Structural anomalies related to changes in the conduction mechanisms of α-Sm2(MoO4)3

Polycrystalline samples of α-phase samarium molybdate were prepared by solid-state synthesis and used for x-ray diffraction (from 300 to 1000 K) and dielectric spectroscopy (from 500 to 900 K and from 10(2) to 10(6) Hz). The electrical conductivity follows the universal dielectric response, and three different regimes of conduction (with semiconductor, polaronic, and ionic characteristics) were ascribed. The polaronic mechanism in the range from 600 to 810 K was probed using the overlapping large polaron model. Above 810 K, the application of scaling laws suggests an ionic conductivity. The thermal dependence of the lattice parameter a shows three different trends in correspondence with the three conduction regimes observed. An analysis using adapted symmetry modes facilitated the Rietveld refinement and the study of the thermal dependence of the distortion arising from the oxygen displacements. We suggest that the transversal displacements of oxygen atoms in Sm-O-Mo bridges joined to the elongation of tetrahedra can help to explain this anomalous behavior. From the calculated bond-valence contour maps, new sites for the oxygen atoms, at higher temperatures, were detected which favor oxygen motion and would then be related to the ionic conduction. This correlation has been compared and extended to α-Eu(2)(MoO(4))(3).