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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. The temperature dependence of the optical activity in Pb5Ge3O11:Cu 2+ crystals is experimentally studied. The critical exponents obtained for Pb5Ge3O11:Cu 2+ and Pb5(Ge0.6Si0.4)3O11 crystals are equal to 0.40 and 0.35, respectively. On the basis of the critical exponents evaluated it is shown that the solid solutions (Pb(1-x)Bax)5Ge3O11 with x = 0.05 stay quite close to tricritical point on the x,T-phase diagram ( 0.28 β = ). The critical exponent for the pure lead germanate is also clarified.

ukrainian journal of physical optics

Optical activity and critical exponent of the order parameter in lead germanate crystals.1.The case of diffused phase transition in Pb5Ge3O11 doped with Cu, Ba and Si ions