V$_8$C$_{7-\delta}$ superstructure in nonstoichiometric vanadium carbide powders

Neutron and X-ray diffraction methods have been applied jointly to study the crystal structure of the ordered phase of V$_8$C$_{7-\delta}$ in macro- and nanocrystalline vanadium carbide powders. The deviation of the composition of the ordered V$_8$C$_{7-\delta}$ phase from the stoichiometric composition V$_8$C$_7$ has been established. The magnitudes and directions of the displacements of C and V atoms have been determined. It has been shown that vanadium atoms are displaced towards a vacancy, increasing the electron density of the vacancy position of the carbon sublattice.