Abstract
Deflection of light studies in function of temperature in NaNH4SeO4·H2O crystal is presented. At 180 K, this compound undergoes a para-ferroelectric/ferroelastic phase transition of the second order. It changes the symmetry from the orthorhombic symmetry class 222 (space group P212121) to the monoclinic symmetry class 2 (space group P21). A distinct deflection pattern, resulting from the ferroelastic domain structure, occurs in the low temperature phase. The intensity of deflected beams varies considerably with temperature. Detailed studies revealed the structure of deflected spots during cooling–heating cycle. These spots change intensity, become more dispersed and finally split with decreasing temperature. Moreover, a variation of deflection angles was observed. A possible explanation of these phenomena is given in a framework of crystals optics.
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