Phase transition in Sr₈[Al₁₂O₂₄](MoO₄)₂ aluminate sodalite (SAM)

Abstract

The cubic-tetragonal phase transition at 571 K of the aluminate sodalite Sr8[AI12024](MoO4)2 (SAM) has been studied by following the position of the (pseudo-)cubic {400} reflections as a function of temperature. The high resolution of the synchrotron powder diffraction experiment allowed the temperature dependencies to be followed with good precision. The tetragonal a lattice parameter appears to be a linear extrapolation of the cubic one, with only a small upward shift at the transition, whereas the c parameter decreases strongly below 571 K. These observations can be explained by a model which assumes the superposition of a ferroelastic strain component, and a volume strain component. The volume strain can be rationalized as being the result of a 'shearing' of the sodalite framework. Causes and consequences of the 'shearing' in relation to the sodalite framework are discussed. The weakly first-* Present address:

A Structural Study of Aluminate Sodalite Ca8[Al12O24](CrO4)2(CACr)

“…K-'. This value agrees well with that found for SAM (DEPMEIER et al 1993A) and determined during the same beamtime. The agreement is rather surprising, however, as SAM possesses a fully-expanded framework, whereas CACr is partially-collapsed.…”

“…It is interesting to note that from Tc3 downwards the lattice parameters, and derived quantities, depend almost linearly on temperature, in contrast to the tetragonal regime. This is also different from the behaviour found in SAM and SACr, where power laws were observed for the lower-symmetry phases (DEPMEIER et al 1993A;MELZER 1992).…”

“…In accordance with the evaluation of an experiment of the same kind (DEPMEIER et al 1993A andcompare DEPMEIER et al 1993B) and performed in the same set of experimental runs we estimate the uncertainty to be of the order of, but probably significantly lower than, fO.OO1 hi, with ensuing uncertainties of i0.3 A3 and f 7 .…”

“…In this case exponential laws could be fitted over large temperature ranges, after a volume strain had been corrected for. The exponents thus determined, however, revealed themselves as non-classical and non-critical because of the first order character of the phase transition (DEPMEIER et al 1993A). The same argument applies to the present case.…”

“…This behaviour is now in clear contrast to that of the homologue SAM where a strong volume anomaly was observed. The volume anomaly was explained to result from a specific distortion mechanism of the sodalite framework -the so-called shearing -which in turn could be traced back to repulsive interactions between oxygens of the Moo4 tetrahedra and oxygens of the framework, respectively (DEPMEIER et al 1993A). Figure 1 might suggest that the shearing mechanism might be negligible in the case of CACr.…”