Optical and x-ray studies of the structural phase transition in cryolite (NH4)3GaF6

Optical polarization and x-ray diffraction studies of powders and single-crystal plates of different cuts of an (NH4)3GaF6 crystal are carried out in a broad temperature range, including the phase transition point. The following sequence of symmetry changes is found to occur: \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document} $$O_h^5 - Fm3m (Z = 4) \leftrightarrow C_i^1 - I\bar 1(Z = 16)$$ \end{document}.