Pyrochlore stability

Lumpkin, G. R. Mariano, A. N. 1996. Natural occurrence and stability of pyrochlore in carbona-tites, related hydrothermal systems, and weathering environments. In Murphy, W. M. Knecht, D. A. (eds) Scientific Basis for Nuclear Waste Management XIX. Materials Research Society Symposium Proceedings, 412, 831-838. 

The investigations of platinum pyrochlores have demonstrated the effectiveness of high pressure techniques in the synthesis of anhydrous oxides when one or both reactants have limited thermal stability. The bulk of the work reported here represents a continuation of an exploration of metal oxide-platinum oxide systems at high pressure. 

A2Pt207, similar to those reported for tin, ruthenium, titanium, and several other tetravalent ions. Trivalent ions which form cubic platinum pyrochlores range from Sc(III) at 0.87 A to Pr(III) at X.14 A. Distorted pyrochlore structures are formed by lanthanum (1.18 A) and by bismuth (1.11 A). Platinum dioxide oxidizes Sb203 to Sb2(>4 at high pressure. The infrared spectra and thermal stability of the rare earth platinates have been reported previously and will not be repeated here, except to point out the rather remarkable thermal stability of these compounds decomposition to the rare earth sesquioxide and platinum requires temperatures in excess of 1200 °C. 

Oxygen (O2-) anion conductors - stabilized zir-conia, stabilized - bismuth oxide, - BIMEVOX, doped cerium dioxide, numerous perovskite-type - solid solutions derived from Ln(A)B (B") 03 (A = Ca, Sr, Ba B = Ga, Al, In B" = Mg, Ni, Co, Fe), La2Mo207 and its derivatives, pyrochlores based on Ln2Ti07. 

There is a need to develop new types of oxide electrodes for reactions of technological importance with emphasis on both high electrocatalytic activity and stability. For example, pyrochlore-type oxides, e.g. lead or bismuth ruthenates, have shown excellent catalytic activity for the oxygen evolution and reduction reactions and should be further investigated to elucidate the reasons for such high activity. The long term stability of such ruthenate electrodes is questionable, however. 

Figure 3.9 shows a schemalic representation of the chemical filing process. The first step of the process involves the chlorination of the surface of the pyrochlore-based ceria-zirconia sample. The extent of the chlorination can be controlled by the concentration of the chlorine gas and/or chlorination time and the cerium and zirconium chlorides partially formed on the surface are vaporized and transported by the formation of gaseous complexes with aluminum chloride. This chemical filing process is carried out at 1273 K to stabilize the surface modification effects at high temperatures. A similar effect can also be achieved by chlorination with ammonium chloride followed by dominant vaporization of formed zirconium chloride. ... 

Figure 3 shows that in a 70 h. run at 300°C both the methanol yield and the methanol selectivity are decreased for all the three samples The CuZn-LaZr [ex oxalate] catalyst seems to be the most stable (yield loss 7%) and the less active one. The pyrochlore promoted copper-zinc catalyst (CuZn + LaZr) shows also a good stability (yield loss 3.5%) better in any case than the tested unpromoted Cu-Zn catalyst (9%). Finally the CuZn-LaZr [ex carbonate] samples given its low pyrochlore crystallization undergo to Ae highest deactivation (yield loss 14%). 

Other oxygen ion conductors that have potential use as solid electrolytes in electrochemical devices are stabilized bismuth and cerium oxides and oxide compounds with the perovskite and pyrochlore crystal structures. The ionic conductivity and related properties of these compounds in comparison with those of the standard yttria-stabilized zirconia (YSZ) electrolyte are briefly described in this section. Many of the powder preparation and ceramic fabrication techniques described above for zirconia-based electrolytes can be adapted to these alternative conductors and are not discussed further. 

The thermal stability of all of these nonstoichiometric pyrochlores is limited and is inversely dependent upon the extent of substitution of noble metal cations on the B-site by post transition element cations (1). For example, Pb2[Ru2]05,5 is stable to 850 C in air while Pb2[RuPK]06,5 is only stable to about 400 C. 

A new family of high conductivity, mixed metal oxides having the pyrochlore crystal structure has been discovered. These compounds display a variable cation stoichiometry, as given by Equation 1. The ability to synthesize these materials is highly dependent upon the low temperature, alkaline solution preparative technique that has been described the relatively low thermal stability of those phases where an appreciable fraction of the B-sites are occupied by post transition element cations precludes their synthesis in pure form by conventional solid state reaction techniques. 

Figure 9.4 Total conductivity of stabilized zirconia (a) and doped ceria (b) solid electrolytes [34—40], compared to the oxygen ionic conductivity of pyrochlore-type Cd2Zr2O7 5 [41], (Cd,Ca)2Ti2O7 5 [42], (Cd,Ca)2Sn2O7 s [43], and 241707 a [44].

All the cited literature references to the above compounds have described solid-state syntheses at temperatures of 700-1200°. Such synthesis conditions will always lead to pyrochlore structure compounds in which all of the octa-hedrally coordinated sites are occupied by the noble metal cation, thus requiring the post-transition metal to noble metal molar ratio always to be 1.0. This paper focuses on solution medium syntheses at quite low temperatures (<75°), thereby stabilizing a new class of pyrochlore compounds in which a variable fraction of the octahedrally coordinated sites are occupied by post-transition element ca-tions.5,6 The specific example here involves the Pb2[Ru2 Pb4+]06 s series. The synthesis conditions may be simply adapted, however, to accommodate preparation of a wider range of pyrochlores which can be described by the formula A2[B2 xAx]07.3> where A is typically Pb or Bi, B is typically Ru or Ir and 0 < 1, and 0 < 1. 

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. 

Exactly as for MgSiOs, high pressure stabilizes a variety of compositions in the compact perovskite structure. Competition with other structures, such as ilmenite, defect pyrochlore, and various polytypes of perovskite, as a function of ionic radius, pressure, and temperature has been reviewed. ... 

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