Rietveld Analysis of Incommensurate Composite Crystals with Nominal Compositions Sr_1.10TiS₃ and Sr_1.19TiS₃

抄録

The existence of columnar composite crystals Sr_xTiS₃ (1.05<x<1.22) was revealed. As the composite crystal has, in general, an incommensurate modulated structure, the superspace-group theory should be applied to describe its structure. Before, the Rietveld analysis through the superspace-group approach of Sr_1.145TiS₃ was successfully performed. In this report, the structures of the specimens near the Sr-rich and Ti-rich ends of the existence range have been analyzed on the basis of the four-dimensional formalism using tne Rietveld analysis program PREMOS (A. Yamamoto). Each crystal is composed of columns of face-shared polyhedra (TiS_6/2)_∞ and rows of Sr, and both are parallel to the c axis. The site ratios of Sr/Ti, determined from the results of Rietveld analysis, are 1.129 for Sr_1.10TiS₃ and that 1.162 for Sr_1.19TiS₃. The origin of the discrepancy between the site ratio and the nominal composition is discussed.