Optical excitations and coupling constants in FeO42- and CrO44- complexes in oxides: Density functional study

The experimental crystal field (CF) and charge transfer (CT) spectra due to Fe6+-doped K2XO4 (X=S, Se, Cr) and Cr4+ in oxides have been investigated b y density functional (DF) calculations on FeO42- and CrO44- complexes at di fferent values of the metal-ligand distance, R. To this aim the coupling co nstants, A(alpha) and A(epsilon), with the A(1) and the Jahn-Teller E modes have also been calculated. The present results explain reasonably the tran sition energies and coupling constants observed for Fe6+ and Cr4+ in oxides and account for the following experimental features: (i) The different nat ure of the first CF and CT excitations observed on passing from Fe6+ to Cr4 + in oxides. (ii) Why for Fe6+ in oxides the dominant coupling is with the E mode for the CF states while with the A(1) mode for CT states. (iii) Why on going from Cr4+ to Fe6+ in oxides the coupling constant A(alpha) of CT s tates decreases while the opposite is found for CF ones. The role played by the covalency on these questions is explained on simple grounds, stressing that it drastically decreases the A(alpha) value for CT states. This fact together with the value nu(alpha)=820 cm(-1) accounts for the low value of the Huang-Rhys factor S-alpha=2.5 in the assigned 1t(1)down arrow --> 4t(2) down arrow CT excitation of K2SeO4:Fe6+. As a salient feature the R depende nce of 10Dq (which determines A(alpha) for the T-3(2) CF state) is found to be greatly sensitive to the small admixture (approximate to 0.5%) of 2s(O) orbitals in the antibonding 2e level. The electronic relaxation decreases the energy of the first CT transition of FeO42- by 19 000 cm(-1), the charg e distribution being however the same as that of the ground state. The impo rtant role played by the trigonal distortion for reducing the Jahn-Teller e ffect in the first excited state of Cr4+ in oxides is analyzed through a ph enomenological model. Though no further neighbors of the host lattice are i ncluded, the calculated equilibrium distances for FeO42- and CrO44- are onl y 3% and 8% higher than experimental values found for Cr4+ to Fe6+ in oxide s. Moreover the calculated value of the nu(alpha) frequency at the right eq uilibrium distance essentially coincides with experimental findings indicat ing that force constants between oxygen and further neighbors likely play a minor role. In comparison with recent ab initio plus configuration interac tion results on FeO42- the DF scheme offers a simpler description of ground and CT states. (C) 1999 American Institute of Physics. [S0021-9606(99)0124 1-6].