, the fluorescence wavelengthat ambient pressure indicates a high-pressure state of ruby of approx-imately 10 GPa. This may be due to the effect of elemental substitu-tion in the compound on the ligand field of Cr3+: the so-called“chemical pressure” that has been recently proposed.11 This effectalso has a significant impact on the excitation spectrum. Because theexcitation spectrum is related to the absorption spectrum, it is verystriking that the Ga2O3:Cr3+ crystal exhibits a green color underwhite light, which is the opposite of its deep red fluorescent color.9The pressure scale of the ruby fluorescence method has beendetermined up to approximately 150 GPa.7,8,12 It deviates from lin-earity as the pressure increases and is defined by a non-linearequation.6–8 Therefore, in the present study, we also investigatedthe pressure dependence of the fluorescence wavelength incorundum-type Ga2O3:Cr3+ under quasi-hydrostatic conditions athigher pressures and attempted to determine the non-linear formas a pressure scale.In the case of ruby fluorescence, density functional theory(DFT) calculations report that Al2O3 undergoes a structural phaseJournal ofApplied PhysicsARTICLE pubs.aip.org/aip/japJ. Appl. Phys. 137, 035902 (2025); doi: 10.1063/5.0246260 137, 035902-1© Author(s) 2025 16 January 2025 01:57:00https://doi.org/10.1063/5.0246260https://doi.org/10.1063/5.0246260https://pubs.aip.org/action/showCitFormats?type=show&doi=10.1063/5.0246260http://crossmark.crossref.org/dialog/?doi=10.1063/5.0246260&domain=pdf&date_stamp=2025-01-15https://orcid.org/0000-0001-6980-9279https://orcid.org/0000-0002-0838-8156mailto:yusa.hitoshi@nims.go.jphttps://creativecommons.org/licenses/by-nc-nd/4.0/https://creativecommons.org/licenses/by-nc-nd/4.0/https://doi.org/10.1063/5.0246260https://pubs.aip.org/aip/japtransition from a corundum type (space group (SG): R�3c) to aRh2O3(II) type (SG: Pbcn) above 78–85 GPa:13–15 in high-pressureexperiments, this transition occurs at approximately 95–130 GPa.16–19 This difference in the pressure values is particularlylarge at room temperature, where it is not thermally activated. Theuse of a pressure scale would be limited to the pressure at which thephase transition occurs, but the pressure range would be extended atroom temperature, where the corundum structure is maintained in ametastable state. It is also known that Ga2O3 undergoes a phase tran-sition to the Rh2O3(II) type under high temperature and pressure.20In previous research, the phase transition pressure to the Rh2O3(II)type at high temperatures was experimentally determined to beapproximately 36 GPa at 2000 ± 100 K.20 This value was consistentwith the 30–33 GPa predicted by DFT calculations.20,21 However, thepressure of the phase transition in the metastable state at room tem-perature has not been investigated. Therefore, in the present study,we performed in situ x-ray diffraction (XRD) experiments on thestructural phase transition pressure at room temperature to deter-mine its applicable pressure range as a pressure marker.There have been many reports on temperature dependence inruby fluorescence.22–27 The theoretical background explains that thisis caused by a shift in energy level due to the interaction of phononswith the electronic state,22,25 and the electron–phonon couplingconstant has been determined from the temperature dependence ofthe wavelength shift. In addition, the temperature dependence of thefluorescence spectrum has practical implications in terms of con-structing a temperature scale and is also important for clarifying theeffects of pressure measurements at low temperatures.24,28 Therefore,we measured the temperature dependence of the fluorescence spec-trum of corundum-type Ga2O3:Cr3+ crystals below room tempera-ture and compared it with the ruby fluorescence spectrum. We alsocalculated the electron–phonon coupling constant.As is often mentioned with respect to ruby fluorescencespectra, differential stresses on the crystal have a significant effecton the peak shape, especially on the R1 peak shift.12,29–31 They arecaused by uniaxial compression of the crystals in the limitedvolume of the DAC, such as when the crystals are in contact withthe anvil. Therefore, it is recommended that smaller crystals beused. Crushing of the crystals will also affect the internal strain andinfluence the spectral shape. Rubies can be annealed at high tem-peratures after pulverization to remove internal strain.32 However,Ga2O3:Cr3+ corundum-type crystals cannot be annealed at temper-atures higher than 600 °C because their structure reverts to theβ-Ga2O3 structure.9 Therefore, synthetic crystals that are smallenough not to need crushing are required. It is believed that crystalgrowth progresses with longer synthesis time, so we evaluated syn-thesis for different holding times to find the optimal conditions forcrystal growth for use as a pressure marker in the DAC.II. EXPERIMENTALSyntheses of corundum-type Ga2O3:Cr3+ crystals were per-formed at high temperature and high pressure using a belt-typeapparatus (FB30H) at the National Institute for Materials Science(NIMS; Japan).33 Details of the synthesis method are described in aprevious paper.9 The starting sample was composed of Ga2O3(Kojundo Chemical Laboratory; 99.99% purity) and Cr2O3(Sigma-Aldrich; 99.99% purity) in a molar ratio of 99:1, mixed wellin an agate mortar, and dried in a vacuum electric furnace at130 °C for 3 h. The sample was packed in a gold capsule and placedin a high-pressure cell at 7.7 GPa and 1200 °C for 6, 60, 600 min, or47 h, then quenched by turning off the heater current, and recov-ered after decompression. The grain size analysis of the recoveredsamples was performed for captured images of the crystals underan optical microscope (LEICA Z16 APO) with CCD camera(LEICA Flexacam C3) using ImageJ software.34Structural identification of the synthetic samples by XRD wasperformed at the Synchrotron Radiation Facility (AichiSR, Japan).Monochromatic x rays (λ = 0.72301 Å) collimated to 100 μm in sizewere irradiated onto samples fixed in polyimide capillaries.Diffracted x rays were detected with a hybrid pixel array detector(PILATUS 1M, Dectris, Switzerland). Cooling experiments wereperformed by blowing nitrogen gas from a refrigerator onto thecapillary sample. XRD experiments were performed using the samesystem operated at room temperature. The temperature was cali-brated by diffraction lines from a small amount of gold powdercoated on the sample.35The metastability of corundum-type Ga2O3:Cr3+ structuresunder high pressure at room temperature was investigated by XRDexperiments at the Photon Factory (BL18C and ARNE1), HighEnergy Accelerator Research Organization (KEK), Japan. A gasmembrane was connected to a DAC with anvils of 300 or 250 μmculet size in diameter, and pressure was applied from the rear of theDAC. Experiments were conducted with and without a mixedalcohol pressure media (methanol:ethanol:water = 13:3:1 by volume)to examine the effect of differences in hydrostaticity on phase transi-tion pressure. Monochromatic x rays (50 μm diameter) at a wave-length of 0.62180 and 0.41700 Å were irradiated onto the sample inthe DAC, and diffraction lines were detected by a complementarymetal–oxide semiconductor (CMOS) flat panel (Teledyne VisonSolutions Rad-icon 2022, USA) detector. The pressure was deter-mined from diffraction lines of Au powder mixed with the sampleusing an equation of state.36 All XRD profiles were converted tointensity vs 2θ data using IPAnayzer software.37 The lattice parame-ters were refined by profile fitting with PDIndexer software.37The R1 and R2 fluorescence spectra at room temperature andhigh pressure were detected using a spectrometer equipped with adiffraction grating with 1800 grooves/mm and a charge-coupleddevice detector (CCD; Andor DU401A, USA) with a focal lengthof 500 mm. A light-emitting diode (LED) laser with a wavelengthof 472.8 nm (LASOS, USA) focused at 3 μm diameter was used asthe excitation source. Measurements under high pressure werecarried out using a DAC with anvils of 300 or 400 μm culet size indiameter. Single crystals of approximately 7 μm in size were placedin a rhenium gasket and sealed in the DAC with an argon pressuremedium liquefied with liquid nitrogen. The argon pressuremedium would have relatively good quasi-hydrostatic properties atpressures higher than the solidification pressure.10 Pressure mea-surements were determined by the pressure dependence of theRaman spectrum of the diamond anvil.38 These were measuredusing a separate spectrometer system (Photon Design, Jupiter,Japan) equipped with Ar-ion laser oscillating at 488 nm (Innova,Coherent, USA). To minimize errors due to pressure distributionin the sample chamber, the Raman spectrum of the diamond wasJournal ofApplied PhysicsARTICLE pubs.aip.org/aip/japJ. Appl. Phys. 137, 035902 (2025); doi: 10.1063/5.0246260 137, 035902-2© Author(s) 2025 16 January 2025 01:57:00https://pubs.aip.org/aip/japHITOSHI YUSA挿入テキスト0measured directly above the crystal. Spectra at low temperatureswere also measured using this spectrometer, and measurementswere taken in 10 K increments using a microscope stage with aliquid nitrogen cooling control system that operates at 303 to 83 K(LINCAM THMS600, UK).III. RESULTS AND DISCUSSIONA. Sample synthesis and crystal growthThe samples synthesized under high pressure were confirmedby XRD to consist of crystals with a corundum-type structure(Fig. S1 in the supplementary material). Micrographs of the synthe-sized crystals are shown in Figs. 1(a)–1(d). The correlations betweensynthesis time and averaged, maximum, and minimum grain sizesare shown in Fig. 1(e). The crystals grew in proportion to the synthe-sis time, indicating that large crystals can be synthesized by keepingthe sample in solid-phase reaction for a long time. After about 6minsynthesis time, the average grain size was about 16 μm and manysmall crystals of less than 10 μm were produced, which were suitablecrystal sizes for pressure measurement in the DAC sample chamber.The fluorescence spectra of these crystals were the same as those pre-viously reported for a Cr concentration of 1.0%.9FIG. 1. High-temperature high-pres-sure-synthesized Ga2O3:Cr3+ samples.The synthesis times were (a) 6 min, (b)60 min, and (c) 10, and (d) 47 h. Thephotograph shows the exposed partafter the top of the sample capsulewas removed. (e) Particle sizedistribution.Journal ofApplied PhysicsARTICLE pubs.aip.org/aip/japJ. Appl. Phys. 137, 035902 (2025); doi: 10.1063/5.0246260 137, 035902-3© Author(s) 2025 16 January 2025 01:57:00https://doi.org/10.60893/figshare.jap.c.7602497https://pubs.aip.org/aip/japB. Fluorescence spectra and x-ray diffraction at lowtemperaturesIn the XRD experiments in the low-temperature region downto 84 K, no structural phase transition occurred, and the latticeconstant monotonically decreased (Fig. S2 in the supplementarymaterial). The peak wavelengths of the R1 and R2 fluorescenceshifted to the low-wavelength side as the temperature decreased(Fig. 2). This is the same trend as the fluorescence peaks of ruby,measured in this study and in previous reports,26 but the shift isconsiderably larger for Ga2O3:Cr3+ (1.70 nm) than for ruby(0.95 nm) when cooled from 285 to 84 K. The same trend occurredfor the R2 peak.According to the theoretical background based on electron–phonon interactions,25 the wavelength variation with temperaturecan be described byν0(T)� ν0(0) ¼ α(T/Θ)4ð(Θ/T)0x3(ex � 1)�1dx, (1)where ν0(T) is the wavenumber of R1 at temperature T at ambientpressure and ν0(0) is the wavenumber at 0 K. α and Θ are anelectron–phonon coupling parameter and the effective Debye tem-perature, respectively.As shown in Fig. 3, fitting the wavenumber data measured forruby in this study to Eq. (1) yield α and Θ in ruby to be−453 ± 13 cm−1 and 779 ± 13 K, respectively, which are almostequivalent to the values (α =−400 cm−1 and Θ = 760 K) previouslyreported.25 In Ga2O3:Cr3+, α and Θ were determined to be−643 ± 20 cm−1 and 701 ± 13 K, respectively, indicating that thedegree of electron–phonon coupling is significantly larger than thatin ruby.In addition to the above-mentioned electron–lattice interac-tions under isochoric (constant volume) conditions, the cause ofthe wavelength shift is also related to the thermal expansion of thevolume.2 This can be clarified by examining the change in latticeconstants at low temperatures. Figure 4 shows the degree of volumecontraction from the standard temperature in comparison withcorundum (Al2O3).39,40 The method of deriving the coefficient ofthermal expansion using the volume at each measurement tempera-ture is described in the supplementary material, and the linearlyapproximated values of the