Waldron, J. E. L.; (2001) Synthesis, Structure and Physical Properties of Selected Transition Metal Oxides. Doctoral thesis (Ph.D), UCL (University College London).

Text Synthesis, structure and physical properties of selected transition metal oxides.pdf Download (18MB)

Abstract

Monoclinic Nb12O29 has been synthesised by topotactic reduction of H-Nb2O5. It has a complex crystallographic shear structure made up of 3x4 blocks of corner shared NbO6 octahedra. Nb12O29 is mixed d0/d1 compound which, can be written as Nb4+2NB5+10O29. It has been found, using Rietveld refinement of powder neutron diffraction data, to undergo a charge ordering transition at around 12K resulting in a lowering of symmetry from A2/m to Am. Below this transition, one Nb4+ ion is localised on a crystallographically distinct site in the centre of the 3x4 blocks. This charge ordering transition has an associated transition to an incommensurate one dimensional antiferromagnetic state. The nature of the magnetic order has been investigated using susceptibility and SR measurements. Susceptibility measurements reveal that only 48% of the Nb4+ in the system is localised, and its temperature dependence is well fitted to a Bonner-Fisher model for S=1/2 one dimensional antiferromagnets. It is found that the remaining valence electrons in NB12O29 itinerant and contribute to its metallic conductivity. Further evidence for the unique properties of monoclinic Nb12O29 has been obtained by doping the structure with small amounts of diamagnetic Ti4+, which rapidly causes a destruction of the long range magnetic order at only 3% doping, but leaves the metallic conductivity unaffected, as is expected for a one dimensional chain.

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