Characterization of phase structures in semiconducting SnWO4 powders by Mossbauer and Raman spectroscopies

Stannous tungstate, SnWO4, crystallizes in the low-temperature alpha and high-temperature beta phases. alpha-SnWO4, powders were prepared by heating an equimolar mixture of SnO and WO3 either in vacuum or in argon atmosphere at 600 degrees C for 15 h. The high-temperature beta phase was obtained as metastable at room temperature after heating the mixture at 800 degrees C and rapid quenching. In addition to x-ray-diffraction studies, Sn-119 Mossbauer and Raman spectroscopies were used as "local" probes for the characterization of the phase structures in the semiconducting and gas-sensitive powders. The composition of the powders was measured by the energy-dispersive spectroscopy of x rays. Mossbauer spectroscopy, especially, as a very local probe for tin atoms gave valuable information of small extra phase(s) in the alpha- and beta-SnWO4 powders originating in the oxidation of Sn2+ ions in the SnWO4 structures into the Sn4+ form. The Sn2+ Mossbauer doublet from both ct and beta phases showed some asymmetry not published before. The asymmetry was possible to relate to the Goldanskii-Karyagin effect with calculations based on published results for the atomic positions and thermal displacement parameters in the x-ray temperature factor of the alpha- and beta-SnWO4, structures. Raman spectra are given together with peak frequencies from a curve fit for both alpha- and beta-phase powders. The symmetries and selection rules of the normal modes at the center of the Brillouin zone are also given for both alpha and beta phases.