X-ray diffraction and vibrational Raman spectra of the Li2−xNaxCo2(MoO4)3 (0 ⩽ x ⩽ 1.4) solid solution with a lyonsite structure

Synthesis, X-ray diffraction and Raman spectroscopy studies of Li2–xNaxCo2(MoO4)3 (0 ⩽ x ⩽ 2) compositions have been investigated. These alkali–cobalt molybdates crystallize in the orthorhombic system (space group Pnma (D2h)) with a lyonsite structure (Z = 4). The cell parameters are determined by DICVOL04 software and confirmed by Rietveld refinements. The structure is formed by a framework of MoO4 tetrahedra and (Li/Na/Co)O6 octahedra, the tetrahedra provide links between sheets and columns formed by the octahedra. The structure shows that while octahedra share corners and edges in the sheets, they share faces columns. Alkali and cobalt are distributed statically in both the octahedral and trigonal prism sites. X-ray diffraction shows the presence of a solid solution in the range of compositions (0 ⩽ x < 1.4). These results are in agreement with Raman studies where the spectra of all the compositions are similar and show linear shifts of the frequencies as a function of the composition toward low values due to the substitutions of Li+ by Na+ with a larger radius.