coefficient of thermal expansion at eachtemperature are shown in Fig. S3 in the supplementary material.The coefficient of thermal expansion of Ga2O3:Cr3+ is larger thanthat of Al2O3. However, the effect of the volume change with tem-perature on the wavelength shift is about Δν =∼−2.5 cm−1FIG. 2. Temperature dependence of the peak wavelengths of the R1 and R2fluorescence spectra of corundum-type Ga2O3:Cr3+ and Al2O3:Cr3+(ruby). Thefilled circles and open squares are data from the present measurements, solidlines are the results of fitting using Eqs. (4) and (5), and the broken lines arereported values from Refs. 39 and 40.FIG. 3. R1 fluorescence peak frequencies of corundum-type Ga2O3:Cr3+ andAl2O3:Cr3+ as a function of temperature. The lines were obtained by fittingEq. (1).Journal ofApplied PhysicsARTICLE pubs.aip.org/aip/japJ. Appl. Phys. 137, 035902 (2025); doi: 10.1063/5.0246260 137, 035902-4© Author(s) 2025 16 January 2025 01:57:00https://doi.org/10.60893/figshare.jap.c.7602497https://doi.org/10.60893/figshare.jap.c.7602497https://doi.org/10.60893/figshare.jap.c.7602497https://doi.org/10.60893/figshare.jap.c.7602497https://pubs.aip.org/aip/jap(Δλ∼ +0.12 nm) per ΔT = 100 K, which is very small comparedwith the electron–phonon interaction shown in Eq. (1).From a practical standpoint as a temperature scale, it ishelpful to present this as a simple equation. For the wavenumbers,we used the same equation, R(T) = A1 + A2T + A3T2 + A4T3, as pre-viously used in the low-temperature measurement of ruby fluores-cence.26 The results for R1 and R2 in the measured temperaturerange were as follows:R1(T) ¼ 14363þ 10:185� 10�2 T � 9:430� 10�4 T2þ 8:316� 10�7 T3cm�1, (2)R2(T) ¼ 14396þ 9:346� 10�2 T � 9:047� 10�4 T2 þ 8:595� 10�7 T3cm�1: (3)Similarly, when converted to wavelength and fitted with athird-order polynomial, R(T) = B1 + B2T + B3T2 + B4T3, the equa-tions were determined as follows:R1(T) ¼ 696:211� 0:493� 10�2 T þ 4:558� 10�5 T2� 3:987� 10�8 T3nm, (4)R2(T) ¼ 694:607� 0:450� 10�2 T þ 4:356� 10�5 T2� 4:113� 10�8 T3nm: (5)The errors in the coefficients of Eqs. (2)–(5) are listed inTable I.The change in the spectral profile at low temperatures isshown in Fig. 5. As the temperature decreases, it can be seen thatR1 and R2 can be more clearly separated. Figure 6 shows a plot offull width at half maximum (FWHM) and a comparison with thatof ruby. The temperature effect on the FWHM is greater than thatof ruby and is reversed below 113 K. Thus, the large temperaturedependence and improvement in resolution due to the decrease inthe FWHM indicates that Ga2O3:Cr3+ is a more suitable tempera-ture standard than ruby on the low-temperature side.FIG. 4. Volume thermal contraction of Ga2O3:Cr3+ at low temperatures. Data forcorundum (Al2O3) are shown for comparison.TABLE I. Coefficients determined by the regression analysis of Eqs. (2)–(5) andstandard error and correlation coefficient (R).Peak Coefficient Values Standard error R-squareEq. (2)R1(T) A1 14 363 0.582 0.99995A2 × 102 10.185 1.034A3 × 104 −9.430 0.488A4 × 107 8.316 0.796Eq. (3)R2(T) A1 14 396 0.675 0.99991A2 × 102 9.346 1.201A3 × 104 −9.047 0.584A4 × 107 8.595 0.959Eq. (4)R1(T) B1 696.211 0.028 0.99995B2 × 102 −0.493 0.005B3 × 105 4.558 0.237B4 × 108 −3.987 0.386Eq. (5)R2(T) B1 694.607 0.032 0.99991B2 × 102 −0.450 0.006B3 × 105 4.356 0.282B4 × 108 −4.113 0.463FIG. 5. Typical R1 and R2 spectra of Ga2O3:Cr3+ measured at lowtemperatures.Journal ofApplied PhysicsARTICLE pubs.aip.org/aip/japJ. Appl. Phys. 137, 035902 (2025); doi: 10.1063/5.0246260 137, 035902-5© Author(s) 2025 16 January 2025 01:57:00https://pubs.aip.org/aip/japC. Pressure dependence of fluorescence spectra andstructural stability under high pressureThe pressure scale in the previous paper9 was determinedunder fully hydrostatic pressure conditions up to 11 GPa. In thepresent study, the pressure dependence of the fluorescence spectrawas investigated up to higher pressures, with the aim of determin-ing pressure scales up to 50 GPa. For this purpose, the Raman spec-trum of the diamond anvil is used for the pressure determination,where the pressure scale equation determined in Akahama andKawamura38 is used. This equation is in good agreement with therecently published universal pressure