Strong carrier concentration dependence of pressure effect on bipolaronic transitions in Magneli phase TinO2n-1 (n=4, 5, 6)

Temperature dependent resistivities of Magneli phases TinO2n-1 n = 4,5,6), which contains two Ti3+ and (n - 2) Ti4+ 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 Ti3+. In Ti4O7, 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. Ti5O9 shows much reduced pressure dependence. In Ti6O11 , 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 electronlattice interaction and electron correlations (inter-site Coulomb repulsions).