Compositionally induced phase transitions and symmetry relations between space groups in the layered system...

Symmetry reduction in the basic structure of thortveitite-type compound FeInGe₂O₇-mC22 (C12/m1, No. 12) promoted by the incorporation of yttrium in the formula FeIn_1-xY_xGe₂O₇ gives rise to a derivative structure called thortveitite-like AA'Ge₂O₇-mP44, with symmetry described by the space group P12₁/m1 (No. 11) for x = 0.50, 0.75 and 0.90. The structure remains unchanged within the structural type of thortveitite when x = 0.25. In order to establish structural models for performing a Rietveld refinement to the derivative structure, symmetry relations between space groups connecting the basic and derivative structures were used. The higher contrast to X-rays of Fe³⁺, In³⁺ and Y³⁺ as well as by the behaviour during the refinements of the isotropic thermal displacements, the values of interatomic distances and calculated bond-valence sums for each atom in the asymmetric unit, were helpful for elucidating the relocation of cations in the different available crystallographic sites generated by the symmetry reduction.