High-Pressure Studies of Cs2CuCl4 and Cs2CoCl4 by X-Ray Diffraction Methods

Abstract

Single crystals of Cs2CuCl4 and Cs2CoCl4 have been investigated at high pressures by X-ray diffraction techniques, using a diamond anvil cell. Increasing the pressure to just below 40 kbar caused the crystallinity of Cs2CuCl4 to deteriorate rapidly. This was accompanied by a color change from yellow/orange at lower pressures to deep red, suggesting, e.g., a major change of the coordination geometry around the Cu2+ ions. Decreasing the pressure to a few kilobars below the transition showed that the observed color change is reversible. No similar color change was observed for a Cs2CoCl4 specimen. Diffraction patterns collected at pressures further above the transition pressure do not indicate the formation of more crystalline new high-pressure phases. Single-crystal structure determinations of orthorhombic Cs2CuCl4, space group symmetry Pnma, were performed just below the transition at 27, 33, and 36 kbar. Refinements, based on about 400 unique diffraction intensities to determine 41 structural parameters, converged to R values well below 0.10. The results show that the main features of the structure are rather unaffected at pressures close to the phase collapse. The main effect in the structure is a shortening of the Cs–Cl distances by 0.2 Å. The flattening of the coordination tetrahedra around Cu2+ decreases slightly with pressure as reflected by a decrease of 4° for one of the Cl–Cu–Cl angles. The Cu–Cl distance decreases only by about 0.02 Å from ambient to 36 kbar. Despite the detailed structural information obtained the origin of the observed color change at the phase transition is still unresolved. However, the lack of any phase transition of Cs2CoCl4 in the investigated pressure range suggests that the transition depends on the presence of the highly distorted coordination tetrahedra in Cs2CuCl4. The structures of Cs2CuCl4 and CsCoCl4 both have very high compressibilities, with bulk moduli of 15.0(2) and 17(1)GPa, respectively.