Title: Critical exponents of the order parameter of diffuse ferroelectric phase transitions in the solid solutions based on lead germanate: studies of optical rotation
Author(s):

	D. I. Adamenko (Vlokh Institute of Physical Optics, 23 Dragomanov St., 79005 Lviv, Ukraine),
	R. O. Vlokh (Vlokh Institute of Physical Optics, 23 Dragomanov St., 79005 Lviv, Ukraine)

In this work we show that the critical exponents of the order parameter (CEOPs) of diffuse ferroelectric phase transitions (DFEPTs) occurring in lead germanate-based crystals can be determined using experimental temperature dependences of their optical rotation. We also describe the approach that suggests dividing a crystal sample into many homogeneous unit cells, each of which is characterized by a non-diffuse phase transition with a specific local Curie temperature. Using this approach, the CEOPs have been determined for the pure Pb₅Ge₃O₁₁ crystals, the solid solutions Pb₅(Ge_1-xSi_x)₃O₁₁ (x = 0.03, 0.05, 0.10, 0.20, 0.40) and (Pb_1-xBa_x)₅Ge₃O₁₁ (x = 0.02, 0.05), and the doped crystals Pb₅Ge₃O₁₁:Li³⁺ (0.005 wt.), Pb₅Ge₃O₁₁:La³⁺ (0.02 wt.), Pb₅Ge₃O₁₁:Eu³⁺ (0.021 wt.), Pb₅Ge₃O₁₁:Li³⁺, Bi³⁺ (0.152 wt.) and Pb₅Ge₃O₁₁:Cu²⁺ (0.14 wt.). Comparison of our approach with the other techniques used for determining the Curie temperatures and the CEOPs of DFEPTs testifies to its essential advantages.

Key words: solid solutions, ferroelectric phase transition, critical exponents, optical activity

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