Structure, Raman spectra and defect chemistry modelling of conductive pyrochlore oxides

Mixed ionic–electronic conducting pyrochlore structure oxides, with Pr and Gd on the A site and Zr, Mn, Ce, Sn, In, Mo, and Ti on the B site, were characterised by X-ray powder diffraction and Raman spectroscopy. Mn and In have a solubility around x=0.1–0.2 in Pr 2Zr 2− x Mn x O 7 and Pr 2Sn 2− x In x O 7, respectively. In the series Pr 2M 2− x M′ x O 7, where M=Sn, Zr and M′=In, Ce, we observe dopant–O 6 symmetrical stretch vibrations in addition to the host lattice modes. A defect model of a B site doped pyrochlore is developed with Pr 3+ on the A site; Zr B x (Zr 4+), Ce B′ (Ce 3+), Ce B x (Ce 4+) on the B site; O O x and V O ·· on the O site, interstitial oxygens O i″, and delocalised electrons and electron holes. Four mass action law expressions govern such a model. The defect model can rationalise why homo-valent doping, i.e. substitution of Zr(4+) by Ce(4+), can lead to an increase in ionic conductivity. The calculated Brouwer diagram for Pr 2Zr 1.6Ce 0.4O 7± δ is shown. This composition has a transition from mixed ionic p-type to presumably pure ionic conduction around pO 2=10 −7.5 atm. At pO 2