Ferroelectric and optical properties of Pb₅Ge₃O₁₁ and its isomorphous compound Pb₅Ge₂SiO₁₁

The temperature characteristics of dielectric constants, spontaneous polarization, thermal‐expansion coefficients, indices of refraction, and optical rotatory power of Pb₅Ge₃O₁₁ single crystals have been investigated in detail from room temperature up to above the Curie temperature, 177°C. From the results of these measurements, the following constants were obtained: linear thermal‐expansion coefficients (α_a)_F=7.75×10⁻⁶/deg°C and (α_c)_F=7.79×10⁻⁶/deg°C (ferroelectric phase), (α_a)_P=13.8×10⁻⁶/deg°C and (α_c)_P=13.4×10⁻⁶/deg°C (paraelectric phase); quadratic electro‐optic constants $g33T=0.47×108 cm4 /C2, g13T=0.37×108 cm4 /C2$⁠; and electrogyration coefficient γ₃₃ =8.7 cm²/C. An isomorphous compound Pb₅(Ge₂O₇)(SiO₄) was synthesized and was found to be ferroelectric with T _c=60°C and P_s=1.7 μC/cm² at room temperature. Ferroelectric properties and electrogyration coefficient were studied in the solid‐solution system Pb₅(Ge₂O₇)(GeO₄)_1−x (SiO₄)_x.