Crystal Structures of Endotaxic Phases in Europium Potassium Silicate Having a Pellyite Unit Cell

Abstract

The structures of three phases of the synthesized europium potassium silicate were determined by X-ray diffraction. Two of these phases crystallize in a new structural type. The chemical formulas of the phases were determined. The orthorhombic unit-cell parameters of all three phases are equal: a = 14.852(1) A, b = 15.902(1) A, c = 7.243(1) A, sp. gr. P21212 (phase I) and sp. gr. Pbam (phases II and III). The structures were solved by direct methods and refined from X-ray diffraction data collected from one crystal to R = 0.0271, 0.0479, and 0.0582 based on 4370, 3320, and 2498 reflections, respectively, with ¦F¦ > 3σ(F). The crystal- chemical formulas of the phases (Z = 4) are K3Eu3+[Si6O15] ∙ 2H2O, K3Eu3+[Si6O13(OH)4] ∙ 2H2O, and K3Eu3+[Si4O9.5(OH)](OH)2∙ 5.5H2O. The structure of phase I consists of silicon-oxygen sheets [Si6O15] analogous to those found in the isostructural compound K3Nd[Si6O15] ∙ 2H2O. In the structures of phases II and III, the ribbons [Si6O17] and [Si8O21] run along the shortest c axis and are linked together by Eu3+ octahedra and trigonal prisms to form three-dimensional layered and framework structures containing K atoms between the sheets and in the channels. The fragments are also linked through hydrogen bonds with the participation of OH groups and water molecules.