Effects of chemical pressure on the charge-transfer spectra of complexes formed in -doped (M = Zn, Mn, Cd, Hg; X = Cl, Br)

This work investigates the charge-transfer spectra of and complexes formed in anisotropic (X = Cl, Br) crystals. Attention is paid to the variations of the charge-transfer transition energies induced by chemical pressure effects when substituting along the crystal series. A salient feature of this study is the weak sensitivity of these bands to structural changes of the complex in comparison to the sensitivity of the corresponding crystal field (d-d) bands. The knowledge of these structural-induced shifts is important since they are responsible to a great extent for the thermo- and piezochromic properties exhibited by some compounds containing units as chromophores. We present a tentative model based on a perturbed tetrahedral complex for explaining the weak sensitivity exhibited by the charge-transfer transitions in the title compounds. Furthermore the estimates of this model can also explain the big difference between the energy shift of d-d and charge-transfer transitions due to structural changes of the complex in pure chlorocuprates.