Hybrid density functional theory study of vanadium monoxide

First-principles calculations of nondefective VO in the Fm (3) over barm structure based on a range of single-particle Hamiltonians containing varying amounts of exact exchange indicate that the ground electronic state is that of a d(3) high spin, antiferromagnetic (AF), Mott-Hubbard insulator with an AF(1) spin alignment of the local cation moments. This description remains essentially unchanged down to 10% exact exchange, and only for the pure density functional theory (DFT) potential is the AF(1) phase found to be metallic. Strong spin-lattice interaction is predicted with differences in lattice constant of up to 1.6% between AF(1) and FM (ferromagnetic) order. The AF(1) lattice constant is predicted to be similar to4.37 Angstrom, which is roughly 7% greater than the reported lattice constants for the defective material. The bulk modulus is comparable to those of CaO, MnO, and NiO. A mapping of total energies onto an Ising spin Hamiltonian containing both direct and superexchange interactions reveals the dominant magnetic interaction to be the direct coupling of antiferromagnetically aligned nearest-neighbor cation spins, which leads to the stability of the AF(1) phase. Direct coupling energies are found in the range -11.1 to -44.3 meV as the proportion of exact exchange is reduced, leading to an estimated critical disorder temperature in the range 300-450 K. However, the limitations of such mapping are exposed by a consideration of the relative stabilities of the AF(1) and AF(3) alignments. Orbital projected densities of states reveal filled to unfilled gaps which depend strongly on the proportion of exact exchange and for the B3LYP potential (20% exact exchange) are similar to2.5 eV for the spin-forbidden xy(up arrow)-->xy(down arrow) excitation, similar to3.0 eV for xy(up arrow)-->z(2)(up arrow), and similar to3.5 eV for V-->O charge transfer. Variationally stable, highly local crystal-field excited states ranging in energy from similar to0.6 to similar to2.7 eV are predicted for exchange-correlation potentials down to 30% exact exchange and from comparisons with the corresponding band excitation estimates of similar to1 to similar to2 eV are obtained for the localization energy of Frenkel excitons. From a mapping of the excited crystal-field energies onto a Kanamori Hamiltonian, values are obtained for the lattice Racah B and C parameters and the d-orbital averaged exchange and crystal-field energies. A comparison of mapped and directly calculated energies of the two-electron excitation (xz,yz)-->(z(2),x(2)-y(2)) confirms the validity of Kanamori mapping, notably in the limit of exact exchange.