Spin-Peierls Distortion of TiPO 4 Causing a Transition from a Magnetic to a Nonmagnetic Insulating State and Its Effect on the Thermoelectric Properties: Density Functional Theory Analysis.

Density functional theory calculations were carried out to probe the nature of the electronic structure change in TiPO 4 before and after its spin-Peierls distortion at 74.5 K, which is characterized by the dimerization in the chains of Ti 3+ (d 1 ) ions present in TiPO 4 . These calculations suggest strongly that the electronic state of TiPO 4 is magnetic insulating before the distortion, but becomes nonmagnetic insulating after the distortion. Consistent with this suggestion, the phonon dispersion relations calculated for TiPO 4 show that the undistorted TiPO 4 is stable, while the distorted TiPO 4 is not, if each Ti 3+ ion has a spin moment, and that the opposite is true if each Ti 3+ ion has no spin moment. These observations suggest that the driving force for the spin-Peierls distortion is the formation of direct metal-metal bonds leading to the dimerized chains of Ti 3+ ions. The abrupt change in the electronic structures from a magnetic insulating state to a nonmagnetic insulating state explains why the spin-Peierls distortion of TiPO 4 exhibits a first-order character. Although the two electronic states of TiPO 4 before and after the distortion have a band gap, the substantial spin-Peierls distortion is found to enhance the thermoelectric properties of TiPO 4 .