In the present paper an approach for evaluation of critical exponent of the order parameter at the diffused phase transition is developed on the basis of optical activity data.
β =). The critical exponent for the pure lead germanate is also clarified.
We present the results of studies on the temperature dependence of the
electrogyration (EG) effect, Faraday rotation and natural optical activity in
Pb5Ge3O11
and Pb5Ge3O11:Cr
crystals at the phase transition. A high EG coefficient is found for
Pb5Ge3O11:Cr
crystals. We demonstrate how the Curie–Weiss constant, the critical
exponents of the order parameter and the dielectric permittivity in
Pb5Ge3O11:Cr
crystals, as well as the coefficients of thermodynamic potential, could be
derived from the temperature dependences of optical activity and the EG
coefficient. We also show that the increment of the Faraday rotation in
Pb5Ge3O11
and Pb5Ge3O11:Cr
crystals appearing at the phase transition is caused by a combined magneto-electrooptic
effect induced by spontaneous polarization. It is proportional to the square of
spontaneous polarization. The phenomenon revealed by us corresponds to combined
effects of crystal optics, which appear due to the common action of different fields.
A technique developed recently for determining piezo-optic coefficients (POCs) of isotropic optical media, which represents a combination of digital imaging laser interferometry and a classical four-point bending method, is generalized and applied to a single-crystalline anisotropic material. The peculiarities of measuring procedures and data processing for the case of optically uniaxial crystals are described in detail. The capabilities of the technique are tested on the example of canonical nonlinear optical crystal LiNbO3. The high precision achieved in determination of the POCs for isotropic and anisotropic materials testifies that the technique should be both versatile and reliable.
Abstract. Basing on temperature dependences of the light intensity transmitted through polarisation system and the birefringence of K 0.2 Rb 1.8 Cd 2 (SO 4 ) 3 crystals, we have determined the phase transition temperature for this solid solution. A conclusion is drawn that the monoclinic and triclinic phases on the (x, T)-phase diagram for the mixed crystals K 2x Rb 2(1-x) Cd 2 (SO 4 ) 3 should disappear at the triple points occurring at x < 0.1.
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