X-ray diffraction by a crystal in a permanent external electric field: electric-field-induced structural response in α-GaPO₄

For the first time, site-selective distortion has been investigated for two different structural units in the ternary compound α-GaPO₄ under the influence of a permanent external electric field. Based on 54 measured reflection intensities, the electric-field-induced distortion of PO₄ and GaO₄ tetrahedra in α-GaPO₄ crystals is evaluated using a model of pseudoatomic displacements introduced recently [Gorfman, Tsirelson & Pietsch (2005). Acta Cryst. A61, 387–396]. A stronger variation of the P—O bond lengths in the PO₄ tetrahedron was found compared to the bonds in the GaO₄ tetrahedron. The different distortions of the tetrahedra owing to the electric field were analysed in terms of the valence charge density of α-GaPO₄ and its topological characteristics. The larger charge of the P pseudoatom compared to the Ga atom was recognized as the main reason for the higher sensitivity of the PO₄ tetrahedron to a permanent external electric field.