Electronic properties of the two rutiles TiO2 and VF2: A comparative study

Results of first-principles, density-functional LMTO-ASA calculations on th e two title compounds and of unrestricted Hartree-Fock (UHF) calculations o n VF2 are reported. The calculations on the crystalline materials were perf ormed with special emphasis on optimising all (three) structural parameters , on the valence density of states and the valence electron density, and on the possible existence of spin polarisation. In addition, full-potential L MTO calculations on isolated monomers were carried through in order to exam ine the nearest neighbour (Ti-O and V-F) chemical bonds in detail and to un derstand the consequences of various approximations in describing the poten tial for the crystal. The LMTO-ASA calculations reproduce the correct latti ce constants with, however, some overestimate for VF2. Also the nearest-nei ghbour Ti-O and V-F bond lengths are reproduced correctly by the LMTO calcu lations. whereas the c/a ratios are significantly overestimated. By analysi ng the charge densities of the isolated monomers we conclude that the overe stimates in the c/a ratios art due to the ASA (Atomic-Sphere Approximation) , which does not describe the quadrupole-quadrupole interactions of the com pounds sufficiently accurately. The Hartree-Fock calculations yield lattice constants for VF2 very close to experiment. Also in agreement with experim ent. TiO2 is found to be non-magnetic and insulating/semiconducting. in con trast, VF2 is found to be metallic within the LMTO-ASA method, although the density-functional calculations indicate that magnetic effects will change the compound into an insulator a it is according to experiment. On the oth er hand. in agreement with experimental evidence, UHF yields a non-metallic structure. It further shows the stability of a number of patterns of magne tic ordering.