Crystal structure and optical activity of La$_3$Nb$_{0.5}$Ga$_{5.5}$O$_{14}$ and Sr$_3$Ga$_2$Ge$_4$O$_{14}$ single crystals of the langasite family

Precision X-ray structural studies of the La$_3$Nb$_{0.5}$Ga$_{5.5}$O$_{14}$ and Sr$_3$Ga$_2$Ge$_4$O$_{14}$ single crystals were carried out. The space group $P321$ was confirmed. The anomalous X-ray scattering was taken into account to establish the absolute structures (chirality) of the crystals; they proved to be of different sign. The structural features responsible for the optical activity of crystals were revealed, and the dependence of the magnitude and sign of specific rotation on the structural parameters of these compounds were determined.