We present a combined computational and experimental study of Nb_3Cl_8, a correlated layered material containing Nb trimers, through the lens of competing intra- and intercluster interactions. Different proposed explanations for its magnetostructural transition such as charge disproportionation, antiferromagnetic quenching, and interlayer singlet formation are investigated in light of the various reported low-temperature structures. Our findings rule out the previously proposed charge-disproportionation, suggest an intricate interplay between Mott physics and the formation of interlayer singlets, and also hint at a possible explanation of the observed intratrimer scissoring distortion. We suggest that the physics of Nb_3Cl_8 should be understood in the context of weakly coupled Hubbard dimers.

The cluster compounds M5O4I11 (M = Nb, Ta) and Ta5O4I11(TaI5) were obtained from heterogeneous solid-state reactions and structurally characterised by single-crystal X-ray diffraction. Their crystal structures are based on the novel [M5O4] cluster core with metal (M) atoms arranged following the motif of a square pyramid. Iodide ligands contribute to different connectivities in the structures, resulting in (van der Waals type) waved layer structures. Two structural modifications exist for Ta5O4I11, denoted as o-Ta5O4I11 and m-Ta5O4I11, and the compound Ta5O4I11(TaI5) encloses [TaI5] molecules within voids of the structure. The two-dimensional nature of the structures and the presence of metal-to-metal bonding motivated investigations of the electronic properties through optical band-gap measurements, electrical conductivity studies, electronic band structure calculations, and X-ray photoelectron spectroscopy.

A series of cluster compounds sharing a [ M 5 O 4 ] cluster core ( M = Nb, Ta) is obtained from heterogeneous solid-state reactions. They show semiconducting behaviour with ionic to covalent bonding, according to electronic structure calculations.

The quasi-one-dimensional structure of ANb3Br7S (A = Cs, Rb) appears in the form of flexible, fraying rods. Electronic measurements and calculations indicate Luttinger-liquid-like behaviour.

Crystal structures of ANb_3Br_7S compounds with A = Rb and Cs are represented by chains of the triangular Nb_3 cluster core, as refined by single crystal X-ray diffraction, to form highly anisotropic materials. The Nb_3 cluster core contains seven valence electrons, six of them being assigned to Nb–Nb bonding within the Nb_3 triangle and one unpaired d electron. The presence of this surplus electron gives rise to the formation of long-range ordered states. The connectivity in the structures is represented by one-dimensional [Nb_3Br_7S]^- chains, containing a sulphur atom capping one face (μ_3) of the triangular niobium cluster as Nb_3S, which is believed to induce an important electronic feature. Several types of studies are undertaken to obtain deeper insight into the understanding of this unusual type of material: the crystal structure, morphology and elastic properties are analysed, as well the (photo-) electrical properties and NMR relaxation. Electronic structure (DFT) calculations are performed in order to understand the electronic structure and transport in these compounds, and, based on the experimental and theoretical results, we propose that the electronic interactions along the Nb chains are sufficiently one-dimensional to give rise to Luttinger liquid (rather than Fermi liquid) behaviour of the metallic electrons.

The compound Ta4SBr11 was prepared by a comproportionation reaction of tantalum bromide with tantalum and elemental sulfur. The crystal structure, as refined by single-crystal X-ray diffraction, is composed of clusters with Ta4S cores, arranged in corrugated van der Waals layers. Individual layers appear to be displaced relative to each other along one direction. Successful crystal growth in a melt of CsBr yielded black platelets of Ta4SBr11, which were used to investigate the electrical properties of the compound. The electronic structure was studied by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy and by density functional theory (DFT) band structure calculations, revealing this material to be a small-gap semiconductor. DFT results, in combination with magnetic susceptibility measurements, suggest that metallicity originating from the one unpaired Ta d electron per cluster is most likely suppressed by electronic correlations, forming a cluster Mott insulator.

The synthesis of Nb4S3, a previously undiscovered binary sulfide, was achieved using Nb3Br7S as a precursor. Its structure is composed of Nb6S triangular prisms arranged in a polar (Imm2) configuration, with sulfur atoms lying in channels along the a axis. Electrical resistivity measurements and density functional theory calculations were used to determine that Nb4S3 is metallic and therefore a polar metal, with metallic bands occupied by electrons with primarily niobium character. The electrons near the Fermi level are so closely associated with the niobium sublattice that the sulfur atoms have positive Born effective charges, indicating that the electrostatic interactions between sulfur atoms are unscreened. Calculations of the dependence of the electron density on the sulfur atomic positions confirm that the metallic electrons do not screen the dipole-dipole interactions between sulfur atoms, which allows polarity and metallicity to coexist in Nb4S3. These findings suggest that applied electric fields might be able to reverse the polarity of thin films of Nb4S3.