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Title: Synthesis and properties of CaFe3O5 and related materials
Author: Hong, Ka Hou
ISNI:       0000 0004 7969 2109
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 2019
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The iron oxide family has become one of the most intensively studied transition metal oxide systems since the discovery of the Verwey transition in magnetite (Fe₃O₄) in 1939. The ground state structure of Fe₃O₄ was only recently solved and revealed a complex charge and orbital ordered arrangement with weak Fe-Fe bonding interactions giving rise to trimerons, linear orbital molecule clusters of three Fe ions. A closely related phase, Fe₄O₅, was recently discovered and was found to undergo incommensurate charge order that led to the formation of dimeron and trimeron like groups at low temperature. Apart from Fe₄O₅, very little study has been carried out on this system. This Thesis explores different analogues of M²⁺Fe₃O₅ (with M = Ca, Mn, Co and Ni). Physical property measurements and diffraction techniques were used to study the ground state structures of these mixed Fe²⁺/Fe³⁺ valence state phases, to investigate the charge, spin and orbital ordering phenomena that are involved. The M²⁺ = Ca analogue, CaFe₃O₅ was synthesised using the ceramic method at ambient pressure. Diffraction studies reveal an electronic phase separation when cooled below a magnetic transition at 302 K, where the high-temperature paramagnetic phase separates into two phases with different electronic and antiferromagnetic ordering. One of the phases has charge ordered Fe²⁺/Fe³⁺ with trimeron formation and the other has a charge averaged structure with infinite chains of orbital molecules. High-pressure ceramic methods were used to synthesise M²⁺Fe₃O₅ phases with small M²⁺ cations (M = Mn, Co and Ni). MnFe₃O₅ was synthesised at a pressure of 10 GPa. Magnetisation studies show a rich variety of magnetic states when cooled below 350 K. Spin order of the Fe cation site is observed below 350 K and result in antiferromagnetism. A second transition at 150 K marks the Mn spin order that leads to spin canting of some of the Fe spins and ferrimagnetism. A further magnetic transition at 60 K, driven by charge ordering of Fe²⁺ and Fe³⁺, results in further spin reorientation and an enhancement in the magnetisation of MnFe₃O₅. The crystal structure of MnFe₃₄O₅ remains in the space group Cmcm within the investigated temperature range of 5-400 K. The CoFe₃O₅ phase was stabilised under 12 GPa of pressure. A neutron diffraction study shows Co/Fe cation disorder in CoFe₃O₅. Similar to MnFe₃O₅, an antiferromagnetic transition is observed near room temperature, at 300 K, from the spin order of the octahedral sites. The triangular prismatic site is magnetically ordered when cooled below 100 K and leads to the spin of the octahedral site to cant and ferrimagnetism. CoFe₃O₅ shows semiconducting behaviour, with a negative magnetoresistance effect of 5% at 125 K. The charge of Fe²⁺/³⁺ in CoFe₃O₅ remains disordered down to 5 K. The absence of charge order is likely due to the strong exchange interactions between the cations in the octahedral sites along the a axis. An even higher pressure was used to synthesise NiFe₃O₅. Structure and property studies show an antiferromagnetic transition at ~275 K that marks the spin order of the octahedral sites in NiFe₃O₅. This is followed by an incommensurate magnetic ordering below ~150 K. A further magnetically ordered states is observed at ~20 K, where the spin of the three cation sites are ordered antiferromagnetically and propagate through the lattice with a k-vector of [½ ½ 0].
Supervisor: Attfield, John ; Parsons, Simon Sponsor: European Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: CaFe3O5 synthesis ; high-pressure synthesis techniques