Abstract
The electron-density distribution of single-crystal has been measured as a function of pressure using synchrotron-radiation techniques in order to understand the variation in its static dielectric properties. adopts three different polymorphs at varying pressures and ambient temperature: the ambient pressure phase adopts an orthorhombic structure that transforms to a tetragonal phase at about 7.0 GPa, which then transforms further to a cubic phase at about 10.0 GPa. The cubic phase is paraelectric, while the two lower-pressure phases are ferroelectric. Difference Fourier and maximum entropy method maps clearly show hybridization, which is composed of and states. The ferroelectric-to-paraelectric transition in at high pressure is discussed with reference to the variation in the electron-density distribution with pressure. Covalent bonding is reduced in the tetragonal phase as valence electrons become more localized with increasing pressure. The effective charge calculated from the valence electron density indicates that the tetragonal phase has the largest dipole moment among the three polymorphs. Orientation of the polarization in the tetragonal phase is possible in the [001] direction as a result of strain, but the orthorhombic phase shows a considerably strong polarization in both the [010] and [001] directions. In the cubic phase, a statistical distribution of Nb atoms around the inversion center in the [001] and [110] directions, rather than the [111] direction, results in paraelectric character.
- Received 25 October 2008
DOI:https://doi.org/10.1103/PhysRevB.80.094108
©2009 American Physical Society