Ball milling-induced pyrochlore-to-tungsten bronze phase transition in RbNbWO 6
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Joa˜o A.H. da Jornada Inmetro, 25250-020 Xere´m, Duque de Caxias–RJ, Brazil; and Universidade Federal do Rio Grande do Sul, Instituto de Fı´sica, 91501-970 Porto Alegre–RS, Brazil
Claudio A. Perottonia) Universidade de Caxias do Sul, 95070-560 Caxias do Sul–RS, Brazil; and Universidade Federal do Rio Grande do Sul, Instituto de Fı´sica, 91501-970 Porto Alegre–RS, Brazil (Received 29 September 2008; accepted 19 December 2008)
A set of Bragg peaks consistent with a hexagonal Bravais lattice was observed in the x-ray powder diffraction pattern of cubic pyrochlore rubidium tungstoniobate (RbNbWO6) subjected to high-energy ball milling. The calculated lattice parameters for this hexagonal phase are similar to those of compounds with tungsten bronze structure. In fact, the powder pattern of the hexagonal phase could be refined with a structural model based on the tungsten bronze structure. The hexagonal phase produced by high-energy ball milling of RbNbWO6 transforms back to the pyrochlore structure upon heating to 773 K in air. A similar phase was obtained by ball milling the mixture RbNbWO6 + WO3, but, in this case, the stoichiometric hexagonal tungsten bronze compound thus obtained remained stable up to 1273 K.
I. INTRODUCTION
Pyrochlore compounds have general stoichiometry A2B2X6X’ (where A is a relatively large cation such as Na, K, Rb, Cs, Ca, Cd, Tl, Pb, Sc, Lu, Yb, etc.; B is a smaller cation such as Ti, Nb, V, W, Ta, Re, Ru, etc.; X is O or F; and X’ can be O, S, F, OH, among others).1,2 These compounds exhibit many interesting physical properties, including ferroelectricity, ferromagnetism, high ionic conductivity, superconductivity, and luminescence, and they have been extensively studied over the past decades.1–10 Some pyrochlore ceramics also have useful dielectric properties and are also attractive for electrochemical application as cathode materials in fuel cells with low operating temperature.9–14 The ideal pyrochlore structure is cubic Fd3m (Z = 8, ˚ ), and it is usually described as lattice parameter a 10 A an oxygen corner-sharing octahedral framework, with the B cations at the center of each octahedron. The threedimensional arrangement of these octahedral structures creates cavities in which the large A cations are located.1,2 Several pyrochlores, known as defect pyrochlores, have crystal structures with partially occupied sites.1 Among them, RbNbWO6 belongs to the family of defect pyrochlore with general formula AB2O6.15,16 Rubidium tunga)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0247 J. Mater. Res., Vol. 24, No. 6, Jun 2009
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stoniobate exhibits a slight tetragonal distortion at room temperature that transforms to cubic Fd3m under heating.16–21 Indeed, it has been shown that rubidium tungstoniobate undergoes a transition from the low-temperature tetragonal ferroelectric phase to a phase of unknown symmetry at 356.5 K, followed by a transition to the cubic phase as temperature is increased to
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