Abstract:
In this study, a number of compounds in the Ho2(ZryTi1-y)2O7 pyrochlore system, which undergoes an order-disorder structural transition from pyrochlore to a defect-fluorite type, were investigated. The materials were processed by mechanical milling, followed by a two-step sintering route. The structure was analyzed via Rietveld refinements using neutron powder diffraction (NPD) data, and further supported by X-ray diffraction (XRD) to identify the phases and determine the composition of the pyrochlore-fluorite transformation boundary at room temperature. The NPD refinement and XRD results show that compositions with 0.0 ≤ y ≤ 0.5 crystallized into a pyrochlore structure while the ones with 0.7 ≤ y ≤ 1.0 crystallized into the defect-fluorite structure. The composition with y=0.6 was found to be a two phase (pyrochlore and defect-fluorite) mixture. Neutron refinement results confirmed that the cation disorder was independent of the anion Frenkel disorder and also the degree of Frenkel disorder O(1)-O(3) was different from the O(2)-O(3) anion disorder. The relation between x-parameter and anion Frenkel disorder was found to be linear for the pyrochlore structure. Unit-cell parameters were obtained using Rietveld refinements for the whole series and were also calculated via the Nelson-Riley function. The ionic conductivity studies were undertaken via AC impedance analysis to determine the electronic behaviour and its relationship to the structural change in the available temperature range of 300–700°C. The trends in ionic conductivity and activation energy were explained on the basis of NPD and XRD data. The composition at the pyrochlore-fluorite boundary exhibited the lowest activation energy and highest ionic conductivity.
