Pyrochlore lattice

T. R. Felthouse, P. B. Fraundorf, R. M. Friedman, and C. L. Schosser, Expanded lattice ruthenium pyrochlore oxide catalysts. I. Liquid-phase oxidations of vicinal diols, primary alcohols, and related substrates with molecular oxygen, J. Catal., 127 (1991) 393 120. 

RU2EU2O7. — Rare earth compounds of the pyrochlore type, Ru , belong to the space group FdSm (Oj) and show analogy with the structures of U02 and U4O8. The lattice parameter [333] of Ru2Eu2C>7 is a = 10.252 A. 

Figure 2. SEM-image of zirconate pyrochlore produced from activated mixture. Black - pores, insets - SAED patterns from the (110) and (211) planes of the lattice. Scale bar is 10 microns.

General Chemical and Physical Characterization. The x-ray diffraction data, chemical analyses by x-ray fluorescence and the effects of various synthesis parameters explored in this study lead to the conclusion that a new series of pyrochlores represented by formula 1 has been synthesized. The substitution of the larger post transition element cation for the noble metal cation on the octahedrally coordinated B-site leads to a considerable enlargement of the pyrochlore s cubic unit cell dimension. The relationship between lattice parameter (ag) and extent of substitution of ruthenium by either lead or bismuth is linear as shown in Figure 1. 

In the case of the Bi2[Ru2-x x]07-y series, it is possible to explain the bismuth substituted pyrochlore either as resulting from the substitution of ruthenium by Bi ", with subsequent vacancy formation on the anion lattice, or by substitution of ruthenium by pairs of Bi and Bi5+, with no anion vacancy formation necessary. Any of the above valence distributions for the bismuth ruthenates could account for the expanded lattice parameter since they all involve average B-site ionic radii larger than Ru ". ... 

The COH ] must be maintained at a certain minimum value (pH 10) in order to synthesize crystalline pyrochlore from solution. This implies that a minimum solubility is required for crystallization and that this crystallization of the pyrochlore directly out of alkaline solution may involve a solution-reprecipitation mechanism. Once the restriction of minimum pH has been satisfied, COH ] does not seem to have a significant effect on crystallinity as long as oxidizing conditions are maintained. The solubility of lead rapidly increases with hydroxide concentration therefore, when all else is held constant, the lattice parameter of the product pyrochlore decreases as the pH of the synthesis medium increases. 

Increasing the temperature of synthesis results in enhanced crystallinity as would be anticipated because of improved reaction kinetics. However, this observation is also consistent with a crystallization mechanism involving solubility. Furthermore, as the temperature increases so does the equilibrium concentration of lead in solution thus with all else held constant, increased temperature of reaction results in a smaller lattice parameter for the product lead ruthenate pyrochlore. 

While high surface area and metallic conductivity are beneficial to electrocatalysis, they do not alone explain the high catalytic activity. We speculate that the variable oxygen stoichiometry of the pyrochlore lattice, and the multiple valence states of the cations, particularly the ruthenium, are essential to the catalytic activities of these pyrochlores. 

It should be noted that the exact cation stoichiometry of the product is highly sensitive to the exact metal concentration of the ruthenium source solution and temperature and pH of the reaction medium (inadvertent increases in both of these parameters lead to increased solubility of lead in the alkaline reaction medium and consequently yield solid products of lower lead ruthenium ratios). While synthesis of a pure lead ruthenium oxide pyrochlore is relatively easy, the precise cation stoichiometry of the product is a property that is not always easy to control. A relatively quick check on the cation stoichiometry of the lead ruthenium oxide product can be obtained, however, by using the correlation between lattice parameter and composition that is displayed in Fig. 1. When lattice parameter and cation stoichiometry are independently determined, the relationship shown in Fig. 1 also provides an assessment of product purity since data points that show significant departures from the displayed linear correlation indicate the presence of impurity phases. The thermal stability of the lead ruthenium oxides decreases with increasing occupancy of tetravalent lead on the octahedrally coordinated site, but all of the ruthenium oxide pyro-chlores described are stable to at least 350° in oxygen. 

Rare earth pyrochlores, possessing the general formula Ln2Sn207, where Ln denotes a rare earth, are active catalysts for the oxidative coupling of methane 150]. Enhanced conversion to useful hydrocarbons (e.g., ethene) is observed with pyrochlores containing rare earths with mixed valence behavior, particularly Sm, Eu, and Yb. Since the rare earth site thus appears to be crucially linked to the catalytic behavior, the distribution of such rare earth species within the lattice is of intrinsic interest. 

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