Crystallographic relations between the β and γ forms of (Ca₂,Ca_1.8Sr_0.2)SiO₄

Abstract

coordinated cations, can account for practically any stoichiometry in the composition range between CaUO4 and CaUO3.5o. The given explanation is not only in agreement with all known experimental data, but also with the results of Thornber & Bevan (1970), who studied mixed oxides of the type MO2 (fluorite)-M203, which can accommodate large departures in stoichiometry in fluorite-related structures. The defect complex in these compounds, which enables the variation in composition, is equivalent to type A ordering, one of the possible ways of reducing CaUO4.There is a perfect intrinsic order in the arrangement of O vacancies in both types of microdomains and the observed disorder is only a result of more than one energetically equivalent possibility of stacking the reduced layers along the original inversion triad. If the nearest-neighbour interaction between reduced layers could be extended to the next-nearest neighbours at sufficiently low temperatures, a complete three-dimensional ordering would be achieved. As the favoured alternation of both types of ordering can only be explained on the basis of the most convenient charge balancing during reduction, it is obvious that this kind of ordering is conditioned by the possibility of U taking on a whole series of different ionization states with close ionization energies. AbstractCrystal associations of/3 and 3' forms of synthetic (Ca2,CalsSro2)SiO4 showing a paramorphosis relationship are studied by optical microscopy and X-ray diffraction. The mutual crystallographic orientation is unequivocally established. The cell parameters are: (a sin/3)~ = 5.48, a~ = 5.08; b~ = 6.74, by = 6.76; c~ = 9.30, cv = 11.23/~,. On the basis of experimental evidence the structural relationships

Polymorphous transformations in alkaline-earth silicates

“…Gazzoni et al studied the crystallographic relations and the transition mechanism between the ␤ and ␥ phases in detail. 25 With respect to the phase transition in Sr 2 SiO 4 , an incommensurate phase was found in experiment between the ␤ and ␣Ј phases. 10 In our simulation the ␤ and ␣Ј phases are isostructural to the ␤ and ␣ H Ј phases of Ca 2 SiO 4 , respectively.…”

Ce3+-Activated γ-Ca2SiO4 and Other Olivine-Type ABXO4 Phosphors for Solid-State Lighting

“…During the cooling step of the phosphor synthesis, the material passes through the metastable β polymorph before forming the final γ-phase (see Figure ). Interestingly, the A site in the γ-phase originates from the larger 8-coordinate Ca site in the β-phase, whereas the B site originates from the smaller 7-coordinate Ca . Supposing that Ce 3+ can occupy both 7- and 8-cooridnate sites in the β-phase, potentially with preference for the larger site, this will impact the final distribution in the γ-phase.…”