Equation of state of solid hydrogen and deuterium from single-crystal x-ray diffraction to 26.5 GPa

Pressure-volume equations of state of solid normal hydrogen and deuterium are determined from single-crystal x-ray diffraction measurements to maximum pressures of 26.5 GPa (265 kbar) at 300 K. Experimental data for deuterium are presented and used with the previously reported results for hydrogen. The measurements for deuterium indicate that the structure remains hexagonal closed packed to at least 14.2 GPa at 300 K. The P-V data are analyzed with both phenomenological equation-of-state formalisms and pair-potential models. The thermal contributions are calculated from a Mie-Gruneisen model and from lattice-dynamics calculations using effective potentials. A P-V equation of state is determined that accurately describes both the high-pressure diffraction data (to V/${\mathit{V}}_0$=0.2) and previous low-pressure compression results (V/${\mathit{V}}_0$=1.0 to 0.4). There is a significant softening of the equation of state relative to the predictions of lattice-dynamics calculations using previously reported pair potentials, and several effective potentials are examined. Possible corrections to the equation of state at higher pressures, above the range of the present experiments, include effects associated with order-disorder transitions, vibron softening, and band overlap.