Heat-capacity and entropy changes at the metal-insulator transitions have been measured on ${\mathrm{Ti}}_4$${\mathrm{O}}_7$ single crystals. The 130-K transition is related to a disordering of the ${\mathrm{Ti}}^{3+}$ and ${\mathrm{Ti}}^{4+}$ chains at the unit cell level. ${\mathrm{Ti}}^{3+}$ pairing occurs in this phase but without any long-range order of the bonds. It is shown from both magnetic-susceptibility and specific-heat data that for the 150-K transition, the electronic contribution seems to be of the same order of magnitude as the lattice contribution.

• Received 14 February 1974

DOI:https://doi.org/10.1103/PhysRevLett.32.1318