Modulated and disordered phases in the system LiCdVO4-LiCd4(VO4)3

A solid solution (1 - x)LiCdVO4 - xLi1/3Cd 1/3□1/3CdVO4 has been observed for 0 ≤ x ≤ 1. From the cell parameters evolution, two domains of the solid solution have been distinguished with a discontinuity at x ≈ 0.6. The first one for 0 ≤ x ≤ 0.6 corresponds to the LiCdVO4 substructure of the Na2CrO4-type. For 0.75 ≤ x ≤ 1 satellite peaks are observed which correspond to a modulated structure. The first end-member LiCdVO4 (x = 0) crystallizes in Cmcm space group with a = 5.911, b = 8.975, and c = 6.513 Å. The structure consists of infinite edge-sharing chains of [CdO6] octahedra linked together by [VO4] tetrahedra and [LiO4] tetrahedra. According to the general formula Li1-2x/3Cdx/3□x/3CdVO4 of the solid solution, when x increases cadmium replaces lithium in the [LiO 4] tetrahedra leading to [(Li/Cd/□)O4] disordered tetrahedra. All the phases in the domain 0.7 ≤ x ≤ 1 are one-dimensionally modulated with wavevector q = γc* and γ values of 0.75, 0.733, and 0.722 for x = 3/4, 6/7, and 9/10, respectively. The superspace group is Xmcm(00γ) where X stands for (1/2, 1/2, 0, 1/2) centering. The main structural result is a strong occupation modulation of lithium and cadmium in the tetrahedral site