scale by Eremets et al.41 inthe pressure range up to 50 GPa. Pressure determination bydiamond Raman spectra has one advantage over ruby in the non-hydrostatic regime. Ruby and Ga2O3:Cr3+ crystals cannot be placedin the same position, and moreover, because of the effect of gasketdeformation due to pressurization, the error due to crystal positionbecomes larger in the pressure region where non-hydrostaticityincreases. Therefore, to minimize errors due to pressuredistribution in the sample chamber, the Raman spectrum of thediamond was measured directly just above the Ga2O3:Cr3+ crystal.The results of plotting the peak wavelengths of R1 and R2 areshown in Fig. 7(a). These data were fitted to the following non-linear form,8 which is used as a ruby fluorescence pressure scale, todetermine the coefficients and determine the fluorescence pressureFIG. 6. Temperature dependence of full width at half maximum of the fluores-cence spectra of corundum-type Ga2O3:Cr3+ (filled black circles) and ruby(open squares). (a) and (b) R1 and R2, respectively.FIG. 7. (a) Pressure dependence of the peak wavelengths of the R1 and R2fluorescence spectra of corundum-type Ga2O3:Cr3+. The filled squares are froma previous study,9 which was conducted under hydrostatic conditions with amixed alcohol pressure media, and the open circles and diamonds are from thepresent study, which was performed with argon pressure medium. The dashedline is the linear fit of the previous study.9 (b) Comparison of deviations from thelinear pressure scale. The dot-dashed straight line and the thin line show thelinear scale1 and the latest non-linear scale (IPPS-Ruby2020)8 of ruby,respectively.Journal ofApplied PhysicsARTICLE pubs.aip.org/aip/japJ. Appl. Phys. 137, 035902 (2025); doi: 10.1063/5.0246260 137, 035902-6© Author(s) 2025 16 January 2025 01:57:00https://pubs.aip.org/aip/japscale,P ¼ A� 103Δλλ0� �1þ BΔλλ0� �� �, (6)where λ0 are the R1 and R2 peak wavelengths at ambient pressureand A and B are the parameters determined by fitting the R1 andR2 peak wavelengths. According to the data from this study,A = 2.142 and B =−2.340 were determined for R1 and A = 2.063and B = 0.345 for R2. The errors in the coefficients of Eq. (6) arelisted in Table II.The deviation from the linearity of Eq. (6) is shown inFig. 7(a). The results show that, compared with the pressure scaledetermined up to 11 GPa, the curve for R1 shifts upward as thepressure increases and the wavelength pressure dependencebecomes slightly larger. Figure 7(b) also shows a deviation fromlinearity in the pressure scale based on ruby fluorescence, whichshows the opposite trend.In the previous paper,9 the degree of peak separation wasmeasured under hydrostatic pressure up to 11 GPa and showed atendency to improve as the pressure increased. The changes inTABLE II. Coefficients determined by the regression analysis of Eq. (6) and stan-dard error and correlation coefficient (R).Peak Coefficient Values Standard error R-squareR1(T) A 2.142 0.015 0.99915B −2.340 0.412R2(T) A 2.063 0.013 0.99938B 0.345 0.397FIG. 8. Typical R1 and R2 spectra of Ga2O3:Cr3+ with increasing pressures.FIG. 9. X-ray diffraction patterns of Ga2O3:Cr3+ under high pressure (a) using amixed alcohol medium and (b) without a pressure medium. The bars at thebottom show the position of the diffraction lines representing corundum-typeGa2O3:Cr3+ (upper) and gold (lower) as a pressure marker. The arrow indicatesthe 211-diffraction line of the newly appeared Rh2O3(II) (see the supplementarymaterial). The red broken lines show the estimated transition pressure fromcorundum to the Rh2O3(II) phase. (c) The relative intensity ratio of the Rh2O3(II)211 peak and the corundum 104 peak indicates a degree of conversion fromcorundum to Rh2O3(II).Journal ofApplied PhysicsARTICLE pubs.aip.org/aip/japJ. Appl. Phys. 137, 035902 (2025); doi: 10.1063/5.0246260 137, 035902-7© Author(s) 2025 16 January 2025 01:57:00https://doi.org/10.60893/figshare.jap.c.7602497https://doi.org/10.60893/figshare.jap.c.7602497https://pubs.aip.org/aip/japthe fluorescence spectrum up to 50 GPa under quasi-hydrostaticconditions are shown in Fig. 8. The peak separation worsenedslightly as the pressure increased due to increasing non-hydrostaticity, but