La₃Ga_5.5Ta_0.5O₁₄ (La₃(Ga_0.5Ta_0.5)(1)Ga₃(2)Ga₂(3)O₁₄, LGT) belongs to the langasite family (sp. gr. P321, z = 1). These compounds are of considerable practical interest in the context of the search for "strong" piezoelectrics. A characteristic feature of this structure is that Ga resides at three positions: octahedral, Ga(1) (partially occupied by Ta atoms); tetrahedral, Ga(2); and trigonal-pyramidal, Ga(3). The La atoms sit at the center position of dodecahedra.

The aim of the work is to determine a composition of langatate with different color for the establishment of relationship between the composition of the samples and their color. 

Single crystals were grown by the Czochralski technique in an <0001>  direction and have a different color: as-grown crystal - I.1. (yellow); crystal, annealed in air at 1400°C - I.2. (orange); crystal, annealed in vacuum at 1000°C - I.3. (colorless) (Fig.a); crystal, annealed in vacuum at 1200°: part II.1. (pale-green) and part II.2. (green) (Fig.b). The color of crystals was determined by transmission spectra using a "Specord M-40" with "Color measurement" option.

The samples were ground into fine powders and examined by XRD on HZG-4A diffractometer (room temperature, СuKα, graphite monochromator, t=15 s and a step of 0.02°, 2Θ range 10°-115°). Refinement of crystal structures was accomplished using the Rietveld suite of programs DBWS-9411.

The compositions of the samples:

An appearance of the crystal color for I crystals is due to the ratio of the V_O^·· (oxygen vacancies):(1/2V_O^··,e¢)^x (associates-centers color): by the V_O^··<(1/2V_O^··,e¢)^x the crystals are colored (yellow – I.1. or orange – I.2), by the V_O^··>(1/2V_O^··,e¢)^x  or without vacancies crystals are colorless. At the same time, in contrast to their results we revealed another color of II crystal. Probably, green color is associated with the precipitation of phase with Ta³⁺ ions. This phase is oriented to the host matrix LGT. The oxygen content of part II.1 is higher than that of  I crystals and  part II.2 and color of  part II.1. near to colorless than to colored.

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