By using a thin C13 diamond chip together with a C12 diamond chip as sensors, the diamond Raman spectra provide the means to measure pressure precisely (±0.3 GPa) at any temperature (10–1200 K) and simultaneous hydrostatic (or quasihydrostatic) pressure (0–25 GPa) for any sample compatible with an externally heated diamond-anvil cell. Minimum interference between the Raman spectrum from the diamond anvils and those of the pressure sensors is obtained by measuring pressures with the Raman signal from the C13 diamond chip up to 13 GPa, and that from the C12 chip above 10 GPa. The best crystallographic orientation of the diamond anvils is with the [100] direction along the direction of applied force, in order to further minimize the interference. At 298 K, the pressure dependence of the C13 diamond first-order Raman line is given by ν(P)=νRT+aP for 91 at. % C13 diamond, where νRT(13C)=1287.79±0.28 cm−1 and a(13C)=2.83±0.05 cm−1/GPa. Analysis of values from the literature shows that the pressure dependence of the Raman line of C12 diamond is best described by the parameters νRT(12C)=1332.5 cm−1 and a(12C)=2.90±0.05 cm−1/GPa. The temperature dependence of the diamond Raman line is best described by ν(T)−νRT=b0 for T⩽200 K, and ν(T)−νRT=b0+b1.5Tk1.5 for 200 K⩽T⩽1500 K, where Tk=T−200 K. For 91 at. %C13 diamond, the parameters are b0=0.450±0.025 cm−1;b1.5=−(7.36±0.09)×10−4 cm−1 K−1.5; and for C12 diamond, the parameters are b0=0.467±0.033 cm−1,b1.5=−(7.56±0.10)×10−4 cm−1 K−1.5. Although no quantitative theoretical models are available for calculating the Raman shift as a function of temperature, the excellent fits to the data suggest that the Tk1.5 dependence above has a physical basis.
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