Oxide Ion Conductivity in Geometric Ferroelectric Materials

The rapid expansion of the field of nanoelectronics has resulted in an increased interest in the application of ferroelectric domain walls (DWs). DWs separate regions of different electronic polarisation within a crystal. Their character is distinct from the bulk material and responsible for their unique properties which include enhanced conductivity and rectifying behaviour, when compared to the surrounding bulk.

DWs form spontaneously and contain free charge carriers (typically electrons and holes) which can be manipulated by external electric fields. These free charges interact with the bound charges associated with the ferroelectric polarisation of the bulk material. The polarisation of "proper" ferroelectrics arises from a second order Jahn-Teller distortion mechanism.

However, the electronic polarisation disappears on introduction of high mobility free charge carriers which act to screen the effect. It is hoped by using a "geometric" ferroelectric the geometric distortion responsible for the electrical polarisation should prove robust to the presence of additional free charge carriers.

Lanthanum tantalate, LaTaO4, is an example of an n = 2 member of the Carpy-Galy phases (general formula AnBnX3n+2). These are two-dimensional layered perovskite materials where the network of perovskite polyhedra is cleaved in the {110} plane by introduction of "excess" anions. LaTaO4 is unusual in this family 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.

The successive phase transitions result from a change to the tilting of the octahedral units in the perovskite in response to the under-bonded La3+ cations which are too small to occupy the inter-layer A sites in the absence of any tilting. LaTaO4 is hence an example of a "geometric" ferroelectric.

It is hoped introduction of low(er) mobility oxide ion charge carriers can be achieved through selective doping of LaTaO4 whilst maintaining the geometrically driven mechanism of ferroelectricity. Preliminary investigations have centred on doped composition of La1xLnxTaO4 with Ln = Ce, Pr, Nd and 0 x 0.3. Though the monoclinic phase has been observed to stabilise with decreasing lanthanoid radius, the materials all still transformed to polar, ferroelectric phases on heating.

However, the phase transition temperatures and evolution of lattice parameters as a function of temperature behaved anomalously for Ce-doped compositions and were dependent on the thermal history of the sample. This might suggest the Ce(III) partially oxidises to generate oxide interstitials which could allow for oxide ion conductivity in a geometric ferroelectric, provided the polar distortion endures the presence of the ionic free charge carriers.