Raman study of fresnoite-type materials: Polarized single crystal, crystalline powders, and glasses
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A. S. Bhalla Materials Research Laboratory, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received 1 April 1991; accepted 26 October 1992)
Raman spectra are reported for fresnoite-type [Ba2Ti(Si, Ge) 2 O 8 ] materials. Single crystal and glasses of the endmember compositions, Ba 2 TiSi 2 08 (BTS) and Ba 2 TiGe 2 0 8 (BTG), along with crystalline powders along the pseudobinary join, Ba^iSi^Ge^jOs, are discussed. The polarized single-crystal Raman spectra of BTS show the vibrational bands of the Si2O7 and TiO5 molecular groups. The ^ ( S i - O - S i ) 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(SiO 3 ) modes and the stretching of a short T i - 0 bond, with mixing between them evident. The BTG single crystal polarized Raman spectra show the u s ( G e - O - G e ) mode at 521 cm" 1 , and a strong band at 841 cm" 1 assigned to the stretch of the short T i - 0 bond. The angular dependence of the TO-LO splitting is cataloged for the A\{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) 2 O 7 groups. Raman investigation of the crystalline powders points to the presence of Si 2 O 7 , Ge 2 O 7 , and SiGeO7 groups within the nonendmember compositions. The BTG glass spectra show correspondence with the BTG crystalline Raman spectra; the v f ( G e - O - G e ) 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 /// spectrum. BTS glass spectra also show some similarity with the BTS crystalline spectrum; the strongest band is found at 836 c m ' 1 in the /// glass spectrum. Through comparison with other titania-silicate glasses, we conclude that the BTS glass has a structure similar to the crystalline phase.
I. INTRODUCTION Interest in fresnoite-type materials has stemmed from their acentric properties, unusual structure, and phase transitions. Fresnoite-type materials have the formula Ba 2 TiX 2 O g , where X = V, Ge (hereafter denoted BTG), or Si (BTS; the synthetic analog of the mineral fresnoite). Sr 2 TiSi 2 0 8 (Ref. 1) and K 2 V 3 O 8 (Ref. 2) have also been reported to occur with the fresnoite structure. In this study, we will focus on the endmembers BTG and BTS, both single crystals and glasses, and briefly report on members of the pseudobinary Ba2TiSi;cGe2-*O8, where 0 =5 x =s 2. Both endmembers are polar, and show medium strength pyroelectric and piezoelectric properties.3 BTS and BTG are closely related structurally, with BTS having the space group P4bm45 and BTG occurring in space group Cmm2,6 at room
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'Current address: Hitachi Metals, Ltd., Magnetic and Electronic Materials Research Laboratory, 5200 Mikajiri, Kumagaya, Saitama 360, Japan. J. Mater. Res., Vol. 8, No. 3, Mar 1993 http://journals.cambridge.org
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