Rietveld analysis of the cubic crystal structure of Na-stabilized zirconia
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Rietveld analysis of the cubic crystal structure of Na-stabilized zirconia G. Fagherazzi, P. Canton, and A. Benedetti Dipartimento di Chimica Fisica, Universit`a di Venezia, Calle Larga S. Marta 2137, 30123 Venezia, Italy
F. Pinna Dipartimento di Chimica, Universit`adi Venezia, Calle Larga S. Marta 2137, 30123 Venezia, Italy
G. Mariotto and E. Zanghellini Istituto Nazionale per la Fisica della Materia and Dipartimento di Fisica, Universit`a di Trento, Via Sommarive 14, 38050 Povo (Trento), Italy (Received 1 July 1995; accepted 13 September 1996)
Using x-ray Rietveld analysis the fcc (fluorite-type) structure of a Na-containing nanocrystalline zirconia powder (9.5 nm estimated crystallite size) obtained by precipitation and calcination has been confirmed. The result shows that conventional x-ray diffraction techniques can distinguish the cubic crystallographic form of ZrO2 from the tetragonal one in nanosized powders. These conclusions are supported by independent Raman scattering experiments.
The stabilization of c-ZrO2 with sodium has been fraught with controversy. X-ray diffraction (XRD) of an amorphous hydrated zirconium oxide containing 3 wt. % of Na (a residual impurity of the precipitation process) calcined at 435 ±C showed c-ZrO2 powder having an estimated 9.5 nm crystallite size and an fcc fluorite-type crystal structure (sample III, as reported in Ref. 1), with a unit cell edge, a0 , of 0.5116(2) nm.1,2 However, when the precipitate was washed (to reduce the amount of sodium present to less than 0.5 wt. %) and calcined at 387 ±C (sample II),1 the tetragonal form was evident [a0 0.5090s4d nm; c0 0.5183s4d nm]. The formation of metastable c-ZrO2 was attributed to a stabilization effect involving sodium, which may substitute for zirconium to generate oxygen vacancies required to maintain charge balance.2 This finding agrees with the results of Nishizawa et al.3,4 who succeeded in stabilizing, at room temperature, the cubic form of ZrO2 by inserting Na1 ions into the gel by hydrothermal reactions. The formation of cubic nanocrystalline zirconia was questioned by Srinivasan et al.,5,6 using conventional x-ray diffraction, Raman spectroscopy, and synchrotron x-radiation. Following our procedure, these authors produced a zirconia powder (their sample D) having the same amount of sodium as Ref. 1. They show a conventional XRD pattern (CuKa radiation) 318
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J. Mater. Res., Vol. 12, No. 2, Feb 1997
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in the range of 70–76± 2u having a single peak (slightly asymmetrical) practically identical to that of our sample III. However, they claimed that their sample D does not have the cubic, but rather the tetragonal structure. With reference to other results reported in the literature, it is important to note that the conventional XRD pattern (refined by Rietveld analysis) of a pure tetragonal zirconia powder of 10 nm of crystallite size (similar to the crystallite size of sample III and sample D), published by Lutter
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