Neutron diffraction studies of the ferroelectric phase of CdTiO3

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International Union of Crystallography
Cadmium titanate (CdTiO3) is relatively poorly studied due to the toxicity of cadmium and difficulties in obtaining pure CdTiO3 since it has only a moderate stability with respect to the oxides. CdTiO3 can be synthesised with either an ilmenite or perovskite type structure. The ilmenite-like phase of CdTiO3 is unstable at high temperatures and undergoes an irreversible reconstructive phase transition to the perovskite phase near 900 °C. The perovskite phase decomposes, through the loss of Cd, if heated above 1000 °C. In recent years, there has been growing interest developing thin films of cadmium for a variety of uses including as a photocatalyst. The precise structure of the perovskite phase of CdTiO3 is uncertain. This is a consequence of the combination of its ferroelectric properties and the subtleties in the various octahedral tilting schemes observed for perovskites. A ferroelectric structure for CdTiO3 at room temperature in Pc21n, and a non-polar in Pbnm have been reported. Studies showed that CdTiO3 undergoes a displacive ferroelectric phase transition at about 80 K, with X-ray analysis suggesting the low temperature phase is in Pn21a or P21ma while the room temperature paraelectric phase is in Pbnm. In the present work we have used high resolution neutron diffraction methods to refine the structure of the hree phases of CdTiO3, namely the paraelectric ilmenite and perovskite phases and the ferroelectric perovskite phase. It is expected that neutron diffraction will provide a more accurate and precise description of these structures compared with X-ray diffraction methods due to the presence of the heavy Cd cations. To circumvent the high neutron absorption cross section of naturally occurring Cd we used samples enriched in 114Cd. Cooling perovskite-type CdTiO3 to 4 K induces a ferroelectric phase transition, with the neutron data suggesting the low temperature structure is in Pna21 (Figure 1). Solid solutions of the type Cd1-xCaxTiO3 could be prepared. Invariably this required the use of relatively high temperatures resulting in the formation of perovskite-type oxides and we did not find any evidence to suggest appreciable amounts of Ca could be incorporated into the ilmenite type CdTiO3 structure. Interestingly we could not prepare solid solutions of the type Cd1-xSrxTiO3 using conventional methods. There are only 5% of Sr and 5% of Ca can be doped in CdTiO3 in the solid solution of CaxSrxCd1-xTiO3. This is somewhat remarkable given the relative ease with which oxides of the type Ca1-xSrxTiO3 can be prepared and suggests the A-O bonding is playing a significant, but poorly understood role in stabilising the oxides. There is ample evidence that altering the A-cation significantly alters the hybridisation between the B-site metal t2g d states and the O p π orbitals © International Union of Crystallography.
Cadmium, Titanates, Perovskites, Oxides, Phase transformations, Ferromagnetism
Zhou, Q., Kennedy, B. J., & Avdeev, M. (2011). Neutron diffraction studies of the ferroelectric phase of CdTiO3. Poster presented at XXII IUCr Congress, Madrid, Spain, 22-30 August 2011. In Acta Crystallographica Section A: Foundations and Advances, A67, C246. doi:10.1107/S0108767311093858