Structure and optical properties of langasite family crystals (La_1–xNd_x)₃Ga₅SiO₁₄ (x = 0, 0.4, 0.6, 1)

The structure and optical properties of crystals from the langasite family (La_{1 - x}Nd_х)₃Ga₅SiO₁₄ with different Nd content were investigated. The rotation of the light polarization plane, ρ, was calculated for these crystals from measured transmission spectra in polarized light. It is shown that for small values of ρ (~3-5 degrees/mm), it is necessary to use transmission spectra not with parallel and crossed polarizers, as is usually done, but at different angles between them, for example ±45°, to obtain better results. Circular dichroism measurements of these crystals were performed. Using Kramers-Kronig relations, the connection between the circular dichroism bands and the rotation of the light polarization plane in the absorption band region was determined. Dispersion curves of ρ values were calculated, taking into account absorption in the wavelength range of 400–1000 nm for crystals (La_0.6Nd_0.4)₃Ga₅SiO₁₄, (La_0.4Nd_0.6)3Ga₅SiO₁₄, Nd₃Ga₅SiO₁₄, and compared with the dispersion of ρ for langasite crystal La₃Ga₅SiO₁₄. Average refractive indices and optical activity parameters of these crystals were calculated from structural data. It is shown that the dependence of the average refractive indices and ρ values on the parameters of the elementary cell, calculated under the assumption of no absorption, is linear. However, such a linear dependence is not observed for experimental ρ values, which is associated with the influence of absorption and the peculiarities of the structure (nonlinear change in the geometry of optically active regions of electron density upon replacing part of La with Nd).