Second-order optical nonlinearities, d33 values, of fresnoite-type crystals, Ba2TiSi2O8 (BTS), Sr2TiSi2O8 (STS), and Ba2TiGe2O8 (BTG), have been evaluated by measurements on transparent surface-crystallized glasses consisting of fresnoite-type crystals with the Maker fringe technique. It is found that all these crystals show large d33 values, i.e., d33=13±2 pm/V for BTS, d33=7.2±0.4 pm/V for STS, and d33=22±3 pm/V for BTG. In particular, it is emphasized that BTG has an extremely large d33 value, comparable to LiNbO3 single crystal. It is clarified that the d33 value of fresnoite-type crystals increases with increasing c/a ratio in the lattice parameters of their structures. It is suggested that the increase in the c/a ratio results in the elongation of TiO5 pyramidal units along the c axis, consequently enhancing spontaneous polarizations in the fresnoite-type structure. Transparent surface-crystallized glasses consisting of fresnoite-type crystals are candidates for nonlinear optical materials.

Large second-order optical nonlinearities of fresnoite-type crystals in transparent surface-crystallized glasses

We determined the internal nucleation, crystal growth and overall crystallization kinetics of fresnoite crystal (2BaO · TiO2 · 2SiO2) in an almost stoichiometric fresnoite glass. Due to the extremely high nucleation rates (∼1017 m−3 s−1) that limit the maximum crystal size to ∼700 nm the nucleation densities and crystal sizes were estimated by scanning electron microscopy (SEM). The volume fraction crystallized was measured by X-ray diffraction. The nucleation rates obtained directly from SEM measurements reasonably agree with those calculated from the combination of overall crystallization with crystal growth kinetics. The activation enthalpies for viscous flow, transport of structural units across the nucleus/melt interface (nucleation) and crystal growth: ΔHη, ΔHτ and ΔHU respectively, follow a similar trend to that observed for other stoichiometric silicate glasses that nucleate internally: ΔHη=294>ΔHτ=87>ΔHU=61 kJ/mol. Fresnoite glass displays the highest internal nucleation rates so far measured in inorganic glasses. These rates are comparable to some metallic glasses and can lead to nanostructured glass-ceramics.

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Nanocrystallization of fresnoite glass. I. Nucleation and growth kinetics

Crystal structure parameters have been refined from single crystal x-ray intensity data collected on Ba2TiSi2O8 at 24 and 300°C. Anisotropic refinements in space group P4bm yielded residuals of 0.035 and 0.042 at 24 and 300°C, respectively. Pyroelectric measurements give a room temperature pyroelcctric coefficient of + 10 μC m -2 K -1. A structural mechanism for the pyroelectric effect is discussed in terms of large oxygen displacements. A sign reversal in the pyroeleetric coefficient occurs at 160°C, perhaps caused by the cancellation of primary and secondary effects.

X-ray structure refinement and pyroelectric investigation of fresnoite, Ba2TiSi2O8

Grain-oriented glass-ceramics of Li2Si2o5, fresnoite (Ba2TiSi2O8), and its isomorphs Sr2TiSi2O8, and Ba2TiGe2O8, were prepared by recrystallizing glasses in a temperature gradient. Electromechanical and hydrostatic piezoelectric properties of these glass-ceramics were measured. Piezoelectric voltage coefficients g33 and hydrostatic voltage coefficient gh of these glass-ceramics are comparable to those of polyvinylidene fluoride and an order of magnitude higher than the corresponding values of lead zirconate-titanate. These glass-ceramics seem to be attractive candidate materials for hydrophones and several piezoelectric devices. Hydrostatic piezoelectric properties of Ba2TiSi2O8 and Ba2TiGe2O8 single crystals were also measured. The unusually high values of gh in fresnoite single crystals and glass-ceramics are supposed to be due to positive d31 in these materials. A composite model is proposed to explain the positive sign of d31 in fresnoite based on its crystal structure and internal Poisson's ratio stress.

Grain-Oriented Glass-Ceramics for Piezoelectric Devices

The optical properties of Ba2TiSi2O8 nanocrystals in transparent crystallized glasses were investigated. The transparent nanocrystallized glass with Ba2TiSi2O8 crystals having a particle size of ∼200nm was fabricated by crystallization (at 760 °C for 1 h) of a glass with the stoichiometric composition of Ba2TiSi2O8, i.e., 40BaO∙20TiO2∙40SiO2 glass. Both visible second-harmonic generation (green light, 532 nm) and luminescence (blue light, ∼470nm) could be observed in the transparent Ba2TiSi2O8 nanocrystallized glass, demonstrating the optical multifunctional nanocrystallized material. It was suggested that the blue luminescence from the transparent Ba2TiSi2O8 nanocrystallized glass originated from the oxygen-related defects of the SiO4 unit in the Ba2TiSi2O8 nanocrystals.

Second-order optical nonlinear and luminescent properties of Ba2TiSi2O8 nanocrystallized glass

Growth and physical properties of fresnoite Ba2TiSi2O8

Second harmonic generation in fresnoite, Ba2TiSi2O8

Single crystal growth and optical, elastic, and piezoelectric properties of polar magnesium barium fluoride

Züchtung und physikalische eigenschaften von monoklinem zinndifluorid

Abstract All elastic and thermoelastic constants and the coefficients of thermal expansion of monoclinic SnF2 are measured. These properties deviate strongly from the behaviour of other difluorides such as CaF2 and MgF2. The high anisotropy, the large bulk compressibility and the departures from the Cauchy relations suggest the existence of strong covalent binding components between tin and fluorine atoms. The thermal properties are in accordance with Grünseisen's rule.

S. Haussühl

Papers

Growth and physical properties of fresnoite Ba2TiSi2O8

Second harmonic generation in fresnoite, Ba2TiSi2O8

Single crystal growth and optical, elastic, and piezoelectric properties of polar magnesium barium fluoride

Züchtung und physikalische eigenschaften von monoklinem zinndifluorid

Elastische und thermoelastische Konstanten des monoklinen Zinndifluorids