The synthesis and characterisation of some hexagonal perovskites

The structural chemistry and magnetic properties of a number of manganese containing hexagonal perovskites have been studied by X-ray diffraction, neutron diffraction and magnetometry. Trends in the magnetic properties are investigated as a function of the hexagonal stacking sequence and manganese oxidation state.
The synthesis of the series of BaMnO3-x hexagonal perovskites is discussed. By varying both the partial pressure of oxygen and the firing temperature, stacking sequences with a range of hexagonal to cubic layer ratios can be synthesised. Factors which increase the structural tolerance factor are found to increase the proportion of hexagonal layers in the stacking sequence. The crystallographic properties of the BaMnO3-x compounds are discussed, and the oxide vacancies are found to be exclusively located in the face-sharing hexagonal (h) layers, particularly those which are adjacent to two apex-sharing cubic (c) layers. The preferential localisation of oxide vacancies in chc sites can be used to rationalise the observed stacking sequences, as well as the limiting stoichiometries of BaMnO3-x structures.
The magnetic behaviour of the BaMnO3-x phases is investigated, both individually and as a function of the stacking sequence. A strong direct exchange interaction between face-sharing cations couples all magnetic moments antiferromagnetically above ambient temperature. A slightly weaker interaction (utilising the 180° superexchange pathways between apex-sharing MnOe octahedra) results in three-dimensional antiferromagnetic order at a temperature in the range 230 ≤ T (K) ≤ 280. The strength of this interaction is found to be dependent on the length of the face-sharing chains. A third interaction occurs at T ~ 45K, believed to represent a canting transition. Low temperature neutron diffraction data reveal that the magnetic moments order in a simple antiferromagnetic manner. The magnitude of the ordered moment shows a dependence on the length of the face sharing chains, where phases with shorter chains have a larger ordered moment. Long-range magnetic order is disrupted by charge disorder in the mixed Mn(III)/Mn(IV) system 4H-BaMnO2.6 5(1).
The hexagonal BaMn1-zTiz O3- x and BaMn1-zZrz03-x systems were also studied. Synthesis under argon atmospheres allows a range of new phases to be accessed. These include a novel manganese-zirconium phase, 6H-BaMn0.8Zr0.2O2.81(1), and BaMno.55Ti0.45,/sub>O3-x, which adopts the rare 15R' stacking sequence. The BaMn1-z TizO3-x system confirms that although the tolerance factor controls the proportion of cubic layers, it does not control the arrangement of these layers. This is controlled by the size of the B cations and the proportion of oxide vacancies. Partial cation order occurs in 6H-BaMno.8Zr0.2O2.81(1), which can be rationalised on the basis of the size difference between manganese and zirconium ions.
Topotactic reductions using binary hydrides are carried out in order to decouple the manganese oxidation state from the adopted stacking sequence. Using hydride reductions, 4H-Bao.5Sro.5MnO3-x can be topotactically reduced to 4H-Bao.5Sro.5MnO2.o2(1). This increases the strength of the 180° superexchange interaction, and antiferromagnetic order is achieved above ambient temperature. Using the same techniques, 4H-BaMnO3-x can be reduced to 4H-BaMnO2.47(1) or 4H-BaMnO2.o6(1). These phases undergo a distortion to an orthorhombic unit cell.

Advisor: Hayward, M