The cation-vacancy ordering transition in dehydrated Na-6 sodalite

Variable-temperature synchrotron x-ray powder diffraction data from dehydra ted Na-6[Al6Si6O24] reveal a structural phase transition that involves both the commensurate ordering of the extra-framework Na cations and a one-dime nsional incommensurate modulation of the framework. Peak splittings and sup erlattice reflections implicate an orthorhombically-distorted volume-double d supercell at room temperature (a=12.9432 Angstrom, b=12.8403 Angstrom, an d c=9.1372 Angstrom) similar to(root(2) over bara x root(2) over bar axa). These data also included additional superlattice peaks associated with an i ncommensurate long-period modulation with tau=(3/2 1/2 1)/8.9. All unique o rderings of the 12 Na cations among the 16 available sites of the commensur ate supercell are enumerated. A unique solution is identified that minimize s the structural energy and accounts for the observed superlattice peaks. R ietveld analysis reveals a significant Pauling "partial-collapse" tilt angl e in the cation-ordered phase, that appears to decrease with increasing tem perature. (C) 2000 American Institute of Physics. [S0021-9606(00)00844-8].