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The pyrochlore-type molybdate Pr(1.37)Ca(0.63)Mo(2)O(7).

ABSTRACT: Praseodymium calcium dimolybdenum hepta-oxide, Pr(1.37)Ca(0.63)Mo(2)O(7), crystallizes in the cubic pyrochlore-type structure. In the crystal structure, MoO(6) octa-hedra are linked together by common corners, forming a three-dimensional [Mo(2)O(6)] network. The Pr and Ca atoms and the remaining O atoms are located in the voids of the [Mo(2)O(6)] network. The Pr and Ca atoms are distributed statistically over the same 16c crystallographic position with site-occupancy factors of 0.684?(3) and 0.316?(3), respectively. They are surrounded by eight O atoms forming a ditrigonal scalenohedron. All atoms lie on special positions. The (Pr, Ca) and Mo atoms are, respectively in the 16c and 16d positions with m symmetry, and the O atoms in the 48f or 8a positions with mm or 3m site symmetry, respectively.

SUBMITTER: Gall P

PROVIDER: S-EPMC2961417 | biostudies-literature | 2008 May

REPOSITORIES: biostudies-literature

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The pyrochlore-type molybdate Pr(1.37)Ca(0.63)Mo(2)O(7).

Gall P P Gougeon P P

Acta crystallographica. Section E, Structure reports online 20080524 Pt 6

Praseodymium calcium dimolybdenum hepta-oxide, Pr(1.37)Ca(0.63)Mo(2)O(7), crystallizes in the cubic pyrochlore-type structure. In the crystal structure, MoO(6) octa-hedra are linked together by common corners, forming a three-dimensional [Mo(2)O(6)] network. The Pr and Ca atoms and the remaining O atoms are located in the voids of the [Mo(2)O(6)] network. The Pr and Ca atoms are distributed statistically over the same 16c crystallographic position with site-occupancy factors of 0.684 (3) and 0.3 ...[more]

PMID: 21202443

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Project description:Sm2-xCaxZr2O7-x/2 (x = 0, 0.05, 0.1) and Gd2-xCaxZr2O7-x/2 (x = 0.05, 0.1) mixed oxides in a pyrochlore-fluorite morphotropic phase region were prepared via the mechanical activation of oxide mixtures, followed by annealing at 1600 °C. The structure of the solid solutions was studied by X-ray diffraction and refined by the Rietveld method, water content was determined by thermogravimetry (TG), their bulk and grain-boundary conductivity was determined by impedance spectroscopy in dry and wet air (100-900 °C), and their total conductivity was measured as a function of oxygen partial pressure in the temperature range: 700-950 °C. The Sm2-xCaxZr2O7-x/2 (x = 0.05, 0.1) pyrochlore solid solutions, lying near the morphotropic phase boundary, have proton conductivity contribution both in the grain bulk and on grain boundaries below 600 °C, and pure oxygen-ion conductivity above 700 °C. The 500 °C proton conductivity contribution of Sm2-xCaxZr2O7-x/2 (x = 0.05, 0.1) is ~ 1 × 10-4 S/cm. The fluorite-like Gd2-xCaxZr2O7-x/2 (x = 0.1) solid solution has oxygen-ion bulk conductivity in entire temperature range studied, whereas proton transport contributes to its grain-boundary conductivity below 700 °C. As a result, of the morphotropic phase transition from pyrochlore Sm2-xCaxZr2O7-x/2 (x = 0.05, 0.1) to fluorite-like Gd2-xCaxZr2O7-x/2 (x = 0.05, 0.1), the bulk proton conductivity disappears and oxygen-ion conductivity decreases. The loss of bulk proton conductivity of Gd2-xCaxZr2O7-x/2 (x = 0.05, 0.1) can be associated with the fluorite structure formation. It is important to note that the degree of Ca substitution in such solid solutions (Ln2-xCax)Zr2O7-δ (Ln = Sm, Gd) is low, x < 0.1. In both series, grain-boundary conductivity usually exceeds bulk conductivity. The high grain-boundary proton conductivity of Ln2-xCaxZr2O7-x/2 (Ln = Sm, Gd; x = 0.1) is attributable to the formation of an intergranular CaZrO3-based cubic perovskite phase doped with Sm or Gd in Zr sublattice.

Dataset Information

The pyrochlore-type molybdate Pr(1.37)Ca(0.63)Mo(2)O(7).

Publications

The pyrochlore-type molybdate Pr(1.37)Ca(0.63)Mo(2)O(7).

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