Measurement and Calculation of the Refractive Indices of Langasite-Family Crystals Sr3NbFe3Si2O14, Ba3NbFe3Si2O14, and Ba3TaFe3Si2O14 and the Relationship of Optical Activity with Peculiarities of Electron-Density Distribution

The optical properties of Sr3NbFe3Si2O14, Ba3NbFe3Si2O14, and Ba3TaFe3Si2O14 single crystals (promising multiferroics belonging to the langasite family (sp. gr. P321, Z = 1)) have been investigated. The crystals have been grown by floating zone melting. The measured and calculated refractive indices of these crystals are compared. The optical activity parameters are calculated based on the structural data. The atomic structures are compared proceeding from the precise X-ray diffraction analysis data. A correlation is established between the structural features and optical properties of the crystals.