EFFECTS OF CHEMICAL PRESSURE ON THE CHARGE-TRANSFER SPECTRA OF CUX42-COMPLEXES FORMED IN CU2-DOPED A(2)MX(4) (M=ZN, MN, CD, HG, X=CL, BR)()

This work investigates the charge-transfer spectra of CuCl42- and CuBr 42- complexes formed in anisotropic A(2)MX(4) (X = Cl, Br) crystals. A ttention is paid to the variations of the charge-transfer transition e nergies induced by chemical pressure effects when substituting Mn --> Zn --> Cd --> Hg along the crystal series. A salient feature of this s tudy is the weak sensitivity of these bands to structural changes of t he complex in comparison to the sensitivity of the corresponding cryst al field (d-d) bands. The knowledge of these structural-induced shifts is important since they are responsible to a great extent for the the rmo- and piezochromic properties exhibited by some compounds containin g CuX42- units as chromophores. We present a tentative model based on a perturbed tetrahedral CuX42- complex for explaining the weak sensiti vity exhibited by the charge-transfer transitions in the title compoun ds. Furthermore the estimates of this model can also explain the big d ifference between the energy shift of d-d and charge-transfer transiti ons due to structural changes of the CuCl42- complex in pure chlorocup rates.