Induced and spontaneous optical activity in Pb 5 Ge 3 O 11 single crystals

Temperature dependences of the optical rotatory power ρs 3 in the ferroelectric phase, the field-induced optical rotatory power ρ3(E 3) in the paraelectric phase and of the initial permittivity ε33 have been measured. These results show that dρ3/dE 3 fulfils the Curie-Weiss law being proportional to the value of ε33. The value of the electrogyration coefficient y333=0.77×10−3 m2C−1 as determined from the measurements of dρ3/dE 3 and ε33 agrees well with the coefficients obtained from spontaneous values of εs 3, P3s. εs 3(T) data in the region between Tc · 3 K and room temperature are well described by the empirical relation: εs 3 ∼ (Tc−T)0.35