Al/Si and Ca/Eu short range order in Ca/Eu-bicchulite, studied by FTIR spectroscopy

Abstract

Powder FTIR spectra of the solid solution series |(Ca_{2 – x}Eu_x)₄(OH)₈|[(Al_{2 + x}Si_{1 – x})₄O₂₄]-SOD, 0 ≤ x ≤ 1, were taken at room temperature. All members, including the mineral bicchulite, |Ca₈(OH)₈|[Al₈Si₄O₂₄]-SOD, have a sodalite-type structure. Spectra taken in the hydroxyl region (4000 − 3000 cm⁻¹) yield information related to the local structural states of the pseudo-cubane-like (Ca_{2 + x}Eu_{2 – x})(OH)₄-clusters, 0 ≤ x ≤ 2. Three different patterns of short range order could be identified. Depending on x, different concentrations of Ca₄(OH)₄, EuCa₃(OH)₄, and Eu₂Ca₂(OH)₄ clusters are found, whereas Eu₃Ca(OH)₄ or Eu₄(OH)₄ clusters are not observed. Hence, short range order is maintained such that each SOD cage of an all-alumina framework contains two Ca and two Eu per cluster. The positions of the framework stretching and bending modes shift linearly towards lower wavenumbers with increasing Al content. The shifts are correlated with changes of the framework geometry. Furthermore, Alrich samples show splitting of the stretching and bending modes. This is interpreted as the change of the local environment of Al due to the substitution of Eu³⁺ for Ca²⁺. With the use of deuteration experiments a more detailed assignment could be achieved, especially for the M—OH absorption bands. Finally, changes in the line shape, resulting from cation ordering, are studied by autocorrelation analysis. In particular, samples of intermediate composition show large broadening parameters, Δ_corr, suggesting high degrees of local structural heterogeneity.