the R1 and R2 peaks could still be separated,even at 50 GPa.To confirm the maximum applicability of this pressurescale, structural changes in Ga2O3:Cr3+ were investigated by insitu XRD experiments. Changes in the diffraction patterns inexperiments with and without a mixed alcohol pressure mediumare shown in Figs. 9(a) and 9(b), respectively. Both results con-firmed that the corundum structure initiates a transition to aRh2O3(II)-type structure, and the phase transition progressesslowly with increasing pressure and is not fully converted in themeasured pressure range. The phase transition pressure occurredat 54–65 GPa, depending on the presence of the pressuremedium. This difference is similar to the kinetic effect reportedfor the transition in Al2O3, where there is also a large differencebetween the room-temperature phase transition pressure(109 GPa),17 the calculated phase transition pressure (78–85 GPa),13–15 and the phase transition pressure of high-temperature high-pressure experiments (96 GPa).17–19More interestingly, differences in the transition pressure wereobserved in the present study depending on whether a pressuremedium was used or not. When the pressure medium was used,the stability of the corundum structure was ensured up to approxi-mately 65 GPa; the structural phase transition started at approxi-mately 54 GPa in the experimental results obtained underconditions without a pressure medium. The corundum andRh2O3(II)-type structures are linked by a structural relationship ofcertain twinning-like defects and explained by a displacement-typephase transition.42 Therefore, this difference may be the effect ofnon-hydrostaticity promoting a displacement-type phase transition.Thus, we can conclude from these results that the effective range ofthe pressure-scale equation determined in this study is approxi-mately 50 GPa.IV. SUMMARYThe pressure dependence of the fluorescence spectrum ofGa2O3:Cr3+ was measured up to 50 GPa in the argon pressuremedium at room temperature, and the pressure scale was deter-mined by fitting with a nonlinear curve equation. The curve shiftedupward from linear with increasing pressure. The results of XRDunder high pressure at room temperature indicate that the corun-dum structure is stable up to 54 or 65 GPa, depending on thedegree of non-hydrostaticity, and then begins to transition to theRh2O3(II) structure. Therefore, this pressure scale is applicable upto at least 50 GPa. The temperature dependence of the fluorescencewavelength was measured in the range 303–83 K. The temperaturedependence was found to be about twice as large as that of ruby,which can be explained by differences in electron–phonon couplingrather than thermal expansion. We investigated the size of crystalsproduced by changing the synthesis time under high-temperatureand high-pressure synthesis conditions at 7.7 GPa and 1200 °C andfound that crystals less than 10 μm in size, suitable for DAC experi-ments, could be grown in 6 min.SUPPLEMENTARY MATERIALSee the supplementary material for the XRD profiles of syn-thesized crystals at different crystal growth times and the calcula-tion of the coefficient of thermal expansion of corundum-typeGa2O3:Cr3+ and the temperature function formula. All data for theregression analysis of Eqs. (1)–(6) are summarized in Tables SIII–SV, and furthermore, the details of the peak fitting analysis fordetermining the phase transition of Ga2O3:Cr3+ from corundum toRh2O3(II) type are summarized. Lattice parameters of Au and pres-sure values determined by the equation of state in the compressionexperiment of Ga2O3:Cr3+ are also given in Table SVI.ACKNOWLEDGMENTSThis work was supported by JSPS KAKENHI (Grant Nos.19H05790 and 23K17711) and, in part, by World PremierInternational Research Center Initiative. The synchrotron radiationexperiments were conducted at BL2S1 in AichiSR and BL18C andAR-NE1 in Photon Factory (KEK) with the approval of AichiSR(Proposal Nos. 2023N6005, 2024N1001, and 2024N2002) and KEK(Proposal No. 23G570). Preliminary experiments were conductedat SPring-8 BL10XU (Proposal No. 2023B1344 and 2024A1216).We thank K. Watanabe for his help with spectroscopic measure-ments under low temperature. 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