Equation of state and metallization of CsI

Abstract

Self-consistent, nonrelativistic augmented-plane-wave (APW) calculations for CsI were carried out to generate the band structure, the static-lattice equation of state (EOS), and the volume dependence of the electronic energy-band gap. The theoretical room-temperature isothermal compression curve agrees well with static and ultrasonic measurements at low pressure. Our calculations do not agree with two recent sets of diamond-anvil-cell measurements above 10 GPa. The calculated band gaps are too small at low pressure, but, at high pressure, are consistent with both the experimental results and the Herzfeld-model prediction. These results suggest that the insulator-to-metal transition occurs in the range 100\ifmmode\pm\else\textpm\fi{}10 GPa. A calculation of the shock compression curve of CsI shows that the thermally excited electrons cause a significant softening of the Hugoniot curve. The experimental zero-pressure band gaps of the isoelectronic compounds Xe, CsI, and BaTe are linearly correlated with $\mathrm{ln}(\frac{v}{{v}_{H}})$, where ${v}_{H}$ is the specific volume of metallization predicted by the Herzfeld model. Based on this correlation, and on the similarity of the APW calculated EOS's of Xe and CsI, which agree closely with experimental compression measurements, we predict that BaTe will become metallic at approximately 30 GPa.