Phase formation in the Ag2O-MgO-MoO3 system and the crystal structure of new double molybdate Ag2Mg2(MoO4)3

Phase relations in the subsolidus region of the Ag2O-MgO-MoO3 system were studied, the Ag2MoO4-MgMoO4 section was examined, and its T-x diagram was constructed. New double molybdate Ag2Mg2(MoO4)(3) was found to be formed. This compound belongs to the Na2Mg5(MoO4)(6) structural family. Its structure was established by X-ray diffraction analysis: a = 6.978(1) Angstrom, b = 8.715(2) Angstrom, c = 10.294(2) Angstrom, alpha = 107.56(3)degrees, beta = 105.11(3)degrees, gamma = 103.68(3)degrees, Z = 2, space group P(1) over bar, R = 0.038. The crystal structure is based on a mixed framework consisting of paired MgO6 octahedra and MoO4 tetrahedra. The cavities of the framework are occupied by Ag atoms. The character of disordering of a part of Ag+ ions is analogous to that observed earlier in the crystal structure of Ag2Zn2(MoO4)(3). Possible boundaries of the homogeneity regions for silver magnesium molybdate and its analogs are discussed, and the structural features of these compounds are considered in comparison to isotypical sodium-containing phases.