Synthesis and crystal structures of compounds in the system Ba₂TiGe₂O₈–Ba₂TiSi₂O₈

Abstract

Complete solid solubility was confirmed in the pseudobinary system Ba₂TiGe₂O₈–Ba₂TiSi₂O₈ and phase boundary between orthorhombic and tetragonal phases was determined by DTA and polarizing microscopy. At room temperature, the phase boundary is passing through about 45 at% Si for the tetrahedral cations. The crystal structures of Ba₂Ti(Ge_0.25Si_0.75)₂O₈ and the paraelastic Ba₂TiGe₂O₈ were determined from single crystal X-ray diffraction data. The space groups, cell dimensions and final R values are: P4bm, a=8.583 (1), c=5.268 (1) Å and 0.039 (440 reflexions at room temperature) for the former crystal, and P4bm, a=8.770 (1), c=5.433 (1) Å and 0.076 (182 reflexions at 890°C) for the latter. The structures of both crystals consist of (Ge, Si)₂O₇ groups and TiO₅ square pyramids to form flat sheets perpendicular to the c-axis. Paraelastic Ba₂TiGe₂O₈ has highly distorted GeO₄ tetrahedra. Average (Ge, Si)–O distances are 1.73 and 1.65Å and bridging angles O–(Ge, Si)–O are 131° and 137.4° in the paraelastic Ba₂TiGe₂O₈ and Ba₂Ti(Ge_0.25Si_0.75)₂O₈, respectively.