Refinement of the crystal structures of the La 3 Ta 0.5 Ga 5.5 O 14 and La 3 Nb 0.5 Ga 5.5 O 14 compounds
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CTURE OF INORGANIC COMPOUNDS
Refinement of the Crystal Structures of the La3Ta0.5Ga5.5O14 and La3Nb0.5Ga5.5O14 Compounds A. P. Dudkaa*, B. V. Millb, and Yu. V. Pisarevskya a
Shubnikov Institute of Crystallography, Russian Academy of Sciences, Leninskiі pr. 59, Moscow, 119333 Russia * email: [email protected] b Moscow State University, Moscow, 119992 Russia Received November 11, 2008
Abstract—This paper reports on the results of accurate Xray structural investigations of single crystals La3Ta0.5Ga5.5O14 (a = 8.2260(1) Å, c = 5.1207(1) Å, R/Rw= 1.09%/1.10%, 3868 unique reflections) and La3Nb0.5Ga5.5O14 (a = 8.2237(1) Å, c = 5.1247(1) Å, R/Rw= 1.02%/1.03%, 3735 unique reflections) (space group P321, Z = 1, sinθ/λ|max ≈ 1.34 Å–1 for both compounds). Using sets of data with a large number of highangle reflections makes it possible to reliably compare the crystal structures by applying statistical tests. The structural differences between the La3Ta0.5Ga5.5O14 and La3Nb0.5Ga5.5O14 crystals are weakly pro nounced, which correlates with the closeness of their piezoelectric characteristics. PACS numbers: 61.66.Fn, 42.70.Mp, 66.30.Dn, 77.80.Bh DOI: 10.1134/S106377450904004X
INTRODUCTION Crystals of La3Ta0.5Ga5.5O14 (LTG) and La3Nb0.5Ga5.5O14 (LNG), as well as crystals of La3Ga5SiO14 (LGS, langasite), (Ca3Ga2Ge4O14 struc tural type, space group P321, Z = 1) are the best known and most wellstudied representatives of the large class of piezoelectric materials of the langasite family [1]. Compared to quartz, they exhibit sever alfold higher piezoelectric moduli, the close ther mal stability, and lower acoustic losses. A large number of bulk and surface acoustic wave devices, most of which are based on langasite, have been developed to date. Large crystals of the LGS, LNG, and LTG compounds have been grown from a melt by the Czochralski method. The absence of phase transitions up to the melting temperature in these compounds and the relatively slow increase in the acoustic losses with an increase in temperature make them very attractive materials for use in the design of sensors operating at elevated temperatures [2]. Since LTG and LNG crystals have higher piezo electric parameters and a wider operating range of temperatures as compared to langasite crystals, investigating them in detail is an especially impor tant problem. Considerable interest in materials of the langasite family and the variation in their characteristics over a wide range have stimulated numerous investigations of their crystal structure. More than 40 structural works for 25 compounds of the langasite family are currently available in the literature [3]. The structures of some compounds were refined several times (in particular,
six times for LGS crystals). The majority of the struc tural works are characterized by medium and low quality refinement: R ≈ 3–10%. The structure of LTG crystals was refined by Takeda et al. [4] (R = 4%) and Yunin et al. [5] (R = 4%). The structure of LNG crystals was refined in three structural works [6] (R = 5.3%)
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