Modulated structures in aluminate sodalites

Depmeier W.

Institute of Geosciences, University of Kiel, Kiel, Germany.

 

The general formula of aluminate sodalites can be written |M8(XO4)2|[Al12O24], where M and X are site symbols and M represents bivalent cations like Ca2+ or Sr2+, and X = S6+, Cr6+, Mo6+, W6+. Our interest in aluminate sodalites focuses on structural aspects of aluminate sodalites, their phase transitions, and related properties. Usually, phase transitions in aluminate sodalites from a cubic high temperature phase to one or several non-cubic low temperature phases are of ferroic type: ferroelastic and ferroelectric phases have been identified. The main emphasis of the present contribution will be on a special character of the non-cubic phases. It seems that the majority of these phases can be conveniently described as modulated phases. Commensurately and incommensurately modulated phases have been found, and the dimensions of superspace may vary between (3+1) and (3+3). The sensitivity of the modulations against even small disturbances leads to quite complicated T x phase diagrams. The reason for the occurrence of the modulations lies in the fact, that the structure of sodalites in general can be broken down into three partial structures, viz. i) the sodalite framework, ii) an interpenetrating net of cations, and iii) cage anions at the centres of the sodalite cages. It is important to know that interactions i) ii) and ii) iii) are basically attractive, whereas i) iii) is repulsive in nature. In the particular case of aluminate sodalites of the given composition, the cage anions iii) are tetrahedral oxyanions. Their orientation is not only incompatible with the latent cubic symmetry of the sodalite framework, but leads to marked repulsion effects i) iii), and deformation of the framework. It turns out that interactions i) ii) on the one side, and ii) iii) on the other side are competitive, such that the system is frustrated and its free energy can be lowered by a modulation. Cascades of phase transitions especially in the Ca-bearing members of the aluminate sodalite family can be rationalized by the fact that the phase transitions from the cubic phase usually happen at an N-point of the body-centred Brillouin zone meaning that the corresponding order parameter has six components. In real space the cascades can be rationalized by an interplay of rotational and translational potentials becoming subsequently deeper or shallower as a consequence of the above-mentioned interactions. Chaotic phases and phases due to sliding of modulation waves are also anticipated.

The low symmetry phases are usually characterized by marked pseudo-symmetry with weak superstructure reflections, sometimes very low degree of spontaneous deformation in the case of ferroelastics, and the strong sensitivity against all kinds of defects and disturbances.

The long-standing studies studies were financially supported by DFG granst DE 412/*-*.


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