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

Size effect of the guest cation on the AlO4 framework in aluminate sodalite-type oxides M8[Al12O24](SO4)2 (M = Sr2+ and Ca2+) in the I43m phase

Sr₈[Al₁₂O₂₄](SO₄)₂ (SAS) and Ca₈[Al₁₂O₂₄](SO₄)₂ (CAS) are members of the aluminate sodalite-type oxides with the general chemical formula M₈[Al₁₂O₂₄](XO₄)₂ (M²⁺ is the guest cation and XO₄^2- is the guest anion). To discuss the role of the guest cations (M²⁺ = Sr²⁺ and Ca²⁺) on the rotation of AlO₄ in the oxygen tetrahedral framework in the I43m phase, the crystal structure parameters and the probability density function of the guest ions in SAS and CAS have been investigated via synchrotron radiation X-ray powder diffraction by considering Gram-Charlier expansions. The interatomic distances between the M²⁺ and O^2- ions evaluated from the maximum positions in the probability density distribution are almost equal to the sum of the ideal ionic radii of the M²⁺ and O^2- ions. This result suggests that the geometry of the AlO₄ tetrahedral framework and the fluctuation of the guest ions are mainly caused by steric effects between the M²⁺ and O^2- ions.

New phase transitions in the solid solution system (Ca1−xSrx)8[Al12O24](WO4)2

The intermediate part of the cation substitution series (Ca1−xSrx)8[Al12O24](WO4)2of the aluminate sodalites Ca8[Al12O24](WO4)2and Sr8[Al12O24](WO4)2has been studied as a function of temperature and composition, with emphasis on the range 0.50 ≤x≤ 0.60. A composition-induced phase transition occurs atx≈ 0.56 at room temperature and a temperature-induced phase transition from tetragonal to possibly cubic on cooling was observed below room temperature. A tentativex– T phase diagram is given.

Intermediate phases in the Ca-rich part of the system (Ca1−x Sr x )8[Al12O24](WO4)2

The Ca-rich part of the cation substitution series of aluminate sodalites (Ca1−x Sr x )8[Al12O24](WO4)2 has been studied as a function of temperature and composition. The intermediate β-phase of pure calcium tungstate aluminate sodalite was found to have a (3 + 2)-dimensional commensurately modulated structure with an isometric unit cell. The probable superspace group is Abm2(α 0 γ) 0s0, 000 with α = ¼, γ = ¼, a = b = [unk] · 9.301(1) Å, c = 9.301(1) Å at 636 K. Its average structure was found to be similar to that of the room-temperature γ-phase. In crystals with 0 < x ≤ 0.18 the existence of a new intermediate phase could be confirmed. At variance with the otherwise very similar room-temperature phase with x = 0, this γ′-phase has an incommensurate modulation vector. Its proposed superspace group is Abm2(½ 0 γ) 0ss with γ = 0.05(1). Within the precision of our TEM-experiments γ does not change with temperature or composition. The average structure of the γ′-phase could be refined and was found closely related with the average structure of γ-CAW.