Pressure-induced polymorphism in hypervalent CsI3

Abstract

We report the results of ambient temperature high-pressure synchrotron-based x-ray diffraction, Raman, and electrical resistance study of $\mathrm{Cs}{\mathrm{I}}_{3}$ up to 29, 25, and 8 GPa, respectively, and confirm three-phase transitions under quasihydrostatic conditions. The ambient orthorhombic (space group (SG): Pnma) phase of $\mathrm{Cs}{\mathrm{I}}_{3}$ is stable up to a pressure of $\ensuremath{\sim}1.3\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$, above which a phase transition to a trigonal ($\mathrm{SG}:P\text{\ensuremath{-}}3c1$) phase is observed. The stability region of the trigonal phase has been found to be up to 22.6 GPa, above which the trigonal phase transforms to a cubic ($\mathrm{SG}:Pm\text{\ensuremath{-}}3n$) phase which remains stable until the maximum pressure of 29 GPa achieved in this study. A third-order Birch-Murnaghan equation of state fit to the pressure volume ($P\text{\ensuremath{-}}V$) data yields a bulk modulus of 17.7(9) GPa for the trigonal phase. Raman spectroscopic measurements however indicate three-phase transitions at $\ensuremath{\sim}1.3,\phantom{\rule{0.16em}{0ex}}4.0$, and 22.6 GPa, respectively. The electrical resistance measured in the low-pressure region up to 8 GPa indicates an electronic transition at around 4 GPa confirming the Raman result observed at 4.0 GPa. The $P\text{\ensuremath{-}}V$ data when transformed to the universal equation of state (UEOS) show a deviation from linearity around 4.0 GPa confirming the electronic transition. The present study has thus revealed a three-phase structural sequence in alkali trihalides, viz., orthorhombic (SG: Pnma) to trigonal ($\mathrm{SG}:P\text{\ensuremath{-}}3c1$) to a cubic ($\mathrm{SG}:Pm\text{\ensuremath{-}}3n$) phase.