Local Distortions and Metal-Semiconductor-Metal Transition in Quasi-One-Dimensional Nanowire Compounds AV₃Q₃O_δ(A = K, Rb, Cs and Q = Se, Te)

Metal cluster compounds have garnered renewed interest in the search for novel superconductors and topological semimetals owing to structural instabilities of metal-cluster geometries and broken symmetries. Here we synthesized needle-like crystals of the V-cluster-based quasi-one-dimensional (Q1D) materials AV3Q3Oδ (A = K, Rb, Cs, Q = Se, Te) which can also be viewed as being composed of parallel nanowires. We examine how changes in their average and local structure control their electronic properties. All compounds crystallize in the TlFe3Te3-type structure (P63/m space group) with infinite (V3Q3)- double-walled columnar chains separated by A+ cations. Our single-crystal and synchrotron powder diffraction studies indicate oxygen atoms partially occupy the center site of the V6 octahedral metal cluster cages in KV3Te3O0.33, RbV3Te3O0.32, and CsV3Te3O0.35, whereas KV3Se3 is structurally oxygen-free. Our synchrotron X-ray pair distribution function (PDF) analyses indicate that the oxygen-free V6 cluster octahedra in KV3Se3 are highly distorted perpendicular to the chain direction even at room temperature, reducing the symmetry of the average structure from hexagonal P63/m to monoclinic P21/m. Our theoretical calculation supports this P21/m distortion and suggests the structure further distorts to P21 or P21/c at lower temperatures. In contrast, the oxygen-centered V-cluster in KV3Te3O0.33 exhibits a V3-triangle-trimerization along the chain direction. This feature is discernible from the local PDF and is consistent with lattice dynamical calculations based on density functional theory. Resistivity measurements indicate that KV3Se3 exhibits metallic behavior, whereas a dramatic metal-semiconductor-metal transition emerges in KV3Te3O0.33, RbV3Te3O0.32, and CsV3Te3O0.35 because of oxygen disorder and changes in local structure captured from our electronic structure analyses of the Fermi surface. Our investigation of the AV3Q3Oδ family demonstrates the importance of understanding local changes in structure driven by electronic instabilities, which can guide the search for new quantum materials in other low-dimensional cluster-compound materials.