  We present the results of studies for temperature dependences of
electrogyration (EG) effect and natural optical activity in Pb5Ge3O11:Cr
crystals. Extremely high magnitude of EG coefficient is found for these
crystals. We also demonstrate how the Curie-Weiss constant, the critical
exponents of the order parameter and the dielectric permittivity, as well as
the coefficients of thermodynamic potential, could be derived from the
temperature dependences of optical activity and EG coefficient.
