Cooperative Magnetic Transitions in the Titanium-Oxygen System: A New Approach

Abstract

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.

Electronic and magnetic properties of theTi5O9Magnéli phase

“…The formation of small polarons, localized on one atomic site and randomly distributed over the lattice, explains rather well the unclear charge distribution in higher n Magnéli phases, observed in earlier experimental works 8,12 . The substantial Curie contribution, observed in Ti 5 O 9 at low temperatures 13 , which can be associated with the presence of upaired localized moments, is also in line with the proposed small polaron picture. A distinct pressure dependence of the resistivity in the HT phase of Ti 5 O 9 allowed the authors to assume that the system is just at the crossover region from large to small polarons.…”

TinO2n−1Magnéli phases studied using density functional theory

“…For the former, there is no conclusive experimental data on the band-gap energy and for the latter, as to our knowledge, no reports at all. The band gap of Ti 4 O 7 is reported to be 0.041 eV from conductivity measurements [62], 0.6 eV from spectroscopy data [63], and 0.25 eV from optical transmission data [64]. In principle, both HSE and U = 5 result in compatible band gaps.…”

Charge order, orbital order, and electron localization in the Magnéli phase Ti4O7

“…Both transitions are first-order [15,16,17] and connected with a conductivity change of three orders of magnitude each [14]. While a sharp decrease of the magnetic susceptibility has been reported for the 154 K transition, which has been interpreted as a change from Pauli paramagnetism to van Vleck-like behaviour [16], there is no considerable change at lower temperatures [14,18]. In contrast, whereas the transition at 154 K goes along with increase in volume [17], a distortion of the crystal structure had been first observed only for the second transition [14].…”