Mixed anion control of negative thermal expansion in a niobium oxyfluoride

A significant change in thermal expansion with anion composition has been identified in the niobium oxyfluoride, NbO2−xF1+x from 0 < x < 0.6. Fluorine doping leads to a transition from positive thermal expansion to unusual zero and negative thermal expansion caused by transverse anionic vibrations. This work has consequences for the development of advanced technological materials with tuneable low thermal expansion and is the first example of the use of multiple anions to control thermal expansion

Experimental and computational studies of low-dimensional functional materials

This thesis describes the synthesis, characterisation and density functional theory investigation of a range of MX$_2$O$_4$ compounds related to Schafarzikite, FeSb$_2$O$_4$. Chemical substitution of the M cation has been performed to yield Mn$_x$Co$_1$$_−$$_x$Sb$_2$O$_4$, which is characterised both structurally and magnetically. Additionally, the synthesis and magnetic behaviour of the mineral Trippkeite (CuAs$_2$O$_4$ has been investigated. Density functional theory calculations have been performed for a wide range of MX$_2$O$_4$ compounds, both to investigate structural behaviour at high pressure, and also to rationalise the experimentally observed magnetic order. In addition, the technique has been used to predict the magnetic groundstate of CuAs$_2$O$_4$, before an experimental validation. The structurally-related compounds Versiliaite and Apuanite have been synthesised for the first time, and characterised both structurally and magnetically. Their relation to the Schafarzikite structure is discussed, as are the differences in magnetic ordering

The Structures and Magnetic Properties of FexCo1 xSb2O4 and MnxCo1 xSb2O4, 0 ≤ x ≤ 1

MxCo1−xSb2O4 (M = Mn, Fe) shows rotation of magnetic moments within the structure, tuneable with increasing x.</p

Supplementary data for The Structures and Magnetic Properties of FexCo1 xSb2O4 and MnxCo1 xSb2O4, 0 ≤ x ≤ 1

MnxCo1-xSb2O4 and FexCo1-xSb2O4 have been synthesised for 0 ≤ x ≤ 1 and their structures and magnetic properties examined. For all compounds, neutron powder diffraction (NPD) data reveal a canted AFM structure that changes gradually from C-type (x = 0) to A-type (x = 1). This transition corresponds to a gradual rotation of the moments through 90o, from ±[001] to ± [100]. It is primarily caused by a change in the relative magnitudes of the three types of magnetic exchange that exist between cations. Within a given chain, direct exchange promotes an antiferromagnetic ground state for the two cations and 90o superexchange that favours ferromagnetic order. Between chains, antiferromagnetic order is preferred. However, the observed magnetic moments (from NPD) are significantly lower than expected except for the end-members of the series; this suggests that incomplete magnetic order is present. Magnetic susceptibility data also suggest complex magnetic behaviour except for the end-member compounds. The complex magnetic features appear to originate from composition inhomogeneity, local magnetic order in the chains of octahedra being dependent on small clusters of the same transition metal ion and the delicate energy balance that clearly exists between the two ordered configurations in the mid-composition region where x is near to 0.5