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

The dielectric, piezoelectric and pyroelectric properties of fresnoite (Ba2TiSi2Os) single crystal and polar glass-ceramics were studied in the temperature range - 150 to 200°C. The sign of pyroelectric coefficient is positive at room temperature and becomes negative at 190°C. The dielectric constant, pyroelectric coefficient and planar coupling coefficient show a maximum value at 160°C and the frequency constant shows a minimum at the same temperature. The probable reasons for the anomaly in these properties are discussed.

Unusual pyroelectric and piezoelectric properties of fresnoite (Ba2TiSi2O8) single crystal and polar glass-ceramics

The BaO–TiO 2 –SiO 2 crystallized glass containing Ba 2 TiSi 2 O 8 crystal phase was examined using X-ray diffraction (XRD) measurement and Maker fringe technique. It has been found that the crystallized glasses with a composition of close to the stoichiometric Ba 2 TiSi 2 O 8 showed bulk crystallization, whereas other crystallized glasses with non-stoichiometric composition of Ba 2 TiSi 2 O 8 showed surface crystallization. The parameter Δ T , the difference in temperature between the crystallization onset and the glass transition temperature, strongly affects the crystallization behaviors of the present BTS glasses. The 30BaO–20TiO 2 –50SiO 2 transparent surface crystallized glass with strong c -oriented Ba 2 TiSi 2 O 8 phase showed its second-order non-linear optical constant, d 33  = 6.1 pm/V.

Structure and non-linear optical properties of BaO–TiO2–SiO2 glass containing Ba2TiSi2O8 crystal

Fresnoite glass-ceramics – A review

Fresnoite, Ba2TiOSi2O7, has been predicted structurally to be a new ferroelectric. Detection of both ac and dc dielectric hysteresis verifies the prediction. The spontaneous polarization Ps estimated from the hysteresis at 1.2 MV m−1 ac is ∼0.2 C m−2 at 295 K, comparable with the minimum Ps observed in one-dimensional ferroelectrics. A reproducible calorimetric anomaly with entropy change 0.19(3) J mol−1 K−1 at 433(2) K in polycrystalline material coincides with a small dielectric and pyroelectric anomaly previously reported in single crystals; an entropy change ∼0.2 J mol−1 K−1 at 810(5) K also accompanies a dielectric anomaly observable in ceramic samples at 805(5) K. Both calorimetric anomalies are ∼60 K wide. Observation of dielectric hysteresis at 875 K shows that neither anomaly corresponds to the Curie temperature; both are likely associated with small changes in atomic position, not with symmetry changes. Melting onset in fresnoite is 1703(1) K with undercooling as deep as 435 K.

Fresnoite: A new ferroelectric mineral

All the elastic constants, temperature coefficients of elastic constants, and thermal‐expansion coefficients in Ba2Si2TiO8 were measured. Signs of the piezoelectric constants d31 and d15 were determined to be positive. From the calculations of the orientational dependences on the electromechanical coupling factors k′31 and k′15 and the temperature coefficient of resonance frequency [Tfr]L in the length‐extensional mode, it was found that the largest values of k′31 and k′15 are 0.13 and 0.28, respectively, and the minimum value of [Tfr]L is −47 ppm/°C. Surface‐acoustic‐wave analysis for the crystal showed that the SH‐mode piezoelectric surface wave has a piezoelectric coupling constant, k2=0.0053, larger than that of quartz, and a temperature coefficient of delay Tτ=+38 ppm/°C for the Y‐propagation direction on the X‐cut surface, smaller than that of LiNbO3.

Masakazu Kimura

Papers

Elastic and piezoelectric properties of Ba2Si2TiO8