L(III)-EDGE XANES STUDY ON UNUSUALLY HIGH-VALENT IRIDIUM IN A PEROVSKITE LATTICE

The Ir L(III)-edge XANES spectra have been studied to probe the electronic configurations such as the oxidation state and the spin state of the iridium in an oxide lattice as well as the covalency of the Ir-O bond, on the basis of the one-electron approximation and the crystal field theory. For the systematic study, the perovskite compounds La2MgIrO6, La2LiIrO6, and Ba2CaIrO6 have been prepared under high oxygen pressure, since the iridium ions in such a perovskite lattice are stabilized in the same chemical environment but with different oxidation states, Ir-IV (5d(5)), Ir-V (5d(4)), and Ir-VI (5d(3)), respectively. Two well-resolved features observed in the second-derivative spectra, which correspond to transitions to t(2g), and e(g) states, suggest clearly that the iridium is stabilized in a regular octahedral site, and a chemical shift of transition to the t(2g), state, which is less sensitive to the crystal field than the e(g) one, is very linear against the change of the oxidation state. Especially the reinforced covalency of the Ir-O bond in the La2LiIrO6 lattice due to the relatively weaker competing Li-O bond, compared to the Mg-O and Ca-O ones in La2MgIrO6 and Ba2CaIrO6, might raise the e(g) state to higher energy. From the peak areas corresponding to transitions to t(2g) and e(g) states, it is concluded that the iridium ions in all compounds are present in the electronic configuration of low spin state, and the difference in the peak areas is also found to be proportional to the difference in vacancies for t(2g) and e(g) states following the change of the oxidation state, Ir-IV --> Ir-V --> Ir-VI.