Insulators containing CuCl₄X₂^2- (X=H₂O, NH₃) units: Origin of the orthorhombic distortion observed only for CuCl₄(H₂O)₂^2-

The origin of the difference in structure between compounds containing CuCl4X22- (X=H2O, NH3) units is analyzed by means of first-principles calculations. While NH3-containing compounds display tetragonal symmetry, H2O-containing ones display an orthorhombic distortion at low temperature where the equatorial Cl- ions are no longer equivalent.

Our simulations of optical and vibrational transitions show good agreement with all available experimental optical absorption and Raman data. As a salient feature, the value of the force constant for the B1g mode, K(B1g), driving the orthorhombic distortion in the CuCl4(H2O)22- unit is found to be four times smaller than that calculated for CuCl4(NH3)22-, stressing that CuCl4(H2O)22- is in the verge of the D4h → D2h instability.

The analysis of results obtained for different values of the distortion coordinate, Q(B1g), clearly shows that the softening undergone by K(B1g) in CuCl4(H2O)22- comes mainly from the vibronic admixture of the antibonding a*1g(~3z2-r2) orbital with the b1bg bonding (or charge transfer) level. This mechanism is thus similar to that responsible for distortions observed in some fluoroperovskites and oxoperovskites.

The present results, quantifying the importance of vibronic effects in structural instabilities, clearly demonstrate that, contrary to what was suggested by several authors, the instability in CuCl4(H2O)22- is not related to the Jahn-Teller effect and that the orthorhombic distortion observed in the pure compound Rb2CuCl4(H2O)2 has a local origin.