Bull, Craig Lawrence; (2002) Structure-property relationships of some mixed transition metal perovskites. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis describes the preparation, characterisation and physical properties of a series of binary transition metal containing perovskite materials of the type Ln2CoMn06 and Ln2NiMn06 (Ln=La, Pr, Nd, 'Pm', Sm, Eu and Gd). We show the temperature dependence of x-ray diffraction (XRD), infrared spectroscopy, magnetic, x-ray absorption spectroscopy (XAS), electrical resistivity and neutron diffraction. These techniques have been used to determine the physical characteristics of the materials and structure property relationships have been derived. These are explained using orbital overlap and bond covalency arguments within the metal oxygen sublattice. Changes in electrical resistivity, electrical band gap, magnetic Curie temperature, infrared absorption and unit cell volume are observed upon changing size of lanthanide ion. These properties have been explained by considering orbital overlap arguments. The local and electronic structure have been investigated using X-ray Absorption Spectroscopy (both K-edge EXAFS and L-edge XANES), allowing the determination of oxidation states of the transition metal ions within the sub-lattice which are shown to be non-integral. Magnetism has also been used to tentatively assign oxidation states of the transition metal ions which is in agreement with the XAS. La2CoMn06 and La2NiMn06 have been reported previously as having an orthorhombic or pseudo-cubic structure. Neutron diffraction has shown for the first time that this is not true. Due to B-site ordering of the transition metal ions, the true structure is monoclinic. Furthermore, we have observed a structural phase transition where the symmetry changes from monoclinic to rhombohedral upon heating.

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