Equation of state and high-pressure phase transition of CsI

High-pressure x-ray diffraction patterns of CsI were obtained to 660 kbar. Diffraction peaks of a new phase appear between 370 and 385 kbar. The pressure-volume relationship of the low-pressure cubic phase was fitted to Keane's second-order equation of state with $B_0=118.9\mathrm{}$ kbar, $B_0^{\prime }=5.93\mathrm{}$, and $B_0^{\prime \prime }=-0.131\mathrm{}$ ${\mathrm{kbar}}^{-1\mathrm{}}$ with $B_0$ and $B_0^{\prime }$ values forced to fit the isothermal values obtained from ultrasonic data. The $d$ spacings of the new high-pressure phase are consistent with tetragonal indexing.