High Resolution Structural Studies of Electrically Active Ceramics

Abstract

The gallium ferrite solid solution series Ga(2-x)FexO3 (with x = 0.8-1.3, 0.5 steps) has been synthesised and analysed by very high resolution powder diffraction to obtain average errors (from Rietveld refinement) of  0.00003 A (atomic coordinates),  0.000004 A (lattice parameters) and  0.00005 on the centroids of the metal atom sites. The samples were carefully prepared by etching and the diffraction data were absorption corrected. This led to an average GoF of 1.2 and well behaved thermal displacement parameters. This level of accuracy was sufficient to identify a departure from Vegard's law in the solid solution series with local metal site distortions correlated with chemical composition just above and below the stoichiometric 1:1 composition.Electrical impedance measurements from two independently synthesised sets of Ga(2-x)FexO3 (x ~0.8 - 1.3) identified an increase in conductivity with iron content precisely correlated with a lowering of the metal site distortions. Microstructure investigations by SEM showed grain growth with annealing time but no real pattern change with composition.Atomic site ordering as a function of composition was investigated by SuperSTEM high resolution electron microscopy. The results showed the Ga1 and Fe1 sites were always preferred by gallium and iron respectively whilst excess Fe in the solid solution series was always deposited on the Ga2 site and excess Ga was always deposited on the Fe2 site. The ordering of excess Fe when viewed in the a-c plane showed that every 4th metal layer received the excess iron; the metal sites showed no ordering into the c axis of the sample. Two different systems were designed to apply in-situ external electric fields on two samples, single crystal PMN-PT and polycrystalline PZT. Diffraction data were collected synchronously with the PE loops. The systems were designed to cycle at different frequencies and different waveforms. The resultant reflections were fitted to a number of models however acceptable fits were obtained for at least two of these models making it impossible to distinguish the electric response clearly. More extensive area detector data collected on BM01 at ESRF showed that the structure of the PMN-PT material was single phase monoclinic. The PZT data was used to verify the performance of the polycrystalline system. Micro actuators were placed on the crystal to simultaneously measure strain. The results suggest a synthesis route to an increased magnet electric coupling for the gallium ferrites at room temperature with x~1.1; longer annealing times and slower cooling rates. This is based on the increased resistivity for x ~ 1.1 and a dense sintered solid or thin film to decrease losses.

Date of Award	31 Dec 2016
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Robert Cernik (Supervisor)