Crystallization, texture and second-harmonic generation in TiO2–BaO–B2O3 glasses

Partially crystallized glasses containing non-centrosymmetric β-BaB2O4 (β-BBO) and/or BaTi(BO3)2 crystals were prepared by controlled surface crystallization of x1TiO2–x2BaO–x3B2O3 glasses, where x1 = 4%, 8%, 15% and x2/x3 = 8/9 (mol%). Differential thermal analyses were made to select appropriate heat treatments to partially crystallize some samples. The resulting crystalline phases were determined by X-ray diffraction and micro-Raman spectroscopy. Spontaneous preferred orientation (texture) occurred on the surface of all these glasses. Samples with x1 = 4% and 8% heat-treated from 1 to 32 h showed only β-BaB2O4 precipitated on the glass surfaces, while the glass with x1 = 15% only had BaTi(BO3)2 in samples heat-treated for less than 8 h. Longer treatments also led to crystallization of β-BaB2O4, occurring preferentially between the arms or underneath the BaTi(BO3)2 crystals, but present on the glass surface as well, and this became the predominant crystalline phase. Some surface crystallized glasses yielded a reasonable second-harmonic generation. The non-linear optical coefficient, deff, in the x1 = 15% crystallized sample was 1.1 pm/V, which corresponds to approximately 50% of that observed in the β-BBO single crystal.