Synthesis and properties of CaFe3O5 and related materials

Abstract

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].