The Temperature Evolution of the Atomic Structure and the Influence of the Local Environment of Atoms on the Optical Properties of the Na2SiF6 Crystal

Millimeter-sized crystals of sodium hexafluorosilicate Na₂SiF₆ have been grown by the hydrothermal method. X-ray diffraction analysis revealed that Na₂SiF₆ samples are twinned according to the merohedral law and crystallize in the sp. gr. Р321 with the following unit-cell parameters at 295 K: 〈a〉 = 8.8582(12) Å, 〈c〉 = 5.0396(11) Å, 〈V〉 = 342.47(17) ų (averaged results of repeated measurements). A multitemperature diffraction study of Na₂SiF₆ was performed; the results obtained were used to calculate the temperature dynamics of the optical properties of crystals. Structural similarity was revealed between Na₂SiF₆ crystals and crystals of the langasite family La₃Ga₅SiO₁₄. This made it possible to explain the optical activity of Na₂SiF₆ by considering the electron density helices similar to those in langasite, twisted around the threefold symmetry axis passing through the origin of the Na₂SiF₆ cell. The kinks in the temperature dependences of the refractive index and rotation of the plane of polarization of light are explained taking into account the anomalous features of interatomic interactions along the threefold axis of the unit cell, passing through the Si2(2d) site with the coordinates (1/3, 2/3, z). It was found that main factor affecting the temperature dynamics of optical parameters is the Si2(2d)–F2(6g) distance, which increases abnormally upon cooling.