No abstract

Magnetic-susceptibility measurements of the Magneli phases of the Ti-0 system show a change from weak covalent bond formation, through antiferromagnetic to paramagnetic-like behavior with increasing oxygen concentration. The reciprocal molar susceptibility plotted versus temperature indicates collective electron effects in all phases except TiOi.50 and TiOi.67. These exhibit noncooperative and cooperative semiconductor-to-metal transitions, respectively.

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Semiconductor-to-metal-type transitions were studied by magnetic susceptibility measurements on pure single phases of Ti3O5, Ti4O7, Ti5O9, and Ti6O11. This work represents a more detailed investigation than those reported previously. From this and related studies the following model is proposed to explain the observed phenomena. Below the transition, groups of cations are distributed periodically throughout the lattice. Within these groups the d electrons are delocalized; however, ``constrained-type'' antiferromagnetism sets in between specific neighboring d electrons through homopolar bonding of cations. Neighboring groups interact via thermal excitation of electrons. Above the transition this type of magnetic ordering is modified by changes in crystal structure; here 3d overlap is large enough to bring about a nearly complete delocalization of electrons over the entire lattice. The behavior is described by the free-electron gas model. In addition, in all but Ti3O4, a Curie-Weiss law phenomenon is seen to be superimposed on this transition, which may result from relatively small (<11%) interactions between the unpaired spins. In proposing this model, calculations of the effective electronic mass and Goodenough's ideas have been used.