Ferroelectric crystals in system PbO-GeO2-SiO2

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Abstract
Crystals of some compounds in system PbO-GeO2 and of solid solutions Pb3(Gex.Sil_x)207 were giown by the spontaneous crystallization method. A well-known ferroelectric crystal which was thought to be of a composition Pb5Ge3O11 is shown to have a more precise formula Pb3Ge2O7. For the first time the temperature dependence of pyro- and nonlinear optical characteristics is measured. The crystal does not have a centre of symmetry above Tc = 175°C right up to the melting point (730°C). Below 175°C it possesses a pyroeffect more pronounced than that of LiNbO3. A new transition from one noncentrosymmetrical modification to another one is revealed at 320°C.Dependence of Tc and lattice parameters on x has been studied for solid solutions Pb3(GexSi1_x)2O7. Conclusion is drawn that Pb3Si2O7 is a ferroelectric with Tc = -100°C. It is found that crystal Pb3GeO5 is noncentrosymmetrical and seems being in spontaneously polarized state.

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