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

Abstract

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 Pb_5(Ge_(1-x)Si_x)3O_11 (x = 0.03, 0.05, 0.10, 0.20, 0.40) and (Pb_(1-x)Ba_x)5Ge_3O_11 (x = 0.02, 0.05), and the doped crystals Pb_5Ge_3O_11:Li^3+ (0.005 wt.), Pb_5Ge_3O_11:La^3+ (0.02 wt.), Pb_5Ge_3O_11:Eu^3+ (0.021 wt.), Pb_5Ge_3O_11:Li^3+, Bi^3+ (0.152 wt.) and Pb_5Ge_3O_11:Cu^2+ (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.

Keywords

• solid solutions
• ferroelectric phase transition
• critical exponents
• optical activity