Midinfrared spectrum of undoped cuprates: d-dtransitions studied byab initiomethods

We present results of ab initio calculations for d-d transitions, which arise in the midinfrared spectrum of undoped cuprate compounds. It has been suggested that these transitions arise at energies as low as 0.4 eV in La₂CuO₄ and Sr₂CuO₂Cl₂. We study the differences in d-d transition energies in a series of cuprates that contains compounds in which the Cu ions are sixfold, fivefold, or fourfold coordinated. Furthermore, we analyze the dependence of the 3d_x²-y²→3d_z² excitation energy on the ratio of the in-plane and apex copper---ligand distances in the model system CuO. Our cluster calculations do not support the assignment of the 0.4--1-eV band to phonon and magnon sidebands of a d-d transition. On the other hand, we confirm the interpretation of the peak around 1.7 eV observed in CuGeO₃ as arising from d-d transitions.