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Systematics in the X-ray photoelectron spectra (X. p. e. s.) of Ti, V, Cr, Mn and Nb oxides with the metal ion in different oxidation states as well as of related series of mono-, sesqui- and di-oxides of the first row transition metals have been investigated in detail. Core level binding energies, spin-orbit splittings and exchange splittings are found to exhibit interesting variations with the oxidation state of the metal or the nuclear charge The 3d binding energies of the monoxides show a proportionality to Goodenough’s ( R — R c ). Other aspects of interest in the study are the satellite structure and final state effects in the X. p. e. s. of the oxides, and identification of different valence states in oxides of the general formulae M n O 2 n -1 and M 3 O 4 . The nature of changes in the 3d bands of oxides under-going metal-insulator transitions is also indicated.
Single crystals of Nd0.5Ca0.5MnO3 and Pr0.6Ca0.4MnO3 show current-induced insulator–metal (IM) transitions at low temperatures. In addition, the charge-ordering transition temperature, Tco, decreases with increasing current. The electroresistive ratio, defined as ρ0.5/ρI where ρ0.5 is the resistivity at a current of 0.5 mA and ρI the resistivity at a given applied current, I, varies markedly with temperature and the value of I. Thermal hysteresis observed in Nd0.5Ca0.5MnO3 and Pr0.6Ca0.4MnO3 at the IM transition indicates that the transition is first order. The current-induced changes are comparable to those induced by magnetic fields, and accordingly the IM transition in Pr0.6Ca0.4MnO3 is associated with a larger drop in resistivity.
