B3LYP density functional calculations on the ground-state structure, elastic properties, and compression mechanism ofα−ZrW2O8

Abstract

The structure of $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Zr}{\mathrm{W}}_{2}{\mathrm{O}}_{8}$ was optimized at different pressures and its elastic constants were calculated at the B3LYP density functional level of theory. Overall, the relative stiffnesses of the atomic bonds (and linkages), ranked in terms of bond compressibilities, decrease according to the sequence $\mathrm{W}\text{\ensuremath{-}}\mathrm{O}>\mathrm{Zr}\ensuremath{\cdots}\mathrm{W}>\mathrm{Zr}\text{\ensuremath{-}}\mathrm{O}$. The tetrahedra around tungsten atoms are found to be much stiffer than the $\mathrm{Zr}{\mathrm{O}}_{6}$ octahedra. These latter are, in fact, more compressible than the $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Zr}{\mathrm{W}}_{2}{\mathrm{O}}_{8}$ unit cell. The elastic constants calculated in the athermal limit are in excellent agreement with recent experimental results obtained near $0\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Zr}{\mathrm{W}}_{2}{\mathrm{O}}_{8}$ compression mechanism around W1 and W2 atoms is quite different. While the former can be described essentially in terms of a correlated polyhedral rotation, the latter involves correlated rotation of the first coordination polyhedra and translation of $\mathrm{W}{\mathrm{O}}_{4}$ units downward along the ⟨111⟩ axis. These modes of deformation should bear some resemblance to the low-energy modes responsible for the negative thermal expansion in zirconium tungstate, and thus could give some insight on the microscopic mechanism behind this phenomenon.