The formation mechanism of ferroelastic Ba2TiGe2O8 crystals in 30BaO·15TiO2·55GeO2 glass and second order optical non-linearity of transparent crystallized glasses have been examined. It has been confirmed that the BaTiGe3O9 crystalline phase is initially precipitated and acts as a nucleation site for the formation of the Ba2TiGe2O8 crystalline phase. Transparent surface crystallized glasses consisting of grain-oriented Ba2TiGe2O8 crystals have been successfully fabricated by means of heat-treatments (690-750 °C, 3 h) for the first time, and second harmonic (SH) generation has been clearly observed, demonstrating that Ba2TiGe2O8 is a non-linear optical crystalline phase. The SH intensity of transparent crystallized glasses (bulk state) with crystalline layers of ∼10 μm thickness is comparable to that of Z-cut α-quartz single crystal with a thickness of 0.6 mm. The SH intensity of Ba2TiGe2O8 crystals (powdered state) prepared through solid-state reaction is about 130 times as large as that of pulverized α-quartz.
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