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The valence state, hybridization and electronic band structure of charge ordered AlV2O4 are investigated by measuring the electron energy loss spectra (EELS) and performing band structure calculations using the WIEN2k code. White line ratio and O K edges of V2O5, VO2, V2O3 and AlV2O4, obtained using electron energy loss spectroscopy, are analysed specifically to probe systematically the VO6 octahedra in all of them. The systematic decrease of the L2 intensity and the O K edge intensity from V(5+) in V2O5 to AlV2O4 indicates a progressive increase in the occupancy of the hybridized states, which is corroborated by the absence of a transition from O 1s to hybridized 2t(2g). Band structure calculations on the parent charge frustrated cubic phase and the charge ordered rhombohedral phase clearly document a band gap in the charge ordered state. From the structural information obtained after convergence and the spectroscopic information from EELS, it appears that partial orbital occupancy may lead to a deviation from an integral valence state on all the vanadium in this exotic charge ordered spinel system.

AlV2O4 is the only spinel compound so far known that exists in the charge ordered state at room temperature. It is known to transform to a charge frustrated cubic spinel structure above 427 ° C. The presence of multivalent V ions in the pyrochlore lattice of the cubic spinel phase brings about the charge frustration that is relieved in the room temperature rhombohedral phase by the clustering of vanadium into a heptamer molecular unit along with a lone V atom. The present work is the first demonstration of pressure-induced frustration in the charge ordered state of AlV2O4. Synchrotron powder x-ray diffraction studies carried out at room temperature on AlV2O4 subjected to high pressure in a diamond anvil cell show that the charge ordered rhombohedral phase becomes unstable under the application of pressure and transforms to the frustrated cubic spinel structure. The frustration is found to be present even after pressure recovery. The possible role of pressure on vanadium t2g orbitals in understanding these observations is discussed.

We calculate properties like equilibrium lattice parameter, bulk modulus and monovacancy formation energy for nickel (Ni), iron (Fe) and chromium (Cr) using Kohn-Sham density functional theory (DFT). We compare the relative performance of local density approximation (LDA) and generalized gradient approximation (GGA) for predicting such physical properties for these metals. We also make a relative study between two different flavors of GGA exchange correlation functional, namely PW91 and PBE. These calculations show that there is a discrepancy between DFT calculations and experimental data. In order to understand this discrepancy in the calculation of vacancy formation energy, we introduce a correction for the surface intrinsic error corresponding to an exchange correlation functional using the scheme implemented by Mattsson et al (2006 Phys. Rev. B 73 195123) and compare the effectiveness of the correction scheme for Al and the 3d transition metals.