Synthesis, structure and properties of bismuth based electroceramic perovskites

The work presented in this thesis focuses on the effect on structure and properties of the rhombohedral (R) perovskite system, Bi(Ti3/8Fe2/8Mg3/8)O3 (BTFM), when solid solutions with CaTiO3 (CTO) are made. Bismuth-based perovskites are established as candidates to replace the piezoelectric material Pb(Zr1-xTix)O3 as they have high Curie and depolarisation temperatures which are necessary for applications and are not toxic. BTFM has a Curie temperature of ~730 °C; however it shows poor electromechanical properties. In Chapter 3 of this thesis is shown that a range of xBTFM-(1-x)CTO compositions were synthesised successfully in the range 0.6 ≤ x ≤ 1 obtaining the perovskite structure. They were proved to be polar by second harmonic generation measurements. Compositions in the range 0.6 ≤ x ≤ 0.825 adopt an orthorhombic (O) structure. For x ≥ 0.95 the R structure crystallises in the polar space group R3c with polarisation along [111]p as BTFM while in the range 0.837 ≤ x ≤ 0.925 a mixed phase, O + R, region is achieved. In Chapter 4 of this thesis, the study of the space group assignment for the O phase based on the reflection conditions and microscopy studies is shown. By using the Maximum Entrophy Method / Rietveld method a six sites disordered model for the A-site has been built based on the charge density distribution using synchrotron X-ray diffraction (SXRD) data. A joint refinement using SXRD and neutron data was performed on composition x = 0.8 and the structure of this new polar phase crystallising in Pna21 with polarisation along [001]p has been solved. However, the O phase despite of being polar displays poor physical properties being the maximum piezoelectric coefficient obtained of 2.8 pC/N for x = 0.835 . In Chapter 5, preliminary structural analysis outcomes for the mixed phase, O + R, compositions with different polarisation directions along [001]p and [111]p respectively are shown along with their physical properties measurements results. By making solid solutions of BTFM and CTO, a maximum piezoelectric coefficient of 53 pC/N has been achieved for x = 0.835. This value is about two orders of magnitude greater than for BTFM (0.6 pC/N). A preliminary phase diagram of xBTFM-(1-x)CTO materials versus temperature has been built