The lithium phosphidosilicates Li10Si2P6 and Li3Si3P7 are obtained by high-temperature reactions of the elements or including binary Li-P precursors. Li10Si2P6 (P21/n, Z = 2, a = 7.2051(4) A, b = 6.5808(4) A, c = 11.6405(7) A, beta = 90.580(4)degrees) features edge-sharing SiP4 double tetrahedra forming [Si2P6](10-) units with a crystal structure isotypic to Na10Si2P6 and Na10Ge2P6. Li3Si3P7 (P21/m, Z = 2, a = 6.3356(4) A, b = 7.2198(4) A, c = 10.6176(6) A, beta = 102.941(6)degrees) crystallizes in a new structure type, wherein SiP4 tetrahedra are linked via common vertices and which are further connected by polyphosphide chains to form unique 82[Si3P7](3-) double layers. The two-dimensional Si-P slabs that are separated by Li atoms can be regarded as three covalently linked atoms layers: a defect alpha-arsenic type layer of P atoms sandwiched between two defect wurzite-type Si3P4 layers. The single crystal and powder X-ray structure solutions are supported by solid-state Li-7, Si-29, and P-31 magic-angle spinning NMR measurements.