Structural variations in the solid solution series of sodalite-type |(Eu_xCa_2–x)₄(OH)₈|[(Al_2+xSi_1–x)₄O₂₄]-SOD with 0 ≤ x ≤ 1, determined by X-ray powder diffraction and ²⁷Al MAS NMR spectroscopy

Abstract

The mineral bicchulite, |Ca₈(OH)₈|[Al₈Si₄O₂₄]-SOD, has sodalite-(SOD)-type structure. It is one of the few minerals contradicting Loewenstein’s rule, as it contains twice as much Al as Si at the topologically unique tetrahedrally coordinated position, with both species being randomly distributed. The solid solution series |(Eu_xCa_2–x)₄(OH)₈|[(Al_2+xSi_1–x)₄O₂₄]-SOD with 0 ≤ x ≤ 1 and Δx = 0.125, was synthesized in order to clarify, whether the continuous violation of Loewenstein's rule is possible in an aluminosilicate framework structure like SOD, or whether the reported examples of SOD-type compounds with an Al : Si-stoichiometry of 2 : 1 represent a preferred composition on the tetrahedrally coordinated site of the structure. Here, we report on the variation of the lattice parameter and of some structural parameters in the series, determined by X-ray powder diffraction and results of ²⁷Al MAS NMR spectroscopy. We found Loewenstein's rule to be systematically and continuously violated in this series. The observed structural variations mainly reflect stronger electrostatic interactions between the cage cations and the sodalite framework with increasing (Al + Eu)-content. While the evaluation of the X-ray diffraction data indicate a global adaptation of the sodalite framework to the cage guest size, the significant line-broadening of the NMR spectra with increasing (Al + Eu)-content points towards local adaptations of the framework to the shape of the cage guests.