Cluster calculations of the electronic d-states in VO2

An ab-initio cluster calculation of the electronic d-states in VO2 has been made by using the self-consistent statistical exchange multiple scattering method. A VO8-6 octaedron was used to represent the tetragonal phase, and a V2O12-10 cluster with paired vanadium atoms was used for the monoclinic phase. Our re ults are in conflict with an explanation of the semiconducting gap in terms of simple distortion-induced band splitting. Vanadium dioxide (V02) undergoes a transition from a high temperature metallic phase to a low temperature semiconducting phase at 340 K [1]. These two phases have different crystallographic structures. Metallic V02 has a rutile tetragonal structure, each vanadium atom being located at the center of an oxygen octahedron (Fig. la). The low temperature phase is a monoclinic distortion of this rutile structure involving a pairing and off axis displacement of alternate vanadium ions along the rutile cr-axis (Fig. 1 b). As a result, the crystal unit cell is doubled. Starting with the work of Adler and Brooks [2], there have been several theoretical I descriptions in terms of band models [3, 4]. In these, the band-gap of semi-conducting V02 originates completely from a modification of the band structure by the distortion. In order to investigate the validity of such descriptions we have made several cluster calculations of the electronic structure, both in the rutile and distorted (*) Present address : Laboratory of Chemistry, University of Groningen, Groningen (Holland). configurations, by using the self-consistent statistical exchange multiple scattering method of Slater and Johnson [5]. This is an ab-initio calculation, which yields the splitting of the vanadium 3d-levels in the distorted structure. A complete detailed description of the calculations will be given elsewhere. We present here only the data and results relevant to the problem of the metal-insulator transition. We compare the band scheme inferred from our calculation to experimental data. A disagreement is found which is considered as evidence for the importance of intra-atomic correlations in inducing a band-gap. Our calculations simulate a neutral cell of V02 in the solid by surrounding each atomic species of the cluster with non-overlapping muffin tin spheres, and by enclosing the cluster with a so-called outersphere [5]. Three complete cluster calculations were performed, using a different geometry and/or a different number of atoms in each case. The quantities of main interest coming out of the calculation are the energies of the vanadium d-levels and the corresponding charge distributions. The clusters have been chosen Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyslet:01975003605015700