Electron State of Solute Atom and Lattice Distortion in Two-dimensional Metals (Ta_1−xT_x)Se₂

Abstract

Transition metal dichalcogenide TaSe₂ with layered structures, in which Ta atomic planes are sandwiched by Se atomic plances, has two-dimensional properties, and causes Peierls trasformation attributed to the periodic lattice distortion at low temperatures. In this study, the relationship between the electron states of solute atoms and lattice distortions in solid solutions are discussed on the basis of two-dimensional metallic TaSe₂ at room temperature. Ta atoms in TaSe₂ are substituted by Ti atoms and the lattice distortions occur to disorder the potential in Ta metallic planes. The correlation between the electron states localized at Ti atoms and lattice distortions caused by the electron localization are studied. The specimens are (Ta_1−xTi_x)Se₂ (x=0, 0.2, 0.4, 0.8). Both transmission electron microscopy and Auger electron spectroscopy were used to investigate the atomic and electronic structures. In the case of Ti content x≤0.2, lattice distortions take place. From the viewpoint of the electronic structures of solute Ti, there are the evidences of the untransferable charge between Ta and Ti atoms and the similarity in valence state between solute Ti and elemental Ti. The periodic potential fluctuates, since solute Ti in Ta planes is characteristic of metallic Ti itself. The conduction electrons are localized under such a potential due to the effects of the two-dimensional structure. Consequently, the lattice distortions are attributable to the electron-lattice interaction. In the case of x>0.2, lattice distortions are inhibited, because the electron remarkably transfers from the Ti to the Ta atom and atomic arrangements are ordered by alloying effects.