B. A. Moral, F. Rodriguez, R. Valiente, M. Moreno and H. U. Güdel
Local Structure of Cu²⁺ in the (C₂H₅NH₃)₂MCl₄:Cu²⁺ (M = Cd, Mn) Layer Perovskites.
Influence of Hydrostatic Pressure in the 0-60 kbar Range
Z. Phys. Chem. 201, 151-158 (1997)           

Abstract: This paper deals with the effect of pressure on the charge-transfer spectra of CuCl₆^4- complexes formed in Cu²⁺ doped (C₂H₅NH₃)₂MCl₄ (M = Cd, Mn). A pressure-induced redshift is observed for the first charge-transfer band in both crystals. While the shift is continuous for the Mn crystal at a rate of -40 cm^–1/kbar in the 0-60 kbar range, it experiences an abrupt jump of -1400 cm^–1 around 26 kbar for the Cd crystal. Such a discontinuous behaviour is interpreted in terms of a structural change in the CuCl₆^4- coordination geometry from an axially elongated octahedron to a more compressed geometry. The present results are compared with those reported for the pure (C₂H₅NH₃)₂CuCl₄ crystal.