Crystal chemistry of K+-conducting solid electrolytes based on rare-earth silicates. Hydrothermal synthesis and crystal structures of K3YbSi3O8(OH)(2) and K6Tm2Si6O18

Orthorhombic layered K3YbSi3O8(OH)(2) (I) and framework K6Tm2Si6O18 (II) si licates were synthesized by the hydrothermal method. Because of the identic al structures of the [M2X5](infinity infinity) layers, both silicates have close unit-cell parameters: a = 13.088, b = 13.505, c = 5.843 Angstrom, spa ce group Pnma (I) and a = 14.053, b = 12.455, c = 5.864 Angstrom, space gro up Pm2(1)n (II). The layers in I and II consist of the YbO6 (TmO6) octahedr a and Si tetrahedra forming triortho groups Si3O8(OH)(2) (I) or Si3O9 "brac kets" (II). In II, the m plane combines the [M2X5](infinity infinity) layer s into a framework with the formation of a six-section silicooxygen chain [ Si6O18](infinity), with the corresponding Si-O-S1 bridges being nearly line ar (179 degrees). The K+ ions in structure I occupy two positions, and the same number of K+ ions in II are distributed over seven positions, of which four are by far not fully occupied. The K(5)-K(7) positions are intermedia te: the intercation distances between them are shortened to 1.27, 2.40 Angs trom. Three-dimensional system of conductivity channels correlates with the properties of solid K(6)Ln(2)Si(6)O(18) electrolytes.