Langasite (LGS, sp. gr. P321, z=1) is the most attractive materials for acoustic-volume (surface)-waves devices. Observable colouring of crystals can influence spectral-luminescent properties, is especial if it is imposed on area of generation or excitation, and also on factor transmission crystals LGS.

The aim of this work is to find a relationship between composition, color and hardness of LGS. One structural peculiarity of La₃Ga₅SiO₁₄ - La₃Ga(1)Ga₃(2)(GaSi)(3)O₁₄ is three positions for atoms of gallium: Ga(1) – octahedral, Ga(2) – tetrahedral, (GaSi)(3) - trigonal-pyramidal. Lanthanum atoms are in the centre of dodecahedron [1].

Powder samples prepared by grinding single crystals were grown by the Czochralski technique from La₃Ga₅SiO₁₄ (crystal I – colorless, atmosphere Ar and growth direction <01-11>; crystals: II – yellow, III – light orange, IV – orange, atmosphere Ar + O₂ and growth direction <0001>). The intensity data for Rietveld analysis were collected on a HZG-4 diffractometer (Ni – filter, CuK_α, t = 15 s, s = 0.02°, 2q 10-115°). In all calculations, we used DBWS-9411. After refining the scaling factor, zero counter position, lattice parameters, and sample displacement, the structural and profile parameters were refined by gradually introducing profile parameters at a refined background. The structural parameters were refined in several steps: first, only atomic position coordinates; then, isotropic thermal parameters at fixed positional parameters; and, finally, site occupancies at fixed positional and thermal parameters.

ii. It may be stated that the experimental data prove the role of oxygen vacancies in the color of crystals. The oxygen content of yellow crystal is higher than that of colorless one.

iii. The presence of point defects lead to changes not only in lattice parameters and color but also in the hardness of these samples.

iv. The growth conditions influence the properties of LGS.

1. Mill B.V., Butashin A.V., Khodzhabagyan G.G., Belokoneva E.L., Belov N.V. “Modified rare-earth gallates with a Ca₃Ga₂Ge₄O₁₄ structure” // Dokl. Akad. Nauk SSSR, (in Russian) 1982, V. 264 (6), P. 1385-1389

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