Incommensurate crystal structure of LiCd4(VO4)3

Abstract

Abstract Single crystals of LiCd4(VO4)3 were prepared by melting a powder sample of LiCd4(VO4)3 at 950 °C followed by slow cooling at 5 °C/hours down to room temperature. The crystal structure was determined by single crystal X-ray diffraction using the 4D superspace formalism. The superspace group is Cmcm(10γ) with unit cell parameters a = 5.8513(3) Å, b = 9.027(5) Å, c = 6.7721(7) Å and modulation vector q = a* + 0.7034c*. The LiCd4(VO4)3 structure is related to the Na2CrO4 structure type, which contains two independent tetrahedral sites that are occupied by sodium and chromium atoms and one octahedral site that is occupied by the second sodium atom. In the LiCd4(VO4)3 structure, the octahedral site is fully occupied by a cadmium atom, and one of the two tetrahedral sites is fully occupied by a vanadium atom. The second tetrahedral site is only partially filled: one third lithium, one third cadmium and one third vacant. To reflect the site occupancies, the overall formula can be expressed as Li1/3Cd1/3[ ]1/3CdVO4. One of the unusual features of the structure is the large anisotropy in the modulated occupancy of the tetrahedral site by lithium and cadmium. Structural relationships with the olivine structure type are discussed.