Calcium fluoride as an internal pressure standard in high-pressure crystallography

Cleavage plates of fluorite, CaF₂, provide a simple and sensitive pressure calibration based on variation of the cubic unit-cell volume in high-pressure/high-temperature (PT) single-crystal diffraction experiments, when temperature is measured independently by thermocouple. Over the range 0 ≤ T ≤ 500°C and P ≤ 0.04 Mbar (4 GPa), the temperature–pressure–volume equation of state for CaF₂ is approximately

, where ΔV/V_o is the fractional change in cubic unit-cell volume from T = 0°C and P = 0 Mbar, T is temperature in °C, and P is pressure in Mbar. Crystal structures of CaF₂ determined at several PT conditions are evidence of reversibility of single-crystal unit-cell parameters with heating and compression. Nonreversible changes occurred in observed temperature factors and extinction parameter.