Structural, electronic and magnetic properties of V2O5−x: An ab initio study

Abstract

Pure V(2)O(5) is a diamagnetic layered semiconductor with many applications such as catalysis. In this paper, we study oxygen vacancy-induced changes in the atomic and electronic structures as well as magnetic properties of V(2)O(5-x) within spin density functional theory with generalized gradient approximation. Both the supercell approach and virtual crystal approximation are used to simulate the oxygen-deficient V(2)O(5-x) with vacancy concentration x up to 0.5. The 1x2x2 supercell calculations with one O vacancy predict that the formation energies of the apical (O(1)), bridge (O(2)), and chain (O(3)) oxygen vacancies are, respectively, 2.48, 4.17, and 4.44 eV/vacancy, and hence that the O vacancies in V(2)O(5-x) would be predominantly of the O(1) type. The local structural distortions of the V atoms next to the O vacancies are found to be large for high vacancy density x(x>0.25), and for x approximately 0.5, even the crystal lattice changes from the orthorhombic to monoclinic symmetry. In all the cases considered, an O vacancy-induced stable or metastable ferromagnetic state with spin magnetic moment of approximately 2.0mu(B)/vacancy is found. For x below approximately 0.13 and 0.19<x< approximately 0.45, the ferromagnetic state would be the ground state, while for 0.45<or=x<or=0.5, the antiferromagnetic state with the V spins on neighboring rungs (AF-2) being antiparallel is the ground state. Importantly, this suggests that undoped V(2)O(5-x) with x<or=0.13 and 0.19<x< approximately 0.45 would be a diluted ferromagnetic semiconductor. The AF-2, however, disappears for x<or=0.25, while the antiferromagnetic state with the V spins on neighboring ladders being antiparallel (AF-1) occurs for the entire range of x studied. Nevertheless, the AF-1 is energetically more favorable than the ferromagnetic state only in 0.13<x< approximately 0.19. For low O vacancy concentrations (x<0.25), the electronic structure of V(2)O(5-x) is very similar to that of the perfect bulk V(2)O(5), except that 2x electrons now occupy the low V d(xy) dominant conduction bands which are exchange split. Majority of the magnetization is located on the d(xy)-orbitals of the V atoms near the O vacancy site. For larger x values, however, the electronic structure may change significantly, and, in particular, the V d-orbital character of the low conduction bands can be altered completely. Analysis of the calculated electronic structure reveals that the oxygen vacancy-induced magnetization in V(2)O(5-x) results primarily from the Stoner mechanism.