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 cha racterised by X-ray powder diffraction and Raman spectroscopy. Mn and In ha ve a solubility around x = 0.1-0.2 in Pr2Zr2-xMnxO7 and Pr2Sn2-xInxO7, resp ectively. In the series Pr2M2-xMxO7, where M = Sn, Zr and M' = In, Ce, we o bserve dopant-O-6 symmetrical stretch vibrations in addition to the host la ttice modes. A defect model of a B site doped pyrochlore is developed with Pr3+ on the A site; Zr-B(x) (Zr4+), Ce-B' (Ce3+), Ce-B(x) (Ce4-) on the B s ite; O-O(x) and V-O on the O site, interstitial oxygens O-i", and delocalis ed electrons and electron holes. Four mass action law expressions govern su ch a model. The defect model can rationalise why home-valent doping, i.e. s ubstitution of Zr(4+) by Ce(4+), can lead to an increase in ionic conductiv ity. The calculated Brouwer diagram for Pr2Zr1.6Ce0.4O7+/-delta is shown. T his composition has a transition from mixed ionic p-type to presumably pure ionic conduction around pO(2) = 10(-7.5) atm. At pO(2) < 10(-15) atm the m aterial gradually changes into the n-type regime. Typical magnitudes are fi nally given for the four equilibrium constants, leading to cases of pure p- type, p- to n-type and pure electrolytic behaviour of doped pyrochlores. (C ) 2000 Elsevier Science B.V. All rights reserved.