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Ta2Ni is Khatyrkite structured and crystallizes in the tetragonal I4/mcm space group. The structure is three-dimensional. Ta is bonded in a 4-coordinate geometry to four equivalent Ni atoms. All Ta–Ni bond lengths are 2.64 Å. Ni is bonded in a 10-coordinate geometry to eight equivalent Ta and two equivalent Ni atoms. Both Ni–Ni bond lengths are 2.44 Å.
Ta2Ni crystallizes in the hexagonal P6_3/mcm space group. The structure is three-dimensional. there are four inequivalent Ta sites. In the first Ta site, Ta is bonded to six Ta and six equivalent Ni atoms to form distorted TaTa6Ni6 cuboctahedra that share corners with eighteen equivalent NiTa9Ni3 cuboctahedra, edges with eighteen TaTa6Ni6 cuboctahedra, faces with six equivalent NiTa9Ni3 cuboctahedra, and faces with fourteen TaTa6Ni6 cuboctahedra. All Ta–Ta bond lengths are 2.89 Å. All Ta–Ni bond lengths are 2.83 Å. In the second Ta site, Ta is bonded to eight Ta and four equivalent Ni atoms to form distorted TaTa8Ni4 cuboctahedra that share corners with twelve TaTa8Ni4 cuboctahedra, edges with eight equivalent NiTa9Ni3 cuboctahedra, edges with sixteen TaTa6Ni6 cuboctahedra, faces with eight equivalent NiTa9Ni3 cuboctahedra, and faces with ten TaTa6Ni6 cuboctahedra. There are four shorter (2.78 Å) and two longer (2.92 Å) Ta–Ta bond lengths. All Ta–Ni bond lengths are 2.84 Å. In the third Ta site, Ta is bonded to eight Ta and four equivalent Ni atoms to form distorted TaTa8Ni4 cuboctahedra that share corners with twelve TaTa8Ni4 cuboctahedra, edges with eight equivalent NiTa9Ni3 cuboctahedra, edges with sixteen TaTa6Ni6 cuboctahedra, faces with eight equivalent NiTa9Ni3 cuboctahedra, and faces with ten TaTa6Ni6 cuboctahedra. There are two shorter (2.78 Å) and one longer (2.92 Å) Ta–Ta bond lengths. All Ta–Ni bond lengths are 2.84 Å. In the fourth Ta site, Ta is bonded to eight Ta and four equivalent Ni atoms to form distorted TaTa8Ni4 cuboctahedra that share corners with twelve TaTa8Ni4 cuboctahedra, edges with eight equivalent NiTa9Ni3 cuboctahedra, edges with sixteen TaTa6Ni6 cuboctahedra, faces with eight equivalent NiTa9Ni3 cuboctahedra, and faces with ten TaTa6Ni6 cuboctahedra. All Ta–Ni bond lengths are 2.84 Å. Ni is bonded to nine Ta and three equivalent Ni atoms to form distorted NiTa9Ni3 cuboctahedra that share corners with nine equivalent TaTa6Ni6 cuboctahedra, corners with nine equivalent NiTa9Ni3 cuboctahedra, edges with six equivalent NiTa9Ni3 cuboctahedra, edges with twelve TaTa8Ni4 cuboctahedra, faces with five equivalent NiTa9Ni3 cuboctahedra, and faces with fifteen TaTa6Ni6 cuboctahedra. All Ni–Ni bond lengths are 2.83 Å.
(TaF6)2NiN6 is Fluorite structured and crystallizes in the cubic Fm-3m space group. The structure is zero-dimensional and consists of four nickel ammine bromide molecules and eight TaF6 clusters. In each TaF6 cluster, Ta5+ is bonded in an octahedral geometry to six equivalent F1- atoms. All Ta–F bond lengths are 1.92 Å. F1- is bonded in a single-bond geometry to one Ta5+ atom.
Abstract During a band-gap-tuned semimetal-to-semiconductor transition, Coulomb attraction between electrons and holes can cause spontaneously formed excitons near the zero-band-gap point, or the Lifshitz transition point. This has become an important route to realize bulk excitonic insulators – an insulating ground state distinct from single-particle band insulators. How this route manifests from weak to strong coupling is not clear. In this work, using angle-resolved photoemission spectroscopy (ARPES) and high-resolution synchrotron x-ray diffraction (XRD), we investigate the broken symmetry state across the semimetal-to-semiconductor transition in a leading bulk excitonic insulator candidate system Ta 2 Ni(Se,S) 5 . A broken symmetry phase is found to be continuously suppressed from the semimetal side to the semiconductor side, contradicting the anticipated maximal excitonic instability around the Lifshitz transition. Bolstered by first-principles and model calculations, we find strong interband electron-phonon coupling to play a crucial role in the enhanced symmetry breaking on the semimetal side of the phase diagram. Our results not only provide insight into the longstanding debate of the nature of intertwined orders in Ta 2 NiSe 5 , but also establish a basis for exploring band-gap-tuned structural and electronic instabilities in strongly coupled systems.