Silicate-germanates with the doubled mica-like layers [Pb(Si,Ge)O-4](2-)(infinity infinity )in centrosymmetric Rb2Pb2[(Ge0.33Si0.67)(2)O-7] and isolated in acentric, optically nonlinear Li6Pb2[(Ge0.4Si0.6)O-4](2)(OH)(2)

摘要

Silicate-germanates, Rb2Pb2[(Si0.67Ge0.33)(2)O-7], P (3) over bar m1, and Li6Pb2[(Ge0.4Si0.6)O-4](2)(OH)(2), P6(3)mc, were synthesized in the multi-component hydrothermal systems. According to single crystal XR data, both structures contain identical mica-like layers [Pb(Ge,Si)O-4 ](2-)(infinity infinity), double or single, comprised of [(Si, Ge)O-4](4-)-tetrahedra and umbrella-like groups [PbO3](4-): the latter play a unique role of (Si,Ge)-tetrahedra being of the same charge, dimension and the Pb-lone-pair as the forth apex. Predicted earlier polar ortho-silicate structure appears with small Li atom, while the centrosymmetric diortho-silicate structure type A(2)Pb(2) [B2O7] is stable with large A = K, Rb, Cs. Layers with (Li3O7}(11- )atomic clusters separate single tetrahedral layers. Under Nd:YAG laser illumination the powder of acentric Li, Pb-silicate-germanate yields intensive light of second harmonic. The maximum second harmonic intensity 10 times more in comparison with quartz standard is achieved in the powder with a grain size of about 20 mu m. The space-averaged second-order nonlinear coefficient = 1.1 p.m./V. The high optical nonlinearity is connected with the one-side directed umbrella-like polar units [PbO3](4-) in the crystal structure of Li,Pb-silicate-germanate.