Abstract
The present study demonstrates the capabilities of the Rietveld procedure to track the structural transformations and framework flexibility on the example of the natural water-containing zirconosilicate elpidite, subjected (in bulk) to thermal treatment from room temperature to 300 °C. The methodological approach to the performed refinements and the obtained results are in accordance with the previously reported data from in situ single crystal X-ray diffraction studies on heated samples of the same mineral. More light has been drawn on the temperature interval in which the non-reconstructive topotactic phase transition occurs upon partial dehydration. The framework flexibility observed as a response to the water loss and subsequent thermal expansion was evaluated in terms of intentionally introduced set of geometric parameters characterizing the spatial orientation of symmetrically related zirconium octahedra in the structure, the coordination polyhedra volumes, their distortion indices, and bond angle variances.
Highlights
Elpidite is a natural microporous zirconosilicate with heteropolyhedral framework and ideal formula Na2 ZrSi6 O153H2 O
This study presents powder X-ray diffraction (PXRD) investigations and results obtained by application of the Rietveld method in an attempt to track the structural transformations and to evaluate the framework flexibility of the natural water-containing zirconosilicate elpidite, subjected to thermal treatment from room temperature to 300 ◦ C
The PXRD patterns simulated from the in situ single crystal studies of Cametti et al [9] (CIF files were taken from the supplementary material of the online version) provide valuable guidance as to how to distinguish the elpidite materials before and after the phase transition in this study
Summary
Elpidite is a natural microporous zirconosilicate with heteropolyhedral framework (i.e., a framework formed by SiO4 tetrahedra and ZrO6 octahedra) and ideal formula Na2 ZrSi6 O153H2 O. The structure of this mineral from the Lovozero complex was re-refined by different authors, in some cases in the acentric space group Pbm2 [3,4] or in Pbcm with one of the unit-cell parameters doubled [5]. The crystal structures of elpidite samples from another locality—Khan Bogdo (KB), Mongolia—have been solved either in Pbm2 [6] or in Pbcm [7]. Despite the observed differences in the space groups of different samples belonging to one mineral species and taken from the same or various localities, all studied compounds retain an equal structural topology that can be presented as a heteropolyhedral framework consisting of double Si6 O15 chains (ribbons) connected by isolated ZrO6 octahedra [4]
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