First-principles investigation of intrinsic defects and self-diffusion in ordered phases of V2C.

The self-diffusion behavior of vanadium subcarbide (V2C) is investigated using density functional theory calculations, owing to its potential application as a diffusion barrier in nuclear applications. Three ordered V2C structures, two of which correspond to experimentally observed phases, are characterized in terms of their equilibrium structural, electronic and elastic properties. Our model for self-diffusion in V2C considers diffusion of carbon and vanadium to occur separately on each sublattice. Two sets of self-diffusion coefficients are calculated for each structure: one for vacancy-mediated diffusion of vanadium and the other for interstitial diffusion of carbon. Calculated activation energies and diffusion prefactors are compared to experimental data for the cubic transition metal carbides as there is no experimental self-diffusion data for any of the hexagonal subcarbides.

First-principles investigation of ferroelectricity in LaBGeO5.

Density functional theory calculations are performed to characterize the structural, electronic and vibrational properties of both the low-temperature ferroelectric and high-temperature paraelectric phases of LaBGeO5. Phonon dispersion calculations for the high-temperature phase reveal an unstable mode whose zone-center eigenvector corresponds to a rigid rotation of the BO4 tetrahedra, in agreement with previous calculations based on a short-range model potential. A possible switching path between two symmetry-equivalent ferroelectric phases that goes through the high-temperature paraelectric phase is identified and used to calculate the spontaneous polarization. The theoretical value for the spontaneous polarization calculated using the modern theory of polarization is 4.9 µC cm(-2) for the PBEsol + U functional, which lies within the experimental range.