The structure and optical properties of a crystal of the langasite family (La1–xNdx)3Ta0.5–yGa5.5+yO14 were studied. An accurate X-ray diffraction study of this crystal was performed in the temperature range of 85–450 K at a resolution of 1.36 Å–1. The distribution of cations over mixed crystallographic positions was determined by the anomalous scattering method, the formula was set as (La0.863(9)Nd0.137(9))3Ta0.483(4)Ga5.517(4)O14. The transmission and absorption spectra of the crystal were measured. The dispersion dependence of the rotation of the plane of polarization of light ρz during light propagation in the optical axis direction at T = 295 K is calculated from the experimental transmission spectra. The changes in the Δρz and Δn values of ρz and the refractive index no in the region of absorption bands in the wavelength range of 350–1000 nm were calculated using the Kramers–Kronig relations; the maximum values are found to be Δρz ~ 3 deg/mm, Δn ~ 0.6 × 10–4. The temperature dependences of the refractive indices and rotation of the plane of polarization of light in the directions parallel (ρz) and perpendicular (ρx) to the optical axis are calculated from the structural data. It is shown that the temperature evolution of the optical activity of crystals correlates with the experimentally detected anomalous expansion of the Ta, GaO6 octahedron, located at the origin of the unit cell. This expansion also manifests itself in the abnormal thermal expansion along the cell c axis, observed below 120 K.
Дудка et al. (Sun,) studied this question.