Anisotropy of electrooptical properties of berlinite is studied at a wavelength of 633 nm, Temperature dependences of the electrooptical coefficient #delta#n_(111) and spontaneous birefringence are studied in a wide temperature range from 290 to 700 K and from 290 to 950 K, respectively. By approximation of the experimental data, the Curie-Weiss temperature is estimated (T_C = 872 K), and the form of the dependence #delta#n_(111) approx (1 - T/T_C)~(1/3) is identified. Fragments of the AlPO_4 structure are separated that are responsible for the appearance of multipole moments and determine the deviation from the inversion symmetry.
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