Electron-phonon coupling in charge-transfer and crystal-field states of Jahn-Teller CuCl64- systems

In this work we present an octahedral perturbative model to explain the dependence of the crystal-field (CF) and the charge-transfer (CT) energy with respect to structural distortions in Jahn-Teller (JT) CuCl64- systems. The method provides a simple way to express the variation of electronic energy to complex distortions of the totally symmetric mode Q(a1g) and the JT mode Q(theta) as a function of the corresponding electron-vibration coupling constants, [partial derivative E/partial derivative Q(i)]o(h) (i = Q(a1g) and Q(theta)). A value of 9100 cm(-1)/Angstrom for the linear JT coupling constant A(1) has been obtained for the octahedral E-2(g)(x(2) - y(2), 3z(2) - r(2)) CF State (e x E) from structural correlations along a series of copper compounds. The corresponding JT coupling for the T-2(1u)(pi) CT State (e x T), A(2) =3000cm(-1)/Angstrom, has been derived from hydrostatic pressure measurements performed on the (C3H7NH3)(2)CuCl4 perovskite layer. A noteworthy conclusion of this model is that a redshift of the intense e(u)(pi)-->b(1g)(x(2) - y(2)) CT band is possible in axially elongated CuCl64- systems upon anisotropic volume reduction, if the axial distance decreases more rapidly than the equatorial distance by \Delta R-ax\>30 \Delta R-eq\. These results are discussed in light of recent pressure experiments carried out in wide-gap CT semiconductors of the A(2)CuCl(4) family.