The Chemistry and Crystallography of Doped Bixbyite, Braunite and Novel Anion-Deficient Fluorite Oxides

The properties of any one material can be purely attributable to its chemical constitution and structure. The bixbyite and many related defective fluorite structures have received an increasing amount of attention for at least the last decade for their application as semiconductors, optoelectronics, superconductivity, magnetism, and catalysis. The bixbyite structure has deep historical roots within solid-state chemistry and crystallography, the first correct crystal structure was solved by Linus Pauling one year after he released The Principles Determining the Structure of Complex Ionic Crystals containing his now famous 5 rules. Of the bixbyite related structures Cu3TeO6 has possibly received the most attention recently for its magnetic properties. A review of the literature reveals multiple different magnetic structures being cited, studies with samples of varying quality and often limited sample characterisation. Flux free vapour aided abnormal grain growth (AGG) methods were developed to grow large neutron quality single crystals which were used to conduct the first low temperature single crystal Laue diffraction experiments to verify the magnetic structure. This was coupled with neutron powder diffraction (NPD) and magnetic susceptibility measurements. A single crystal X-ray diffraction (XRD) study comparing these crystals with crystals grown through traditional chemical vapour transport (CVT) allowed for insight in to the frequently overlooked atomic displacement parameters which appear to be anharmonic in nature. This observation appears not to be unique to Cu3TeO6 but rather is present in most bixbyites likely due to a small rattling type behaviour. Powders of the solid-solution Cu3-0.75xTe1-xSbxO6-0.125x has been previously synthesised, single crystal growth of Sb doped Cu3TeO6 was attempted here with varying success. NPD experiments were conducted between 4 and 1200 K to explore the temperature dependent effects of Sb doping on the magnetic structure, crystal structure and oxygen occupancy in conjunction with magnetic property measurements. The solid-solution exhibits re-entrant and canonical spin glass behaviour with increasing Sb concentration. The ordering/freezing temperature decreases with doping as magnetic frustration is maximised which can be primarily attributed to increasing disorder from mixed site occupancy. Previously a colour transition with heating had been observed which was hypothesised to occur due to a thermochromic phase transition, possibly related to oxygen occupancy. There was no identifiable phase transition at higher temperatures, however the oxygen deficiency is removed with heating. X-ray absorption spectroscopy (XAS) experiments at the Cu K and L-edge confirm the differences in cation disorder between the pure copper tellurate and antimonate and evidence of Cu1+ with increasing Sb concentration. O K-edge spectra reveal a complicated environment for the copper antimonate and some evidence of oxygen holes in Cu3TeO6 at least on the surface. The unexpected depth and complexity which resulted from the doping of Cu3TeO6 with Sb inspired the investigation into alternative dopants. The solid-solutions of Cu3-xMnxSbO6, Cu3-xFexTeO6, and Cu2Mn1-xFexSbO6 were synthesised and characterised with various techniques. Cu3-xMnxSbO6 showed complicated doping behaviour, it was found through powder XRD and NPD that the addition of Mn removes the previously observed oxygen deficiencies. It was found that there are two separate doping regimes above and below x ≃ 1 which can be related to the preferential 24d site filling of Mn coupled with changes in redox. This was confirmed with XAS of the Mn, Cu L and O K-edge where Mn2+ concentration increases with Mn concentration. All compositions lacked long-range magnetic ordering down to 4 K from NPD experiments with the exception of Cu2MnSbO6 where significant ordered domains are present. Magnetic property measurements would show a temperature dependent hysteretic behaviour possibly indicating a domain restricted non-collinear ordering. Attempts to prepare powders and grow crystals of the Cu3-xFexTeO6 solid-solution had limited success with very limited solubility of Fe in powders (x ≃ 1) and less so in CVT grown single crystals. This is the first