THE DISORDERED SPINEL NAAGMOO4 AND ITS HIGH-TEMPERATURE, ORDERED ORTHORHOMBIC POLYMORPH

Four polymorphs of NaAgMoO4 have been discovered by differential thermal analysis. The well-known spinel phase (NaAgMoO4-I) is stable between room temperature and 455°C. It is cubic, a=9.220 angstrom, with a disordered distribution of the Na and Ag cations. One of the high temperature polymorphs (NaAgMoO4-II) is quenchable and has been investigated by Raman spectroscopy and X-ray difractometry. It is orthorhombic, space group Pnma, with a=10.384(1), b=7.1220(8), and c=5.5933(6) angstrom; Z=4. Its structure has been deduced from the diffraction peak intensities: it is isotypic with Ag2CrO4, with an essentially ordered distribution of the cations; it is thus a rare example of cation ordering with increasing temperature. Some of the unexpected aspects of the I-II transformation are briefly discussed.