Oxide ion conductivity in a geometric ferroelectric

Lanthanum tantalate, LaTaO4, is one of the few oxide examples of an n = 2 member of the Carpy-Galy phases of general formula AnBnX3n+2. These are 2-dimensional perovskite-related materials where the perovskite network disrupted by introducing ‘excess’ anions resulting in the formation of perovskite layers n-octahedra deep. LaTaO4 is unusual in that on heating it successively transforms from non-polar monoclinic to incommensurate polar orthorhombic at ca. 440 K to commensurate polar orthorhombic at 500 K. These transitions result from a change in tilting of the octahedral units in the perovskite layers and is therefore an example of a ‘geometric’ ferroelectric. Selective doping with Ce at the La-site and subsequent controlled partial oxidation of Ce (+III) to Ce (+IV) gives rise to interstitial oxide ions in between the perovskite layers and results in oxide ion conductivity. It is proposed that the geometric mechanism of ferroelectricity in this structure should therefore be robust to the presence of these additional charge carriers. The aim of this study is to determine the influence of both Ce(III)-doping and partial oxidation to Ce(IV) on the structural evolution, and stability of the ferroelectric phase, of LaTaO4.