CRYSTAL-FIELD AND CHARGE-TRANSFER SPECTRA OF (MG,FE)SIO3 PEROVSKITE

The optical and near-infrared absorption spectrum of a synthetic silic ate perovskite (Mg0.94Fe0.06SiO3) was obtained in the range from 1000 to 25000 cm-1. The spectrum shows three main features. (1) A system of crystal-field bands is centered at 7000 cm-1, which is caused by the transition 5E(g) --> 5T2g of Fe2+ in the dodecahedral site of the stru cture. (2) A broad band due to Fe2+ --> Fe3+ intervalence charge trans fer occurs at 14900 cm-1. (3) A sharp increase in absorption above 200 00 cm-1 probably results from O-Fe3+ charge transfer. From these data, the crystal-field parameters of Fe2+ in the dodecahedral site of the perovskite structure can be extracted. This yields a crystal-field spl itting (DELTA) = 6835 cm-1 and a crystal-field stabilization energy (C FSE) = 4350 cm-1. If the splitting of the ground state (estimated as 5 00 cm-1) is neglected, one obtains DELTA = 7085 cm-1 and CFSE = 4251 c m-1. This means that the CFSE of Fe2+ in perovskite approximately equa ls the CFSE in magnesiowustite (4320 cm-1). Therefore, the preferentia l partitioning of Fe2+ into the wustite phase cannot be explained by a simple crystal-field model. The existence of strong absorption bands over the entire range from mid-IR to UV indicates that perovskite is a very poor thermal conductor at high temperatures where radiative heat transfer dominates.