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Title: Phase transitions in Li-doped Ag(NbxTa₁-x)O₃ perovskite ceramics
Author: Khan, Hidayat Ullah
ISNI:       0000 0004 2720 9322
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2011
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Ceramics in the solid solution series, Ag(NbxTal-x)03 were fabricated to study their phase transitions and functional properties. As Ta concentration increased, the onset temperature of the M-phase transitions decreased in agreement with the Pawelcyck phase diagram. Electron diffraction and Raman spectroscopy revealed that the reduction in onset temperature may be attributed to a decrease in the correlation length and magnitude of cation displacements as the less polarisable Ta replaced the highly polarisable Nb ion in the B06 octahedra. For compositions with x = 0.75, a previously unreported phase transition was observed at ~ 3 l5K. Further studies were also performed on Li doped AgNb03, AgNbl/2 Ta 1/203 and AgTa03 compositions to determine whether their functional properties could be modified or enhanced. For > 0.05% Li doped AgNb03, the structure transformed from orthorhombic, Pbcm (√2ap,√2ap,4ap) to a new structure with a unit cell, √2ap√2ap,6ap, where ap is the pseudocubic perovskite lattice parameter. The new cell may be envisaged as arising from a complex modulated tilt system in which blocks of 3 in- phase tilted octahedra are rotated in antiphase with respect to each other around the c- axis. The onset of this transition resulted in asymmetry of the M3-M2 transition but there was no discernible improvement in ferroelectric properties at room temperature. For Li doped AgNbl/2 Ta1/2O3, the correlation length of cation displacements and to a lesser extent octahedral tilting decreased according to electron diffraction and Raman spectroscopy as Li concentration increased. The decrease in correlation length manifested itself as a highly perturbed domain structure at the solubility limit (~ 15 mol% Li) and resulted in convergence of the M-phase transitions to give a single broad dielectric response at the solubility limit. For Li doped AgTaO3, there were no apparent changes in the structure at room temperature. However, temperature dependent dielectric measurements clearly indicated that Li substitution had induced the onset of a ferroelectric phase transition in contrast to the incipient behaviour of AgTaO3. The temperature of the ferroelectric transitions increased with increase in Li concentration up to the solubility limit (~ 15 mol %.). The onset of the ferroelectric transition was also observed by Raman spectroscopy but further work is required to determine its precise nature.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available