Structural, Magnetic And Electrical Studies On Some Mixed Metal Perovskite Oxides

Abstract

This thesis describes crystallographic, magnetic and electrical studies of some mixed metal perovskites. In the first part, the charge distribution, magnetic and electrical properties of mixed metal rhodium-copper perovskite oxides were investigated. Various series with the general formula Ln1-yAyRh1-2xCuxBxO3 in which (Ln = La3+, Tb3+; A = Ca2+, Sr2+, Pb2+, Bi3+; B = Sc3+, Cu2+, Zn2+ ; y ≤ 0.3, x ≤ 0.25), have been synthesised by solid state reaction, and characterised by X-ray diffraction, scanning electron microscopy and physical property measurements and, as available, neutron diffraction and X-ray absorption near edge structure measurements. Structures were invariably orthorhombic with space group Pbnm with the Rh and Cu ions disordered on the same site. X-ray diffraction measurements of selected samples showed that the orthorhombic structure persisted over a wide temperature range, 30 to 900 ºC. All the samples are semiconductors and paramagnetic over the temperature range 4-300 K. Doping a divalent cation onto the B site appears to have a significant impact on charge delocalization between Rh3+/4+ and Cu2+/3+ ions due to the oxidation of Rh3+ to Rh4+ required to maintain the overall charge. That was most evident from the Rh L3 XANES measurements of the LaRh1-2xCu2xO3 and La1-xPbxRh0.5Cu0.5O3 series. Powder neutron diffraction measurements of La0.75Pb0.25Rh0.5Cu0.25Zn0.25O3, LaRh0.5Cu0.25Zn0.25O3 and La0.75Pb0.25Rh0.5Cu0.5O3 show no evidence for anion vacancies and it is postulated the oxides do not contain appreciable amount of oxygen vacancies. The magnetization curves show negative values for Weiss constants indicating weak antiferromagnetism may be present but there is no indication for long range coupling in the oxides. There are several factors that may influence the magnitude of the cell volume, octahedral distortion, octahedral tilting, magnetic interactions and electronic properties. These include ionic size, effective charge, electron configuration and electronegativity. In addition the charge delocalization and local ordering effects can play a role. The present work has demonstrated that: The changes in the unit cell volume and the octahedral distortion of the isovalent doped oxides such as La0.75A0.25Rh0.7Cu0.3O3 where A = Ca2+, Sr2+ and Pb2+ are consistent with the increase in the ionic radii, whereas the decrease in magnetic moments of these is correlated with the increase in the electronegativities of the dopant cation. The unit cell volumes for the terbium oxides are somewhat smaller than found in the analogous lanthanum oxides reflecting the small ionic size of Tb3+. The divalent cation doped oxides LnRh1-2xCu2xO3 and LnRh1-2xCuxZnxO3 display lower cell volumes and octahedral distortions but higher magnetic moments and electrical conductivities than the trivalent cation doped oxides LnRh1-2xCuxScxO3 as consequence of charge delocalization. The electrical conductivity of the oxides increases as the divalent dopant content increases possibly because of an increase in carrier concentration that occurs as consequences of the formation of ionic defects due to the oxidation of Rh3+ (3d6) to Rh4+ (3d5). The electron configuration influences the spin coupling and the band gap and this is most evident in the Pb2+ and Bi3+ (6s2) doped LaRh1-2xCu2xO3 oxides which exhibited the lowest magnetic moments and the highest activation energies among the oxides studied. Compared with the analogous lanthanum oxides, the magnetic susceptibilities of the terbium oxides increased as a consequence of the contribution of Tb3+ 4 f8 electrons. Changing the A site composition resulted in anomalous changes in the cell volumes, octahedral distortions, electrical resistivity and magnetic susceptibility of the La1-xPbxRh0.5Cu0.5O3 and La1-xBixRh0.5Cu0.5O3 perovskites. This is likely a consequence of charge delocalization and short-range local ordering effects. Increasing the doping on the B-site resulted in either a decrease or increase in the cell volumes and the magnetic moments, depending on the dopant type cation. The final part of this thesis describes the structure of some Ba2-xSr1+xBO5.5 (B = Nb5+ and Ta5+) perovskites. These were characterised by scanning electron microscopy, thermogravimetric analysis, X-ray and neutron diffraction. The preparation of these used solid state methods but the initial reactions were conducted under different media. Four compounds were prepared and these all have a face centred cubic structure with space group Fm3 ̅m. The two synthetic methods produce monophasic powders and these differ in color, particle size, and hardness. The cell edges of the oxides obtained by mixing the reactants with water are larger than these obtained when the mixing was conducted with acetone. The neutron diffraction profiles demonstrate that the A cation and oxygen ions are disordered in the BaSr2NbO5.5 and BaSr2TaO5.5 structures. The unusual thermal expansion of the unit cell is due to the presences of water and anion deficiency into the oxides structure. The oxides were found to absorb CO2 atmosphere during storage.