Pyrochlore formation, phase relations, and properties in the CaO-TiO{sub 2}-(Nb,Ta){sub 2}O{sub 5} systems

Journal Article · Sat Mar 15 04:00:00 EDT 2008 · Journal of Solid State Chemistry

DOI:https://doi.org/10.1016/j.jssc.2007.12.005· OSTI ID:21043905

Roth, R S ^[1]; Bordet, P ^[2]; Grey, I E; Mumme, W G ^[3]; Cai, L; Nino, J C ^[4]

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1. Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899 (United States)
2. CNRS Laboratoire de Cristallographie, BP 166, 38042 Grenoble (France)
3. CSIRO Minerals, Box 312, Clayton South, Vic. 3169 (Australia)
4. Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611-6400 (United States)

Phase equilibria studies of the CaO:TiO{sub 2}:Nb{sub 2}O{sub 5} system confirmed the formation of six ternary phases: pyrochlore (A{sub 2}B{sub 2}O{sub 6}O'), and five members of the (110) perovskite-slab series Ca{sub n}(Ti,Nb){sub n}O{sub 3n+2}, with n=4.5, 5, 6, 7, and 8. Relations in the quasibinary Ca{sub 2}Nb{sub 2}O{sub 7}-CaTiO{sub 3} system, which contains the Ca{sub n}(Ti,Nb){sub n}O{sub 3n+2} phases, were determined in detail. CaTiO{sub 3} forms solid solutions with Ca{sub 2}Nb{sub 2}O{sub 7} as well as CaNb{sub 2}O{sub 6}, resulting in a triangular single-phase perovskite region with corners CaTiO{sub 3}-70Ca{sub 2}Ti{sub 2}O{sub 6}:30Ca{sub 2}Nb{sub 2}O{sub 7}-80CaTiO{sub 3}:20CaNb{sub 2}O{sub 6}. A pyrochlore solid solution forms approximately along a line from 42.7:42.7:14.6 to 42.2:40.8:17.0 CaO:TiO{sub 2}:Nb{sub 2}O{sub 5}, suggesting formulas ranging from Ca{sub 1.48}Ti{sub 1.48}Nb{sub 1.02}O{sub 7} to Ca{sub 1.41}Ti{sub 1.37}Nb{sub 1.14}O{sub 7} (assuming filled oxygen sites), respectively. Several compositions in the CaO:TiO{sub 2}:Ta{sub 2}O{sub 5} system were equilibrated to check its similarity to the niobia system in the pyrochlore region, which was confirmed. Structural refinements of the pyrochlores Ca{sub 1.46}Ti{sub 1.38}Nb{sub 1.11}O{sub 7} and Ca{sub 1.51}Ti{sub 1.32}V{sub 0.04}Ta{sub 1.10}O{sub 7} using single-crystal X-ray diffraction data are reported (Fd3m (no. 227), a=10.2301(2) A (Nb), a=10.2383(2) A (Ta)), with Ti mixing on the A-type Ca sites as well as the octahedral B-type sites. Identical displacive disorder was found for the niobate and tantalate pyrochlores: Ca occupies the ideal 16d position, but Ti is displaced 0.7 A to partially occupy a ring of six 96g sites, thereby reducing its coordination number from eight to five (distorted trigonal bipyramidal). The O' oxygens in both pyrochlores were displaced 0.48 A from the ideal 8b position to a tetrahedral cluster of 32e sites. The refinement results also suggested that some of the Ti in the A-type positions may occupy distorted tetrahedra, as observed in some zirconolite-type phases. The Ca-Ti-(Nb,Ta)-O pyrochlores both exhibited dielectric relaxation similar to that observed for some Bi-containing pyrochlores, which also exhibit displacively disordered crystal structures. Observation of dielectric relaxation in the Ca-Ti-(Nb,Ta)-O pyrochlores suggests that it arises from the displacive disorder and not from the presence of polarizable lone-pair cations such as Bi{sup 3+}. - Graphical abstract: Crystal structures of the pyrochlores Ca{sub 1.46}Ti{sub 1.38}Nb{sub 1.11}O{sub 7} and Ca{sub 1.51}Ti{sub 1.32}V{sub 0.04}Ta{sub 1.10}O{sub 7} were refined using single-crystal X-ray diffraction data. Both Ca{sup 2+} and Ti{sup 4+} occupy the A-type sites; Ca occupies the ideal 16d site and Ti is displaced 0.7 A to partially occupy a ring of six 96g sites. The O' oxygens are disordered among a tetrahedral cluster of 32e sites displaced 0.48 A from the ideal 8b site. Both pyrochlores display dielectric relaxation similar to that observed for analogous Bi-based systems.

• OSTI ID: 21043905
• Journal Information: Journal of Solid State Chemistry, Journal Name: Journal of Solid State Chemistry Journal Issue: 3 Vol. 181; ISSN 0022-4596; ISSN JSSCBI
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
BISMUTH IONS
CALCIUM IONS
CALCIUM OXIDES
COORDINATION NUMBER
CUBIC LATTICES
DIELECTRIC MATERIALS
MONOCRYSTALS
NIOBIUM OXIDES
PEROVSKITE
PHASE DIAGRAMS
PYROCHLORE
SOLID SOLUTIONS
TANTALATES
TANTALUM OXIDES
TITANIUM IONS
TITANIUM OXIDES
X-RAY DIFFRACTION
ZIRCONOLITE