New silicate-germanate Cs2Pb2[(Si0.6Ge0.4)2O7] from the series A2Pb2[B2O7], AÂ = K, Cs, BÂ = Si, Ge with the umbrella-like [PbO3]4- group

New Cs2Pb2[(Si0.6Ge0.4)2O7], sp. gr. P-3m1, belonging to the series A2Pb2[B2O7] (A = K, B=Si or Ge), is synthesized and investigated on single crystal. Playing roles of polar tetrahedra in mica-like layers, the umbrella-like [PbO3]4- groups in this crystal are similar to those in nonlinear crystals Pb2(BO3)(NO3), Pb2(BO3)Cl.84