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 $80\mathrm{GPa}$ and from $300\text{to}900\mathrm{K}$. The atomic volume of neon is reported in the same $P\text{−}T$ range and also up to $208\mathrm{GPa}$ at room temperature. The crystal structure of neon remains face-centered cubic over the domain of investigation. We show that a Mie–Grüneisen–Debye formalism reproduces very well the present $P\text{−}V\text{−}T$ 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–Grüneisen–Debye model. Its thermal expansion decreases faster with increasing pressure than the predictions of simple thermodynamic models.