time an Fe containing member of the A3TeO6 type has been synthesised. Powders of Cu2Mn1-xFexSbO6 were synthesised, from NPD Rietveld refinements there is a clear preference for Mn over Fe on the 8b site when Sb is displaced which remains true until x ≥ 0.8. Similar to the Cu3-xMnxSbO6, there is no apparent long-range magnetic ordering in Cu2Mn1-xFexSbO6 showing frustrated glassy behaviour. The first ever single crystals of Cu2FeSbO6 were grown through AGG methods and characterised with single crystal XRD. Attempts to grow single crystals of either the pure copper antimonate Cu2.77Sb1.23O5.87 or Cu2MnSb2O6 through CVT and/or vapour aided AGG resulted in the growth of the parwelite related structure Cu5Sb2SiO12 and a new braunite related phase Cu4MnSb2SiO12, respectively. The unintentional synthesis of Cu5Sb2SiO12 and Cu4MnSb2SiO12 both from bixbyite related precursor powders possibly highlighted the energetic similarities between the structures. The relationship between defective fluorite related structures, in particular bixbyites and braunites was first noted by Pauling from comparison of unit cell parameters. Cu4MnSb2SiO12 can be considered a cation ordered variant of the braunite Mn,Mn6SiO12. The solid solution Cu5-xMnxSb2SiO12 was synthesised, there was no intermediate phase found as would be predicted through group theory, rather both the orthorhombic Cu5Sb2SiO12 and tetragonal Cu4MnSb2SiO12 phases co-existed with higher synthesis pressures and the use of Mn2+ favouring the tetragonal phase. Single crystals of various compositions of Cu5-xMnxSb2SiO12 were grown, most contained super structure reflections which can be identified as epitaxial intergrown Cu5Sb2SiO12 within the majority tetragonal Cu4MnSb2SiO12 phase crystals. This polysomatic relationship can be rationalised through an order-disorder (OD) relationship between Mn and Cu and their occupation of the sole cubic coordination site of v Cu4MnSb2SiO12 which dictates the local site symmetry. This would also predict the preferential occupancy of the cubic site by Mn2+ over Mn3+ which partially alluded from varying the synthesis conditions and partially supported by XAS experiments. The intimate OD relationship observed in this polysomatic system builds upon analogues observations that have been made with regarding the bixbyite-braunite-braunite II relationship. On exploring other potential copper containing bixbyites Cu2GaSbO6 was found. Although previously described, the synthesis conditions for Cu2GaSbO6 were optimised as it was found that higher synthesis pO2 stabilised the structure by minimising the reduction of Cu2+. The first single crystals of Cu2GaSbO6 were grown via CVT and AGG while also producing twinned crystals of a new structure Cu3Ga3SbSiO12. Cu3Ga3SbSiO12 is a new crystal structure type containing the longest recorded Ga-O bond and Ga in the rare 5-fold coordination and is another example of a Cu Kagome staircase compound. From closer inspection it can be seen to be rationalised as being structurally related to the bixbyite-braunite-parwelite defective fluorite structures, likely the most complex of all. The remarkable coordination and cation ordering of Cu3Ga3SbSiO12 gives evidence for the possibility of many other alternative cation ordered structures related to the Cu3Ga3SbSiO12 and Cu4MnSb2SiO12 structure. Although magnetic property measurements could not be conducted for Cu3Ga3SbSiO12 it is generally accepted that the more geometrically distorted a Kagome lattice is, the lower the magnetic frustration is expected to be. With this assumption an empirical method is suggested and tested for comparison of different Kagome lattices with a Kagome distortion index suitable for both 2D and buckled/staircase lattices.

Name thesis:degree_name

PhD

Level thesis:degree_level

Doctoral

Discipline thesis:degree_discipline

Chemistry

Grantor dc:publisher

ResearchSpace@Auckland

Year dc:date.issued

2022

Author dc:creator

• Spasovski, Martin

Advisor dc:contributor.advisor

• Söhnel, Tilo

Statement dc:rights

• Items in ResearchSpace are protected by copyright, with all rights reserved, unless otherwise indicated.

Licence dc:rights.uri

• https://researchspace.auckland.ac.nz/docs/uoa-docs/rights.htm

Handle dc:identifier.uri

https://hdl.handle.net/2292/67267

OAI identifier oai:identifier

oai:researchspace.auckland.ac.nz:2292/67267