Gold as a reliable internal pressure calibrant at high temperatures

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Abstract
Energy-dispersive x-ray diffraction measurements on a mixture of powdered NaCl and Au were carried out at high temperatures up to 600 °C (at atmospheric pressure) and also at simultaneously high temperatures and high pressures (to 425 °C and 10.2 GPa). A modified diamond-anvil high pressure cell with a mini resistance-wire heater was used in conjunction with the synchrotron radiation at the Stanford Synchrotron Radiation Laboratory. The temperature of the sample was measured directly by a precalibrated Pt-Pt 10% Rh thermocouple adjacent to the sample. The pressure of the sample was determined by measuring the molar volume changes in NaCl and deriving the pressure through an equation of state. The measured thermal expansion from 25 °C up to 600 °C for NaCl and Au at 1 atm can be expressed by second-order polynomials: V/V0(NaCl)=1+(1.08±0.05)×10−4 (T−25)+(7.5±1.1)×10−8 (T−25)2 and V/V0(Au) =1+(4.26±0.85)×10−5 (T−25)+(5.0±20)×10−9 (T−25)2, where T is in °C. These results are in excellent agreement with previously published data. The measured molar volume ratios of Au at simultaneously high temperatures and high pressures have been found to be in good agreement (within 0.2%) with the values based on the calculated isochores from shock wave data. The pressures determined from the compression of Au at high temperature are compatible with those based on the compression of NaCl (within 2.7% at pressures up to 10.2 GPa). These results enhance the reliability and the scope of the use of Au as an internal pressure calibrant at high temperatures.