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(Ge0.25Si0.75)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) A and 0.039 (440 reflexions at room temperature) for the former crystal, and P4bm, a=8.770 (1), c=5.433 (1) A 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 1.65A and bridging angles O–(Ge, Si)–O are 131° and 137.4° in the paraelastic Ba2TiGe2O8 and Ba2Ti(Ge0.25Si0.75)2O8, respectively.
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