Formation mechanism of ferroelastic Ba₂TiGe₂O₈ and second order optical non-linearity in transparent crystallized glasses

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.