| dc.description.abstract | The thesis is primarily based on detailed studies of transition metal oxide systems employing X?ray absorption spectroscopy, X?ray photoelectron spectroscopy and magnetic susceptibility measurements.
Chapter I deals with a detailed study of a variety of transition metal oxides and related solids by X?ray absorption spectroscopy. The systems include oxides and other compounds of Cr, Mn, Te, Co and Ni in a wide range of oxidation states. The study has provided a functional relation between the effective atomic charge, q, of the metal and the chemical shifts of K?absorption discontinuities, ?E, of the metal. The ?E values of any given metal series vary in the same direction as the 2p?/? binding energies obtained from X?ray photoelectron spectroscopy. The relation is given by:
?E = aq + bq²
This parabolic relation is shown to be superior to the linear relation ?E = aq proposed by earlier workers. Theoretical considerations are also shown to justify the parabolic relation.
Chapter II is concerned with a study of transition metal oxide perovskites by X?ray and UV photoelectron spectroscopy and X?ray absorption spectroscopy. Systematics in the core levels and in the valence bands in a series of LaBO? solids (B = transition metal) have been discussed. Lanthanum chemical shifts in the X?ray absorption spectra of this series of compounds have also been measured. Photoelectron spectra of the solid solutions LaNi???Co?O?, LaNi???Fe?O? and LaFe???Co?O? in the valence?band region suggest that the rigid?band model is applicable to these systems. It is also shown that X?ray photoelectron spectroscopy can be employed to identify multiple oxidation states of transition metals in oxide perovskites.
A magnetic?susceptibility study of the spin?state transition in rare?earth cobaltates, LnCoO? (Ln = rare earth), forms Chapter III. Unlike LaCoO? which shows a plateau in the ?–temperature plot, the cobaltates of Pr, Nd, Tb, Dy and Yb exhibit low?to?high?spin transitions of cobalt characterised by a maximum in the ??–temperature plots, where ?? is the cobalt contribution to the susceptibility. The temperature at which the ??–T curve shows a maximum (spin?state transition temperature) increases with decrease in the size of the rare?earth ion. The La???Nd?CoO? system has also been investigated by magnetic?susceptibility measurements, to see whether the behaviour characteristic of NdCoO? changes to that of LaCoO?.
The systematic variation in the spin?state transition temperature of LnCoO? with Ln can arise from variation in the crystal?field parameter or the Raccah parameter. Since the latter would be accompanied by a variation in the Co–O distance, we considered it worthwhile to determine the Co–O distances in the series of rare?earth cobaltates. EXAFS (extended X?ray absorption fine structure) was employed to obtain the Co–O distances since it has not been possible to obtain single crystals of these materials. The results of the EXAFS study are presented in Chapter IV.
The influence of small amounts of magnetic and non?magnetic dopant ions such as Al³?, Ga³?, Cr³? and Fe³? on the spin?state equilibrium in LaCoO? has been examined in Chapter V. Al³?, Ga³? and Fe³? seem to stabilise the low?spin state of cobalt while Cr³? and Mn³? stabilise the high?spin state.
Itinerant electron behaviour and long?range magnetic ordering in the system La???Sr?CoO? forms the subject matter of Chapter VI. Electrical resistivity of La???Sr?CoO? is shown to be reduced by the substitution of Ni²? in place of cobalt ions. In the La???Sr?Co???Ni?O? system, for small values of x (= 0.05), superparamagnetic clusters seem to be formed at low temperatures (~300 K). When x is large, the occurrence of long?range ordering seems to depend on the itinerancy of charge carriers.
In Chapter VII, a magnetic?susceptibility study of spin?state transitions in cobalt oxide systems of K?NiF? structure is reported. Solids studied are Sr?Nb?.??Co?.??O? and Sr?Ta?.??Co?.??O?.
The research work carried out for the Ph.D. programme was planned to provide a broad?based training in research methodology and in various experimental techniques like magnetic susceptibility. | |
