Figure 6
Orbital model for Ta\(_\textrm{2}\)NiSe\(_\textrm{5}\): (a) In the orthorhombic phase at 350 K, and (b) in the monoclinic phase at 80 K. In both sketches the Ni \(d_{xy}\) and \(d_{yz}\) orbitals symbolize the states relevant for the hybridization in the NiSe\(_4\) tetrahdron. Similarly, the Ta \(5d_{1/ \sqrt{2} \times (d_{xz} + d_{yz})}\) orbitals (coordinates were defined with respect to the unit cell) pointing between the Se neighbors and towards the Ni atoms symbolize the states relevant for the hybridization in the TaSe\(_6\) octahedron and for the direct Ta-Ni bond. Among the Ta 5d \(t_2\) states, these orbitals are the only ones that can effectively hybridize simultaneously with both the Ni \(4d_{yz}\) and Se \(4p_z\) orbitals. In the orthorhombic phase, hybridization between Ni and Ta states are forbidden due to their opposite mirror parity, while in the monoclinic phase, two mirror symmetries are broken, permitting hybridization between the relevant Ta, Ni, and Se orbitals. Note, the perspective view and the projections along the main directions change between (a) and (b), as indicated by the red and orange rectangles in the figures.
