High pressure–high temperature equations of state of neon and diamond

We report accurate measurements of the equation of state of diamond and neon, measured by x-ray diffraction in a resistively heated diamond-anvil cell. The atomic volume of single crystals of diamond embedded in neon pressure-transmitting medium has been measured between 0 and 800.3em0exGPa and from 3000.3em0exto0.3em0ex9000.3em0exK. The atomic volume of neon is reported in the same P-T range and also up to 2080.3em0exGPa at room temperature. The crystal structure of neon remains face-centered cubic over the domain of investigation. We show that a Mie--Gr\"uneisen--Debye formalism reproduces very well the present P-V-T data for neon as well as low pressure--low temperature data available in the literature. This makes neon a well calibrated x-ray pressure gauge, suitable for high pressure--high temperature studies. The thermal behavior of diamond is more complex and cannot be completely described by a Mie--Gr\"uneisen--Debye model. Its thermal expansion decreases faster with increasing pressure than the predictions of simple thermodynamic models.