Structural investigations along the join CaTiOSiO₄-CaSnOSiO₄

High resolution synchrotron X-ray data were used to structurally characterize a series of compounds along the solid-solution titanite (HT-phase)-malayaite. The refinements reveal a high sensitivity of the crystal symmetry on the amount of octahedral Sn present. This suggests a weak interaction between dipoles formed through Ti out-of-centre distortion. The structural evolution along the solid-solution also allows the identification of structural strain, which is caused by the geometrically competing arrangement of corner sharing octahedral chains occupied by Ti and Sn on one hand and edge sharing polyhedral chains occupied by Ca on the other hand. This strain is expressed in a systematic variation of structural parameters, such as Ti-O-Ti angles, Ca displacement parameters and Ca-O bond length distortion, as a function of octahedral occupation.