Volume and pressure dependences of the electronic, vibrational, and crystal structures of Cs2CoC l4: identification of a pressure-induced piezochromic phase at high pressure

Abstract

This work investigates the high-pressure structure of Cs2CoCl4 and how it affects the electronic and vibrational properties using optical absorption, Raman spectroscopy, x-ray diffraction, and x-ray absorption in the 0-15 GPa range. In particular, we focus on the electronic and local structures of Co2+, since compression of Cs2CoCl4 yields structural transformations associated with change of coordination around Co2+, which are eventually responsible for the intense piezochromism at 7 GPa. This study provides a complete characterization of the electronic and vibrational structures of Cs2CoCl4 in the Pnma phase as a function of the cell volume and the local CoCl4 bond length, RCo-Cl, as well as its corresponding equation of state. In addition, our interest is to elucidate whether the phase transition undergone by Cs2CoCl4 at 7 GPa leads to a perovskite-layer-type structure where Co2+ is sixfold coordinated, decomposes into CsCl+CsCoCl3, or it involves an unknown phase with different coordination sites for Co2+. We show that Co2+ is sixfold coordinated in the high-pressure phase. The analysis of optical spectra and x-ray diffraction data suggests the formation of an interconnected structure of exchange-coupled Co2+ through edge-sharing octahedra at high pressure.