Strong Carrier Concentration Dependence of Pressure Effect on Bipolaronic Transitions in Magneli Phase Ti_nO_<2n-1> (n = 4, 5, 6)

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Temperature dependent resistivities of Magneli phases Ti_nO_<2n-1> (n=4, 5, 6), which contains two Ti^<3+> and (n - 2) Ti^<4+> per unit formula, are measured under various pressures up to 3.5 GPa, to investigate the nature of the two step transitions from a metal to bipolaronic insulators. We found that the pressure dependence of the transition temperatures is critically changes with the density of Ti^<3+>. In Ti_4O_7, the low temperature bipolaronic phase can be suppressed very rapidly with pressure and the high temperature metallic state is stabilized down to almost T = 0 limit at 3.5 GPa. Ti_5O_9 shows much reduced pressure dependence. In Ti_6O_11, eventually, the transition temperatures hardly depend on the applied pressure. We propose that this critical change of the pressure effect originates from a subtle interplay between electron-lattice interaction and electron correlations (inter-site Coulomb repulsions).
社団法人日本物理学会の論文
2002-06-15