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 Pr2Zr2−xMnxO7 and Pr2Sn2−xInxO7, respectively. In the series Pr2M2−xM′xO7, where M=Sn, Zr and M′=In, Ce, we observe dopant–O6 symmetrical stretch vibrations in addition to the host lattice modes. A defect model of a B site doped pyrochlore is developed with Pr3+ on the A site; ZrBx (Zr4+), CeB′ (Ce3+), CeBx (Ce4+) on the B site; OOx and VO·· on the O site, interstitial oxygens Oi″, 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 Pr2Zr1.6Ce0.4O7±δ is shown. This composition has a transition from mixed ionic p-type to presumably pure ionic conduction around pO2=10−7.5 atm. At pO2<10−15 atm the material gradually changes into the n-type regime. Typical magnitudes are finally given for the four equilibrium constants, leading to cases of pure p-type, p- to n-type and pure electrolytic behaviour of doped pyrochlores.