Exploring the synthesis and characterisation of oxychalcogenide, oxypnictide and chalcogenide materials
Keywords: antiferromagnetism
long-range magnetic ordering
neutron powder diffraction

This thesis details various studies of the structure, characterisation and physical properties of some oxychalcogenide, oxypnictide and chalcogenide materials, with a particular focus on the long-range magnetic ordering of the local 3d transition metal moments.
A wide range of characterisation methods are used to investigate the crystal structure of the Ca2FeO2Fe1.6OS2 oxysulfide, including synchrotron XRPD, high-resolution NPD, XANES, XPS and Mössbauer spectroscopy. The ‘Fe1’ layer hosts Fe3+ in FeO4S2 distorted octahedra and the ‘Fe2’ layer contains Fe2+ in FeO2S4 distorted octahedra. Long-range Fe vacancy ordering exists in the ‘Fe2’ layer, resulting in a supercell which is a 2√2𝑎𝑛𝑢𝑐 × 2√2𝑎𝑛𝑢𝑐 × 2𝑐𝑛𝑢𝑐 expansion of the basic nuclear unit cell. This vacancy ordering is accompanied by movement of the Fe and O atoms in this layer, all of which can be described using the Aea2 space group. Long-range ordering of the two Fe sublattices occurs simultaneously at 330(10) K and analysis of NPD data reveals that the Fe moments order in the ab-plane in a manner which ensures all Fe-O-Fe superexchange pathways contain antiferromagnetic coupling of spins. A second antiferromagnetic ordering transition occurs at 70(2) K, which can be modelled by canting of the spins in both Fe layers.
The Ca2FeO2Fe1.65OS1.5Se0.5, Ca2FeO2Fe1.7OSSe and Ca2FeO2Fe1.75OS0.5Se1.5 solid solution phases are isostructural to the Ca2FeO2Fe1.6OS2 material but do not exhibit long-range ordering of the Fe vacancies in the ‘Fe2’ FeO2Ch4 layer. The long-range magnetic ordering models obtained from NPD data are comparable to that deduced for Ca2FeO2Fe1.6OS2, where all Fe-O-Fe superexchange pathways have antiferromagnetic interactions of the Fe moments. The most selenide-rich Ca2FeO2Fe1.75OS0.5Se1.5compound undergoes spin-reorientations of the Fe3+ spins on the ‘Fe1’ sublattice with changing temperature.
First reports of the structure and properties of the newly synthesised Sr2CrO2Cr2OAs2 phase are also presented. This material is again isostructural to the Ca2FeO2Fe1.6OS2 material, however in this case no transition metal vacancies or any deviations from the formula stoichiometry are evident. Anothercontrast to the Ca2FeO2Fe1.6OS2 material is that Cr2+ is present in the ‘Cr1’ CrO4As2 layers and the ‘Cr2’ CrO2As4 layers host Cr3+ (i.e., the transition metal oxidation state site preferences are reversed). The Cr sublattices order independently, with the ‘Cr2’ (Cr3+ d3) moments ordering just above 600 K and the ‘Cr1’ (Cr2+ d4) moments ordering at 530(10) K, all parallel to the c-axis. It is likely that antiferromagnetic direct exchange interactions between Cr centres in the ‘Cr2’ sublattice are the driving force behind the nature of the ordering observed in this layer, whereas antiferromagnetic Cr-O-Cr superexchange interactions dictate the checkerboard ordering adopted by the ‘Cr1’ sublattice.
Long-range magnetic ordering of the Fe3+ spins in the Ba2FeSbS5 and Ba2FeBiS5 quaternary sulfides is modelled from NPD data and the arrangement of the moments can be explained by Fe-S…S-Fe super-superexchange (SSE) pathways. These ensure that Fe centres coupling over the shortest distance are antiferromagnetic and the relatively weak interactions between the Fe spins via these pathways results in the very low Néel temperatures of 12.1(10) K and 34.5(10) K observed for Ba2FeSbS5 and Ba2FeBiS5 respectively. Small magnetic Bragg peaks in the NPD pattern of Ba2FeSbS5, which are not positioned on a commensurate k-vector, suggest an additional incommensurately modulated component is present in the long-range ordering of the Fe moments.