Spin-Peierls Distortion of TiPO₄Causing 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 TiPO₄before and after its spin-Peierls distortion at 74.5 K, which is characterized by the dimerization in the chains of Ti³⁺(d¹) ions present in TiPO₄. These calculations suggest strongly that the electronic state of TiPO₄is magnetic insulating before the distortion, but becomes nonmagnetic insulating after the distortion. Consistent with this suggestion, the phonon dispersion relations calculated for TiPO₄show that the undistorted TiPO₄is stable, while the distorted TiPO₄is not, if each Ti³⁺ion has a spin moment, and that the opposite is true if each Ti³⁺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 Ti³⁺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 TiPO₄exhibits a first-order character.