Geikielite–ecandrewsite solid solutions: synthesis and crystal structures of the Mg_1−xZn_xTiO₃ (0 ≤ x ≤ 0.8) series

The crystal structures of members of the geikielite–ecandrewsite solid solution series, Mg_1 − xZn_xTiO₃ (0 ≤ x ≤ 0.8 a.p.f.u. Zn; a.p.f.u. = atoms per formula unit), synthesized by ceramic methods in air at ambient pressure, have been characterized by Rietveld analysis of X-ray powder diffraction patterns. These synthetic titanates adopt an ordered R structure similar to that of ilmenite. The maximum solubility of Zn in MgTiO₃ is considered to be ∼ 0.8 a.p.f.u. Zn, as compounds with greater Zn content could not be synthesized at ambient conditions. Data are given for the cell dimensions and atomic coordinates, together with bond lengths, volumes and distortion indices for all the coordination polyhedra. Within the solid-solution series unit-cell parameters and unit-cell volumes increase with Zn content. All compounds consist of distorted (Mg,Zn)O₆ and TiO₆ polyhedra and, in common with geikielite and ilmenite (sensu lato), TiO₆ polyhedra are distorted to a greater extent than (Mg,Zn)O₆. The displacements of (Mg,Zn) and Ti from the centers of their coordination polyhedra vary insignificantly with increasing Zn content. The interlayer distance across the vacant octahedral site in the TiO₆ layer decreases slightly with the entry of the larger Zn²⁺ cation into the ^viA site. The empirically obtained upper limit of the Goldschmidt tolerance factor (t) for A²⁺BO₃ compounds adopting an ordered R structure is 0.755. The absence of natural solid solutions between geikielite and ecandrewsite seems to be due to the contrasting geochemistry of Mg and Zn rather than for crystallochemical reasons.