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Title: The chemistry and magnetism of some rhenium and rhodium perovskite oxides
Author: Hasanli, Nijat
ISNI:       0000 0004 9355 6437
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2019
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This work focuses on synthesis and characterisation of novel 4d and 5d materials with attention given to spin, orbital and lattice effects seen in metal oxides with heavy transition-metals. The first half of the work is devoted to topochemical reduction of rhenium-containing perovskites following from the work on double perovskites with the composition Sr2BB’O6 (B = 4d/5d; B’ = 3d), that exhibit spin-polarisation and ferromagnetism. An attempt was made to make Re5+/6+ phases via low temperature methods to overcome synthetic challenges of cited phases. Treatment of Sr2ReLiO6 with 5%/95% H2/N2 gas resulted in formation of Sr2Re6+LiO5.5, a cation ordered double perovskite with partial oxygen vacancy ordering localised on alternate layers of a tetragonal perovskite unit cell. The phase showed extensive crystallographic twinning to minimize lattice strain built as a result of unusual vacancy positions. The phase showed high spin-orbit coupling effects despite the breakage of local Re symmetry. Following this, B’-site doped Sr2ReLi0.64Co0.36O6 was treated with dilute hydrogen gas to form Sr2ReLi0.64Co0.36O5.5, a solid-solution of Re5+, Re6+, Co2+, and Li1+ with oxygen vacancies distributed across the lattice in a disordered manner. This experiment confirmed that inclusion of Li can stabilise the 5d-3d metal oxide systems for topochemical reduction. The second half is devoted to the study of electric transport and magnetic properties of mixed-metal rhodium perovskite oxides. It is shown that La0.5Sr0.5Mn0.5Rh0.5O3 crystallizes in the Pnma space group with Rh and Mn distributed evenly in the B-site to form a solid-solution. Electric transport measurements show Mott’s Variable Range Hopping behavior across the whole temperature range but magnetic measurements show a transition to a ferromagnetic state at TC = 55 K suggesting ferromagnetic insulator type behavior. A study of structural and physical properties concludes the cations are found in Mn3+ and Rh4+ oxidation states and the ferromagnetism is a result of superexchange interactions between Mn3+ – Mn3+ centers with Rh4+ cations not involved in ferromagnetic exchange. A’-site doping with Ca2+ to make La0.5Sr0.5-xCaxMn0.5Rh0.5O3 (x = 0.15, 0.25) shows expected changes to unit cell parameters including decrease in volume, bending of the B-O-B angle, and a decrease in TC. The synthesis of the Ruddlesden-Popper analogue La0.5Sr1.5Mn0.5Rh0.5O4 is studied in detail and shown to crystallize in the I4/mmm space group when made in air but with phase separation due to oxygen inhomogeneity. Treating the anion deficient phases under flowing oxygen gas restores the stoichiometry. Finally the synthesis and characterisation of B-site disordered spin-glass La0.5Sr0.5Fe0.5Rh0.5O3 with the Fe3+ and Rh4+ oxidation state combination confirms the relative stability of Rh4+ over the generally accepted Rh3+ oxidation state.
Supervisor: Hayward, Michael A. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available