Equation of state of lithium deuteride from neutron diffraction under high pressure

Abstract

Time-of-flight neutron-diffraction spectra of powder samples of $^{7}\mathrm{LiD}$ have been recorded at 300 K, up to 10 GPa at the ISIS spallation source. The experimental setup consists of a 2.5-MN hydraulic press, 50 kg in mass, acting on a toroidal anvil cell. This allows compression of 100-${\mathrm{mm}}^{3}$ samples to over 10 GPa, with better stress homogeneity than flat-anvil devices. The equation of state of $^{7}\mathrm{LiD}$ has been related to the NaCl pressure scale with a precision of 1.5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}3}$ on the volume variation under pressure. Comparison of existing calculations with the present data shows variable agreement, depending on the calculational hypotheses. Reassessment of the divergent theoretical estimates for the pressure of band crossing in lithium hydrides reconciles them to a common prediction of about 180 GPa for the pressure of semi- metallization. However, the present work cannot reduce the wide separation of the estimates for the B1\ensuremath{\rightarrow}B2 structural-transition pressure.