On the Nature of the Ground State of Cr(V) Defects in YVO_4: An Ab Initio Model Poential Embedded-Cluster Study

摘要

In this article we present the results of ab initio model potential (AIMP) embedded cluster calculations of the electronic structure of (CrO_4)~(3-) in the lattice of YVO_4, in order to study the factors which determine the relative energy position of the two lowest lying electronic states of the Cr(V) ion as a dopant in this host latice: ~2A_1 and ~2B_1. Electron paramagnetic resonance (EPR) and optical spectroscopy studies have reported these two states to be in reversed order from what should be expected following crystal field theory (CFT) considerations, taking into account the elongated distortin of the (VO_4)~(3-) cluster characteristic of the YVO_4 lattice. The relative order predicted by CFT (~2B_1 < ~2A_1) is found when self-consistent field (SCF) (CrO_4)~(3-) cluster calculations are done in vacuo. When AIMP embedded cluster calculations are performed using large multiconfiguration expansions for the ~2A_1 and ~2B_1 wavefunctions, the ~2A_1 state is found to be the ground state, in agreement with experimental evidence; the energy difference with respect to the ~2B_1 state is calculated to be 1650 cm~(-1). We have analyzed the results producedusing different embeddings and different wavefunction expansions and we havae reached the conclustion that embedding effects [both direct and indirect, through the local distortion produced by the Cr(V) impurity] and covalency (beyond SCF molecular orbital formation) are responsible for the energy reversal. Our results do not support previous interpretations of the energy reversal in terms of unusually stronog interactions between the Cr(V) impurity and the two Y~(3+) cations in the second coordination sphere of the Cr(V).