New Investigations of the Crystal Structure of Lead Germanate Pb 5 Ge 3 O 11

— Perfect stoichiometric Pb 5 Ge 3 O 11 single crystals have been synthesized by the flux method. Their cationic and anionic stoichiometry is confirmed by chemical analysis. The specificity of the ferroelectric phase transition at 450 K is determined using dielectric spectroscopy and second-harmonic laser generation. Precise X-ray diffraction analysis has been used to study the changes in the structure and ferroelectric properties in dependence of temperature. Difference residual electron-density maps have been constructed for the first time; the possible sites of lone electron pairs for some Pb +2 cations are indicated. The polar displacements are calculated, and their temperature dependence is estimated. Peculiar features of polyhedra for all symmetrically independent cations in the structure, including calculation of their distortions and valence state, are analyzed. A set of ferroelectric active cations is selected based on the data obtained. Some important aspects of the problem of structural conditionality of ferroelectric properties in Pb 5 Ge 3 O 11 are discussed additionally.