Optical studies of phase transitions in oxyfluoride (NH$_4$)$_2$NbOF$_5$

Polarization-optical studies and measurements of the birefringence $\Delta n$ and the angle of rotation of the optical indicatrix for the (NH$_4$)$_2$NbOF$_5$ crystal have been carried out in the temperature range 100–350 K. Two anomalies of the birefringence have been revealed at the temperatures $T_{01}$ = 258 K and $T_{02}\approx$ 219 K. According to the twinning pattern, the crystal undergoes successive changes in symmetry: orthorhombic $\leftrightarrow$ monoclinic 1 $\leftrightarrow$ monoclinic 2. The twofold axis of the monoclinic phases (or the normal to the plane) is directed along [001]$_{\mathrm{or}}$. The effect of the uniaxial compression along [011]$_{\mathrm{or}}$ and the electric field $E\approx$ 25 kV/cm along [100]$_{\mathrm{or}}$ on the twin structure has been studied. The ferroelastic phase transition at $T_{01}$ is due to the appearance of the shear deformation $x_4(T)$ and is accompanied by significant anomalies of the birefringence. Strong pretransition phenomena mask the jumps in the birefringence $\Delta n(T)$ and in the angle of rotation of the indicatrix $\varphi(T)$ at $T_{01}$.