In order to better understand the reconstructive ferroelectric-paraelectric transition of Bi2WO6, which is unusual within the Aurivillius family of compounds, we performed first principlescalculations of the dielectric and dynamical properties on two possible high-temperature paraelectic structures: the monoclinic phase of A2/m symmetry observed experimentally and the tetragonal phase of I4/mmm symmetry, common to most Aurivillius phase components. Both paraelectric structures exhibit various unstable modes, which after their condensation bring the system toward more stable structures of lower symmetry. The calculations confirm that, starting from the paraelectric A2/m phase at high temperature, the system must undergo a reconstructive transition to reach the P 21ab ferroelectric ground state.
We present quantum mechanical calculations of structural, electronic, and magnetic properties of X2AlX′ (X=Fe, Co, Ni; X′=Ti, Cr) Heusler alloys within the generalized gradient approximation. Structural properties, magnetic moments, and stable ground states are well described by the present method and are in good agreement with available experimental and theoretical results. Also, electronic properties are predicted and related to stability and conductivity mechanisms. We also report the polarized densities of states and shown the half metallicity character among these materials.
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