The cation-vacancy ordering transition in dehydrated Na 6 sodalite

Variable-temperature synchrotron x-ray powder diffraction data from dehydrated Na 6 ͓Al 6 Si 6 O 24 ] 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 ͑aϭ12.9432 Å, bϭ12.8403 Å, and cϭ9.1372 Å͒ ϳ(&aϫ&a ϫa). These data also included additional superlattice peaks associated with an incommensurate long-period modulation with ␶ϭ͑3/2 1/2 1͒/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.