Temperature and Pressure Changes of the Refractive Properties of LiNH4SO4 Crystal in $\beta$ Modification

We have obtained a crystal of LiNH
₄
SO
₄
in β modification of good optical quality. Birefringence properties of the crystal under the action of uniaxial pressures in the wavelengths range 300-800 nm at room temperature are investigated. The character of the influence of uniaxial pressures applied along mutually perpendicular directions is defined. Studies of the temperature dependences of birefringence for 500 nm wavelength have shown that the second-order phase transition takes place at temperature 461 K, and that birefringence curves for X and Z directions cross at 489 K. The temperature dependence of the angle between the optical axes was obtained experimentally and compared with the calculated from the refractive indices.