Phase Transitions and Twin Domains in (NH₄)₂SbF₅. Dielectric and Optical Properties

The dielectric and optical properties of (NH₄)₂SbF₅ crystals (APFA) have been studied simultaneously by optical observation in the temperature range 80 to 300 K. The existence of two phase transitions is demonstrated at temperatures 293 and 169 K. The second order of the first one is confirmed. The continuous character of the lower one is demonstrated and the proportionality between electronic density deformation and NH⁺₄ orientations seems clear. A notable phenomenon of brilliant domain walls is discovered at about 145 K (when β = 0°).