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1985) X-ray structure refinement and pyroelectric investigation of fresnoite, Ba2TiSi2O8, Ferroelectrics, 62:1, 17-26, Crystal structure parameters have been refined from single crystal x-ray intensity data collected on Ba,TiSi,O, at 24 and 300°C. Anisotropic refinements in space group P4hm yielded residuals of 0.035 and 0.042 at 24 and 300°C, respectively. F'yroelectric measurements give a room temperature pyroelectric coefficient of + 10 pC m-' K-'. A structural mechanism for the pyroelectric effect is discussed in terms of large oxygen displacements. A sign reversal in the pyroelectric coefficient occurs at 160°C. perhaps caused by the cancellation of primary and secondary effects.

Raman spectra are reported for fresnoite-type [Ba2Ti(Si, Ge)2O8] materials. Single crystal and glasses of the endmember compositions, Ba2TiSi2O8 (BTS) and Ba2TiGe2O8 (BTG), along with crystalline powders along the pseudobinary join, Ba2TiSi−xGe2−xO8, are discussed. The polarized single-crystal Raman spectra of BTS show the vibrational bands of the Si2O7 and TiO5 molecular groups. The v (Si–O–Si) mode is found at 666 cm−1, and is relatively weak. The strongest bands in the BTS single crystal spectra are found at 858 cm−1 and 873 cm−1. These are attributed to the v (SiO3) modes and the stretching of a short Ti–O bond, with mixing between them evident. The BTG single crystal polarized Raman spectra show the vs(Ge–O–Ge) mode at 521 cm−1, and a strong band at 841 cm−1 assigned to the stretch of the short Ti–O bond. The angular dependence of the TO-LO splitting is cataloged for the A1(z) stretching modes. The vibrational characteristics of both BTS and BTG are dominated by the molecular groups, and do not show behavior indicative of a sheet structure composed of TiO5 and (Si, Ge)2O7 groups. Raman investigation of the crystalline powders points to the presence of Si2O7, Ge2O7, and SiGeO7 groups within the nonendmember compositions. The BTG glass spectra show correspondence with the BTG crystalline Raman spectra; the vs(Ge–O–Ge) mode occurs at 518 cm−1 in the glass. Fivefold coordinated titanium may be present in the BTG glass, as revealed by a very strong band at 824 cm−1 in the I spectrum. BTS glass spectra also show some similarity with the BTS crystalline spectrum; the strongest band is found at 836 cm−1 in the I glass spectrum. Through comparison with other titania-silicate glasses, we conclude that the BTS glass has a structure similar to the crystalline phase.

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