Unveiling excitonic insulator signatures in Ta[Formula: see text]NiSe[Formula: see text] through structural and...

The high-temperature phase of Ta[Formula: see text]NiSe[Formula: see text], a near-zero-gap semiconductor ([Formula: see text] = 0), is a promising candidate for an excitonic insulator. Given the dome-like evolution expected for an excitonic insulator around [Formula: see text], we investigated Ta[Formula: see text]NiSe[Formula: see text], the more semi-metallic Ta[Formula: see text](Ni,Co)Se[Formula: see text], and semiconducting Ta[Formula: see text]NiS[Formula: see text] 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 Ta[Formula: see text]NiSe[Formula: see text] and Ta[Formula: see text](Ni,Co)Se[Formula: see text], but no transition in Ta[Formula: see text]NiS[Formula: see text] 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 Ta[Formula: see text]NiSe[Formula: see text] and emphasizing the crucial electronic role of orbitals in the formation of the excitonic insulator state.