The cation-vacancy ordering transition in dehydrated Na6 sodalite

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Abstract
Variable-temperature synchrotron x-ray powder diffraction data from dehydrated Na6[Al6Si6O24] reveal a structural phase transition that involves both the commensurate ordering of the extra-framework Na cations and a one-dimensional incommensurate modulation of the framework. Peak splittings and superlattice reflections implicate an orthorhombically-distorted volume-doubled supercell at room temperature (a=12.9432 Å, b=12.8403 Å, and c=9.1372 Å) ∼(√a×√a×a). These data also included additional superlattice peaks associated with an incommensurate long-period modulation with τ=(3/2 1/2 1)/8.9. All unique orderings of the 12 Na cations among the 16 available sites of the commensurate supercell are enumerated. A unique solution is identified that minimizes the structural energy and accounts for the observed superlattice peaks. Rietveld analysis reveals a significant Pauling “partial-collapse” tilt angle in the cation-ordered phase, that appears to decrease with increasing temperature.