On the nature of the ground state of Cr(V) defects in YVO4: An ab initio model potential embedded-cluster study

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 lattice: 2 A 1 and 2 B 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 distortion of the (VO 4 ) 3 - cluster characteristic of the YVO 4 lattice. The relative order predicted by CFT ( 2 B 1 < 2 A 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 multiconfigurational expansions for the 2 A 1 and 2 B 1 wavefunctions, the 2 A 1 state is found to be the ground state, in agreement with experimental evidence; the energy difference with respect to the 2 B 1 i state is calculated to be 1650 cm - 1 . We have analyzed the results produced using different embeddings and different wavefunction expansions and we have reached the conclusion 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 strong interactions between the Cr(V) impurity and the two Y 3 + cations in the second coordination sphere of the Cr(V).