On isotropic states in α-LiNH4SO4 crystals

The spectral and baric dependences of the birefringence of alpha-LiNH4SO4 crystals are studied. It is found that the birefringence is rather sensitive to uniaxial compressions. A baric shift of the isotropic point to longer or shorter wavelengths depending on the uniaxial compression direction is observed and a generalized temperature-spectral-baric diagram of the isotropic state is plotted. A possibility of the appearance of a pseudoisotropic state under the action of uniaxial pressure sigma (z) or simultaneous pressures sigma (x) = sigma (z) is shown.