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Compositons in the pseudoquaternary system, 1-×(0.35Bi(Mg1/2Ti1/2)O3-0.30BiFeO3-0.35BiScO3)—×PbTiO3 were fabricated and characterised at the morphotropic phase boundary (MPB) between ferroelectric rhombohedral and tetragonal phases. The MPB occurred at × ≈ 0.48 at which composition the ferreoelectric to paraelectric phase transition, TC = 450°C, the piezoelectric constant, d33 = 328 pC/N and electromechanical coupling factor, kp = 0.44. The piezoelectric properties are viable for actuator applications but lower than equivalent high TC piezoelectrics such as 0.36BiScO3–0.64PbTiO3 (d33 = 450 pC/N, TC = 450°C). However, the relative reduction in the Sc2O3 content gives a significant cost saving which may prove a commercial advantage.

Bulk crystallized PbTiO3 (PT) was the major crystalline phase in borate-based glass ceramics with starting composition 39PbO–1BaO–25TiO2–9.8Al2O3–1SiO2–24.2B2O3 (mol%). Crystallization of PT starts at ≥520 °C and the crystal size and c/a ratio of PT were approximately the same for all samples heat treated between 520 and 700 °C. Two transient unidentified phases were induced when crystallized between 520 and 650 °C. The first at 520 °C followed by a second at 550 °C. At >650 °C, PbTi3O7 was present in addition to PbTiO3. The electrical properties of glasses heat treated between 550–700 °C were investigated by fixed frequency capacitance measurements and impedance spectroscopy, IS. No permittivity maximum or Curie temperature, T c, for the PT phase was observed from fixed frequency capacitance measurements for glasses heat treated below 700 °C, however IS data analysis using electric modulus (M″) spectroscopy revealed the PT phase to have T c ∼480 and 440 °C for samples heat treated at ∼550 and 700 °C, respectively. The conductivity of the PT phase and the dc conductivity of the glass heat treated at 700 °C were significantly higher than those of glasses heat treated at lower temperatures suggesting there is significant compositional differences between the glass heat treated at 700 °C compared to those treated at ≤650 °C.

Borate- and silicate-based glasses that devitrify on heating to form ferroelectric lead titanate have been developed. Their potential for use as sintering aids in PbO-based electroceramics has been evaluated by studying the densification of PbTiO3 (PT) with increasing wt.% addition of glass powder. When ≥3 wt.% of a borate-based glass was added, ≥94% theoretical density was achieved at 1,100 °C but for a silicate-based glass system 10 wt.% glass addition was required to achieve the same density. As the wt.% glass content increased the maximum and room temperature permittivity decreased and there was also a decrease in T c and the c/a ratio. The decrease in T c is believed to be due to increased clamping from neighbouring grains as the density of the PT increases. The decrease in maximum and room temperature permittivity is related to the presence of secondary phases.

Hexagonal BaTiO3 materials have been stabilised at room temperature according to the formula BaTi0.95M0.05 O3−δ where M = Mn, Fe, Co and Ni. Dense ceramics (> 96% of the theoretical X-ray density) were sintered at 1450∘C in flowing O2 gas from calcined powders prepared by the mixed oxide route at 1300∘C. All samples were single-phase and the bulk conductivity, σb, measured by Impedance Spectroscopy and Q.f measured by microwave dielectric resonance methods showed a strong dependence on the type of dopant. σb at 300∘C was 10−7, 10−5.5, 10−5.5 and 10−4 Scm−1 for M = Mn, Fe, Ni and Co, respectively and Q.f at ∼ 5 GHz was 7790, 6670, 2442 and 1291 GHz, for M = Mn, Fe, Ni and Co, respectively. The correlation between σb and Q.f is attributed to the presence of oxygen vacancies and/or mixed valency of the dopant ions.

Modelling of impedance spectroscopy (IS) data of electroceramics depends critically on the correct choice of equivalent electrical circuit so that the extracted parameters have physical significance. The strategy proposed to choose the correct circuit involves analysis of IS data in several of the four complex formalisms: impedance, electric modulus, admittance and permittivity together with consideration of the frequency and temperature dependence of data and the magnitude and temperature dependence of extracted resistance and capacitance values. This is demonstrated using IS data from oxide-ion conducting La0.80Sr0.20Ga0.83Mg0.17O2.82 ceramics over the range 182 to 280°C. Low temperature data are fitted first, to allow a full characterisation of the bulk response; some of the bulk parameters may then be fixed to enable fitting of the higher temperature data which increasingly feature lower frequency phenomena such as grain boundary impedances. The most appropriate circuit in this case is found to consist of a parallel combination of a resistor, capacitor and constant phase element (CPE) for the bulk response in series with a resistor and capacitor in parallel for a constriction resistance. The origin of the constriction resistance may be associated with the presence of plate-like LaSrGaO4 secondary phase within the grains and/or with the presence of pores at the grain boundaries. The importance of choosing (a) the correct equivalent circuit and (b) approximately correct input values for the various circuit parameters to be fitted and refined are demonstrated.

The mechanism of doping BaTiO3 with La has been investigated by a combination of X-ray diffraction, electron probe microanalysis, scanning and transmission electron microscopy and impedance measurements. Phase diagram results confirm that the principal doping mechanism involves ionic compensation through the creation of titanium vacancies. All samples heated in oxygen at 1350–1400°C are electrical insulators, consistent with an ionic compensation mechanism. Samples heated in air or atmospheres of low oxygen partial pressure, at similar temperatures, lose a small amount of oxygen and this gives rise to a second, electronic compensation mechanism in addition to the main, ionic compensation mechanism; as a result, samples are dark-coloured and semiconducting. The change from insulating to semiconducting behaviour is reversible, by changing the atmosphere on heating at 1350–1400°C. We find no evidence for any changes in cationic composition of the BaTiO3 solid solutions arising from changes in oxygen content.

Impedance spectra of La0.8Sr0.2Ga0.83Mg0.17O2.815 ceramics were measured using gold and platinum paste, and sputtered gold electrodes. Impedance complex plane plots, Z*, for samples with sputtered gold electrodes consist of a single arc associated predominantly with the bulk response and a low frequency 'spike' associated with electrode-polarisation and diffusion-related phenomena. An additional arc at intermediate frequencies with an associated capacitance of the order of 1 nFcm−1 and a resistance with a similar temperature dependence to that of the bulk resistance is present in Z* plots for samples measured using gold paste electrodes hardened at temperatures between 500 and 850°C. Scanning electron microscopy and impedance spectroscopy show the additional response to be a 'spreading resistance' caused by the electrodes making well-separated discrete contacts with the ceramic due to partial sintering of gold particles in the paste. Z* plots for samples measured using platinum paste electrodes hardened between 725 and 1000°C exhibit a much smaller additional response with similar behaviour to that observed with gold paste electrodes but also showed an apparent increase in sample resistance due to significant loss of contact between sample and electrode in some regions of the interface.

A review is given of some of the problems encountered in the analysis and interpretation of impedance data. The importance of choosing the correct equivalent circuit to represent the data is emphasized and it is shown how ferroelectric materials, with their characteristic temperature-dependent capacitance, are particularly suited to discriminating between plausible equivalent circuits. Results are discussed for two materials, LiTaO3 single crystal and BaTiO3 ceramics.