Raman study of fresnoite-type materials: Polarized single crystal, crystalline powders, and glasses

Abstract

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_−xGe_2−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 v_s (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 v_s(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.