Determination of structural disorder in superionic by neutron total scattering

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Abstract
The structural disorder in the two high-temperature, superionic phases of has been fully characterized using neutron powder diffraction and two complementary analyses. Rietveld profile refinement of Bragg scattering has been used to determine the most probable average structures and radial distribution functions, determined by Fourier transformation of the total scattering structure factors, have been used to refine local structural models based on these average structures. It is found that the -phase, which exists between 420 K and 1075 K is best described by a face-centred cubic arrangement of the tellurium ions with the silver ions randomly occupying the tetrahedral and octahedral voids. There is no local ordering of the filled tetrahedral sites and the silver ions in octahedral sites have a broad distribution and preferentially sit off centre in one of the eight directions away from filled tetrahedra. The -phase, which exists between 1075 K and the melt, has a body-centred cubic structure similar to , and the average structure is best described with silver ions distributed over tetrahedral and trigonal sites and positions which join these two sites. The radial distribution functions in both superionic phases clearly show that silver ions are never closer than Å to each other which means that nearest neighbour 12d tetrahedral sites cannot be simultaneously filled in . The differences in quality of fit to the experimental data between different silver ion ordering schemes are slight and it is proposed that single crystal diffuse scattering may be the only way to distinguish clearly between different models.