2025/09/19 by Nour Maraytta, Peter Nagel, Maraytta, Nour +22
Chemistry · Engineering · Materials Science · #2D Materials and Applications #Chalcogenide Semiconductor Thin Films #FOS: Physical sciences #Inorganic Chemistry and Materials #Materials Science (cond-mat.mtrl-sci) #Strongly Correlated Electrons (cond-mat.str-el)
paper · pdf · doi:10.48550/arxiv.2509.15771
openalex publication_date 2025/09/19 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/03
The high-temperature phase of Ta2NiSe5, a near-zero-gap semiconductor (EG = 0), is a promising candidate for an excitonic insulator. Given the dome-like evolution expected for an excitonic insulator around EG, we investigated Ta2NiSe5, the more semi-metallic Ta2(Ni,Co)Se5, and semiconducting Ta2NiS5 using high-resolution single-crystal x-ray diffraction and near-edge x-ray absorption fine structure (NEXAFS). Our findings reveal a second-order structural phase transition from orthorhombic (space group: Cmcm) to monoclinic (space group: C2/c) in Ta2NiSe5 and Ta2(Ni,Co)Se5, but no transition in Ta2NiS5 down to 2 K. This transition breaks two mirror symmetries, enabling and enhancing the hybridization of Ta, Ni, and Se atoms, shortening bond lengths, and strengthening orbital interactions. NEXAFS data confirm stronger hybridization, significant changes in excitonic binding energies, and a key alteration in orbital character, suggesting an excitonic insulating state in Ta2NiSe5 and emphasizing the crucial electronic role of orbitals in the formation of the excitonic insulator state.