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, andš 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 alsoš 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/*-*.