Optical studies of phase transitions in the (NH4)3VO2F4 crystal

(NH4)3VO2F4 crystals were grown, and polarization-optical studies and measurements of birefringence were conducted on crystal plates of various cuts over a wide temperature range. Phase transitions were detected at temperatures T1↑ = 417 K, T3↑ = 211 K, and T4↑ = 205 K (on heating) and at T1↓ = 413 K, T3↓ = 210 K, and T4↓ = 200 K (on cooling). The transitions are accompanied by anomalies of the birefringence and by twinning. The sequence of changes in the phase symmetry is assumed to be as follows: cubic Fm\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \bar 3 $$\end{document}m ↔ orthorhombic Immm (I2221) ↔ monoclinic 112/m) ↔ triclinic P\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \bar 1 $$\end{document}. Near temperatures T2 ≈ 240–250 K, an additional anomaly of the birefringence is observed, with the crystal retaining the orthorhombic symmetry.