Phase transitions inTi5O9single crystals: Electrical conductivity, magnetic susceptibility, specific heat, electron paramagnetic resonance, and structural aspects

Abstract

The physical properties of ${\mathrm{Ti}}_5$ ${\mathrm{O}}_9$ single crystals, grown by the vapor-transport method have been studied in order to elucidate the nature of the three phases and the origin of the two successive transitions which take place as a function of temperature at ∼ 130 K and ∼ 140 K. Two transitions are clearly observed in the lattice-parameters, the electrical-conductivity, the magnetic-susceptibility and the specific-heat measurements. The enthalpy change of the two transitions have been found to be 135 ± 5 and 110 ± 5 cal/mole, respectively. An intense EPR line is found in the low-temperature phase and it vanishes at 130 K. No changes have been observed in the crystal structure of the three phases, as determined by the classical x-ray diffraction methods. It is suggested that ${\mathrm{Ti}}^{3+}$-${\mathrm{Ti}}^{3+}$ pairing takes place in the low-temperature phase but that the crystal contains microdomains with unpaired ${\mathrm{Ti}}^{3+}$ cations at the domain walls which would be responsible for the strong EPR signal. The intermediate phase is proposed to be a disordered phase as far as the ${\mathrm{Ti}}^{3\pm }$-${\mathrm{Ti}}^{3+}$ pairs are concerned similar to the intermediate phase of ${\mathrm{Ti}}_4$ ${\mathrm{O}}_7$. The nature of the high-temperature phase is not clear.