Metal-insulator transitions inTi4O7single crystals: Crystal characterization, specific heat, and electron paramagnetic resonance

Abstract

${\mathrm{Ti}}_{4}$${\mathrm{O}}_{7}$ single crystals have been studied by different methods in order to elucidate the nature of the three phases and the mechanisms of the two successive metal-insulator transitions. The crystals have been characterized by x-ray diffraction methods together with electron-paramagnetic-resonance (EPR) studies. Heatcapacity and entropy changes at the transition have been measured. On the basis of these data and of previous results, it is proposed that the low-temperature transition is related to a disordering of the ${\mathrm{Ti}}^{3+}$ pairs While in the low-temperature phase, Ti chains are alternatively occupied by ${\mathrm{Ti}}^{4+}$ ions and ${\mathrm{Ti}}^{3+}$ pairs, ${\mathrm{Ti}}^{3+}$ pairing also occurs in the intermediate phase but without any long-range order. A detailed model of disordered chains is given. Good agreement is found between the calculated and experimental values of the entropy change. It is also shown from both magnetic-susceptibility and specific-heat data, that for the high-temperature transition, the lattice contribution to the entropy is of the same order of magnitude as the electronic one. EPR spectra have been studied on untwinned crystals. A ${\mathrm{Ti}}^{3+}$ center, attributed to a stoichiometry defect has been identified through the $^{47}\mathrm{Ti}$-$^{49}\mathrm{Ti}$ hyperfine structure. The intensity of the line follows a Curie-Weiss law at low temperature and decreases sharply at the low-temperature transition. The vanishing of this line in the intermediate phase shows that the disorder of this phase is dynamic. The linewidth increase below the transition is attributed to a multiphonon Orbach relaxation through empty ${\mathrm{Ti}}^{4+}$ levels. The nature of the three phases of ${\mathrm{Ti}}_{4}$${\mathrm{O}}_{7}$ is discussed. The low-temperature phase may be described by an order of the Verwey type for the ${\mathrm{Ti}}^{3+}$ pairs. Interchain electronic correlations are shown to be the dominant mechanism responsible for this order. An atomic level scheme is given for this phase. Bipolarons are shown to be responsible for the transport properties in the intermediate phase and the bipolaronic state is discussed. In the metallic phase, the high Pauli magnetic susceptibility is attributed to an enhancement due to the interatomic electronic correlations.