Structural and electrical characterisation of some ABO₃ perovskites
Powder X-ray diffraction and Raman spectroscopy have been used to characterise the electrolyte materials La₁₋ₓSrₓGaO₃₋δ (x = 0-0.2) and LaGa₁₋yMyO₃₋δ (y = 0-0.2, M = Cr, Mg). XRD of La₁₋ₓSrₓGaO₃₋δ (x = 0-0.2) showed the secondary phase of SrGa₃O₇ at >5 mol% Sr. The cell deformed with increasing Sr dopant concentration. In the XRD patterns of LaGa₁₋yCryO₃₋δ (y = 0-0.2) the cell also deformed with increasing Cr dopant concentration. The peak at ca. 32° showed a second peak at 32.37°, likely to be due to a phase change, since the peaks did not alter in relative intensity with increase in dopant concentration (above x = 0). LaGa₁₋yMgyO₃₋δ (y = 0-0.2) showed, from the powder X-ray diffraction pattern, the formation of the secondary phase of La₄Ga₂O₉ at >5 mol% Mg. Raman spectra of LaGaO₃ showed 18 features at room temperature, and 14 at -196 °C with the disappearance of the band at 92 cm⁻¹. Raman spectra of the doped material, LaGa₁₋yMgyO₃₋δ at y = 0-0.2 showed the formation of a secondary phase of La₄Ga₂O₉ with bands at 243, 295 and 356 cm⁻¹ which become more predominant at 20 mol % Mg. LaFeO₃ was prepared using a reverse-strike coprecipitation method. The Raman spectrum of LaFeO₃ were obtained at both low-temperature and room temperature, with 9 bands observed of a predicted 24 Raman active modes. Mode assignment was determined from comparison of the Raman profile and band position with perovskites with the same structure, namely, SmAlO₃ and LaGaO₃ and are as follows: 102 (B₁g ), ca.140 (B₂g), 150 (B₁g), 176 (Ag), 227 (B₃g), 261 (Ag), 289 (Ag), 413 (Ag), 429 (B₃g). LaFe₁₋ₓMgₓO₃ was prepared using a reverse-strike coprecipitation method. The XRD patterns obtained at room temperature showed single phase, but also included small amounts of impurity of La₂O₃ and La₂MgOₓ. Lattice calculations based on FeLaO₃ showed that Mg dopant concentration had a minor effect on the overall unit cell volume, with the minimum volume achieved being 241.68Å³ at 1450 °C, 15%Mg concentration. In summary, Mg-doped LaFeO₃ produced a single phase material in the range 0-20 mol% Mg as observed from X-ray diffraction. However, a small amount of impurities in each of the samples was observed, containing La₂O₃ and La₂MgOₓ. The presence of the impurities suggested that an extra calcination step be included in the processing of the material. LaGdO₃ was fabricated using reverse strike co-precipitation method and sintered at temperature of 1350, 1400 and 1450 °C. X-ray diffraction showed the presence of a single monoclinic phase. Raman spectroscopy showed a spectrum similar for that reported for the B-type rare earth oxide Gd₂O₃ and the band positions of the low temperature Raman spectrum were assigned in comparison with this. A total of 18 bands were identified. Intense broad profile in the region 1000-2000 cm⁻¹ was observed for LaGdO₃, likely due to fluorescent/luminescent bands. The acceptor-doped perovskite proton conductor SrCe₁₋ₓYₓO₃₋δ (x=0.025 to 0.20, δ = x/2) was prepared and characterised using X-ray diffraction and AC impedance spectroscopy, and the effect of the yttrium dopant concentration on structure and electrical properties has been investigated. X-ray diffraction studies showed a decrease in lattice volume with increasing yttrium content. Electrical conductivity studies were made as a function of oxygen partial pressure, and a partial pressure of water vapour of 0.001 and 0.01 atm. The total conductivity was separated into different components by fitting procedures and regions of ionic and p-type conduction were identified. At 800°C, and at the water vapour partial pressure of 0.01 atm, the ionic conductivity showed a maximum at a doping level of x = 0.10, reaching a value of 5 mS/cm. The conductivity component appearing at low oxygen partial pressure, which according to recent studies may be regarded as protonic rather than n-type, decreased with doping, while the p-type component at high oxygen partial pressure increased.
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