On Isotropic Points in Potassium Ammonium Sulphate Crystals

An effect of uniaxial pressure on the spectral and temperature dependences of birefringence of K 1.75 [NH 4 ] 0.25 SO 4 single crystals has been studied. Different changes in the birefringence are found for different compression directions. The possibility of the occurrence of isotropic points in the crystal under uniaxial compression is discussed. A decrease in the birefringence under crystal heating is established, and the positions of two isotropic points in the range of 290–630 K are determined. The partial isomorphic substitution of NH 4 ions for K ions was found to result in a shift of isotropic points to lower temperatures.