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Title: Suppression of semiconductivity in the B-site ordered perovskite Ba2Bi3+Bi5+O6 by chemical doping
Author: Tan, Chaou Choak
ISNI:       0000 0004 2724 8719
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2010
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An investigation on making a dielectric material from a Bi3+/Bi5+ charge transfer semiconducting Ba2Bi206 (BBO) compound by using Nb5+, Ta5+, Sb5+, La3+ and y 3+ cations to replace either Bi5+ or Bi3+ was conducted. The influence of the processing temperature on the electrical properties of BBO was investigated and then followed by dopant studies to establish the structure-composition-property relationships in doped BBO perovskites. BBO and Ba2Bi2-xMx06 (M = Nb5+, Ta5+, Sb5+, La3+, y3+) were synthesised at temperatures from 800-1100 °C using the mixed oxide method. Control of the processing temperatures and conditions are critical as they can influence Bi volatilisation, oxygen loss, a change in polymorphic phase transition temperatures and a loss of B-site cation ordering. All of these factors affect the dielectric properties. BBO adopts 121m symmetry at room temperature (RT) with a tolerance factor (t) of 0.927. t increases to near unity when doped with pentavalents cations and pentavalent doped BBO series show a change in RT crystal structure from J2lm to R3 when t ;;:=: 0.94. Trivalent doped BBO series show a more complex change in RT crystal structure with changes in sequence from triclinic to monoclinic to rhombohedral and finally to cubic. The change in RT crystal structure from a monoclinic to rhombohedral structure occurs at higher t (0.960) than the pentavalent doped BBO series and the rhombohedral structure exists under a very narrow window of tolerance factor (0.960 s::t < 0.962). BBO is a p-type semiconductor and the total resistivity of ceramics is dominated by the grain boundary component. The bulk resistivity increases when sintered at ;;:=:950 'C. The semiconductivity of BBO can be suppressed by either pentavalent or trivalent dopants. The reduction in conductivity can be ~ 4-7 orders of magnitude as observed in Nb-, Ta- and Y-doped BBO and the activation energy increases from ~ 0.1 eV from undoped BBO to ~ 0.6 eV for Nb- doped BBO. Pentavalent doped BBO compound show higher bulk relative permittivity compared to BBO and in particular, Nb-doped BBO shows the highest permittivity. The values are twice that expected based on the Clausius Mossotti equation.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available