The Effect of Processing Conditions and W-Doping on the Structural and Magnetic Properties of α and γ CuMo_1-xW_xO₄

This thesis reports the results of structural and magnetic measurements of CuMo_1-xW_xO₄. A structural transition occurs in this material between the alpha and gamma phases, induced through a temperature reduction below 200 K, or an applied pressure. The effect of different processing conditions on this transition, and on the structural and magnetic properties of the alpha and gamma phases, is reported. This was done using as synthesised, thermally cycled, air annealed, and tungsten-doped samples.

Thermal cycling leads to a reduction of the transition into gamma phase, proportional to cycling level. Air annealing leads to a complete removal of the transitional capabilities in the undoped sample. Tungsten-doping increases the transition into gamma phase, proportional to the doping percentage (in the range of 0% to 8%). These processing methods all also cause significant changes to the inter-grain region of the crystallites. This prompts the construction of an inter-grain phase nucleation model, which poses that the transition is initialised in this region, from where it spreads to the bulk. We build on this by proposing that the transitional mode is linked to atomic disorder in this inter-grain region (i.e., oxygen vacancies or tungsten ions), based on observations in structural and magnetic data. This picture of the phase transition is original to this thesis.

The low temperature magnetic properties of CuMoO₄ are reported in this thesis, in both mixed phase (alpha and gamma) and, for the first time, in 100% alpha phase. The mixed phase measurements give insight into the unexplained discontinuity between the theoretical Curie constant ratio (between alpha and gamma) of 1/3, and the experimental ratio (between the high temperature and low temperature regimes) of 1/2. The low temperature magnetic properties of the 100% alpha phase are, until now, entirely undocumented. The compound exhibits highly complex magnetic behaviour. Linear magnetic field loops at 2.2 K suggest antiferromagnetic ordering, although, the T_N is not observed in the susceptibility data. However, following application of a high magnetic field at low temperature (6 T at 2.2 K) a well-defined magnetic peak at 6 K, indicating a transition into antiferromagnetic order, is observed. This may be due to the high field inducing a spin reorientation into an ordered state.