Raman spectroscopy of charge-ordered states in Magneli titanium oxides

We have systematically investigated the Raman spectra of the mixed-valence compounds TinO2n-1 (n = 4,5,6) known as the Magneli phases to elucidate the nature of the charge-ordered states in these compounds. It is found that a stable charge-ordered state exists only for Ti4O7. In the case of Ti5O9, the charge order is almost completely lost, and the charge order in Ti6O11 is incomplete. Thus each compound seems to be have quite individually. This result is unexpected in view of the similarity in the crystal structure. We interpret this seeming lack of the systematic trend as being due to the electron correlation effect which becomes less effective with increasing n. Also it is suggested that the charge order is unstable for the compounds with odd n. Finally the effects of temperature on the charge ordering in Ti5O9 and Ti6O11 are discussed. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim