Structure and Bonding of Bi₄Ir: A Difficult-to-Access Bismuth Iridide with a Unique Framework Structure

Crystals of Bi₄Ir, a new intermetallic compound, were obtained by the reaction of an iridium-containing intermetallic precursor with liquid bismuth. X-ray diffraction on a single crystal revealed a rhombohedral structure [<i>R</i>3̅<i>m</i>, <i>a</i> = 2656.7(2) pm, and <i>c</i> = 701.6(4) pm]. Bi₄Ir is not isostructural to Bi₄Rh but combines motifs of the metastable superconductor Bi₁₄Rh₃ with those found in the weak topological insulator Bi₁₄Rh₃I₉. The two crystallographically independent iridium sites in Bi₄Ir have square-prismatic and skewed-square-antiprismatic bismuth coordination with Bi–Ir distances of 283–287 pm. By sharing common edges, the two types of [IrBi₈] units constitute a complex three-dimensional network of rings and helices. The bonding in the heterometallic framework is dominated by pairwise Bi–Ir interactions. In addition, three-center bonds are found in the bismuth triangles formed by adjacent [IrBi₈] polyhedra. Density functional theory based band-structure calculations suggest metallic properties

Many Faces of Ni₃Bi₂S₂: Tunable Nanoparticle Morphology via Microwave-Assisted Nanocrystal Conversion

Several combinations of the microwave-assisted polyol route and conversion chemistry techniques were exploited to access the bimetallic sulfide Ni₃Bi₂S₂ with a variety of morphological features. First, Bi₂S₃ microstructures can be converted into Ni₃Bi₂S₂ at 240 °C; the precursor’s rod-like shape and size pertain to the final product. Second, round Ni₃Bi₂S₂ particles can be obtained directly from a presynthesized NiBi intermetallic precursor; the resultant submicron size particles agglomerate and thus differ from the starting alloy’s shape. Third, microwave reflux of bismuth nitrate and nickel acetate solution in ethylene glycol in the presence of thiosemicarbazide can be employed to produce Ni₃Bi₂S₂ with a peculiar flower-like morphology. The presence and the decisive role of the <i>in situ</i> generated NiBi intermediate are unraveled, confirming that the reaction proceeds via transformation of solid rather than via a solution–dissolution process. NiBi nanoparticles preconfigure the Ni₃Bi₂S₂ product morphology in a wide range of pH values. In turn, the pH value is found to be a key factor that determines the type of impurities accompanying the Ni₃Bi₂S₂ ternary phase. At pH ≈ 4 bismuth precipitates as a main side-phase, while pH ≈ 12 favors the formation of NiS impurity

Crystal Growth and Real Structure Effects of the First Weak 3D Stacked Topological Insulator Bi₁₄Rh₃I₉

A detailed account of the crystal-growth technique and real structure effects of the first 3D weak topological insulator Bi₁₄Rh₃I₉ = [(Bi₄Rh)₃I]­[BiI₄]₂ is given. As recently shown, this compound features decorated-honeycomb [(Bi₄Rh)₃I]²⁺ sheets with topologically protected electronic edge-states and thereby constitutes a new topological class. Meticulous optimization of the synthesis protocol, using thermochemical methods, yielded high-quality crystals of Bi₁₄Rh₃I₉ suitable for the experimental characterization of the structural as well as topological properties. Insightful information about the crystal structure, its pseudosymmetry, and the thereby caused stacking disorder and twinning phenomena, obtained by X-ray diffraction and TEM studies, is crucial for an adequate theoretical modeling of coupling between the topologically nontrivial sheets. As demonstrated here, Bi₁₄Rh₃I₉ is not an exotic anomaly, but a stable, structurally well-defined bulk material, which can be used for gaining experimental knowledge about the yet poorly investigated class of weak 3D topological insulators. It could equally foster the synthesis and understanding of related compounds with the bismuth-based decorated-honeycomb sheets

Synthesis, Crystal and Topological Electronic Structures of New Bismuth Tellurohalides Bi₂TeBr and Bi₃TeBr

Halogen substitution, that is, bromine for iodine, in the series of topological Bi_<i>n</i>TeI (<i>n</i> = 1, 2, 3) materials was conducted in order to explore the impact of anion exchange on topological electronic structure. In this proof-of-concept study, we demonstrate the applicability of the modular view on crystal and electronic structures of new Bi₂TeBr and Bi₃TeBr compounds. Along with the isostructural telluroiodides, they constitute a family of layered structures that are stacked from two basic building modules, _∞²[Bi₂] and _∞²[BiTeX] (X = I, Br). We present solid-state synthesis, thermochemical studies, crystal growth, and crystal-structure elucidation of Bi₂TeBr [space group <i>R</i>3̅<i>m</i> (no. 166), <i>a</i> = 433.04(2) pm, <i>c</i> = 5081.6(3) pm] and Bi₃TeBr [space group <i>R</i>3<i>m</i> (no. 160), <i>a</i> = 437.68(3) pm, <i>c</i> = 3122.9(3) pm]. First-principles calculations establish the topological nature of Bi₂TeBr and Bi₃TeBr. General aspects of chemical bonding appear to be similar for Bi_<i>n</i>TeX (X = I, Br) with the same <i>n</i>, so that alternation of the global gap size upon substitution is insignificant. The complex topological inversion proceeds between the states of two distinct modules, _∞²[Bi₂] and _∞²[BiTeBr]; thus, the title compounds can be seen as heterostructures built via a modular principle. Furthermore, highly disordered as well as incommensurately modulated ternary phase(s) are documented near the Bi₂TeBr composition. Single-crystal X-ray diffraction experiments on BiTeBr and Bi₂TeI resolve some discrepancies in prior published work

Synthesis of a Cu-Filled Rh₁₇S₁₅ Framework: Microwave Polyol Process Versus High-Temperature Route

Metal-rich, mixed copper–rhodium sulfide Cu_3−δRh₃₄S₃₀ that represents a new Cu-filled variant of the Rh₁₇S₁₅ structure has been synthesized and structurally characterized. Copper content in the [CuRh₈] cubic cluster was found to vary notably dependent on the chosen synthetic route. Full site occupancy was achieved only in nanoscaled Cu₃Rh₃₄S₃₀ obtained by a rapid, microwave-assisted reaction of CuCl, Rh₂(CH₃CO₂)₄ and thiosemicarbazide at 300 °C in just 30 min; whereas merely Cu-deficient Cu_3−δRh₃₄S₃₀ (2.0 ≥ δ ≥ 0.9) compositions were realized via conventional high-temperature ceramic synthesis from the elements at 950 °C. Although Cu_3−δRh₃₄S₃₀ is metallic just like Rh₁₇S₁₅, the slightly enhanced metal content has a dramatic effect on the electronic properties. Whereas the Rh₁₇S₁₅ host undergoes a superconducting transition at 5.4 K, no signs of the latter were found for the Cu-derivatives at least down to 1.8 K. This finding is corroborated by the strongly reduced density of states at the Fermi level of the ternary sulfide and the disruption of long-range Rh–Rh interactions in favor of Cu–Rh interactions as revealed by quantum-chemical calculations

Modular Design with 2D Topological-Insulator Building Blocks: Optimized Synthesis and Crystal Growth and Crystal and Electronic Structures of Bi_<i>x</i>TeI (<i>x</i> = 2, 3)

Structural engineering of topological bulk materials is systematically explored with regard to the incorporation of the buckled bismuth layer [Bi₂], which is a 2D topological insulator per se, into the layered BiTeI host structure. The previously known bismuth telluride iodides, BiTeI and Bi₂TeI, offer physical properties relevant for spintronics. Herewith a new cousin, Bi₃TeI (sp.gr. <i>R</i>3<i>m</i>, <i>a</i> = 440.12(2) pm, <i>c</i> = 3223.1(2) pm), joins the ranks and expands this structural family. Bi₃TeI = [Bi₂]­[BiTeI] represents a stack with strictly alternating building blocks. Conditions for reproducible synthesis and crystal-growth of Bi₂TeI and Bi₃TeI are ascertained, thus yielding platelet-like crystals on the millimeter size scale and enabling direct measurements. The crystal structures of Bi₂TeI and Bi₃TeI are examined by X-ray diffraction and electron microscopy. DFT calculations predict metallic properties of Bi₃TeI and an unconventional surface state residing on various surface terminations. This state emerges as a result of complex hybridization of atomic states due to their strong intermixing. Our study does not support the existence of new stacking variants Bi_<i>x</i>TeI with <i>x</i> > 3; instead, it indicates a possible homogeneity range of Bi₃TeI. The series BiTeI–Bi₂TeI–Bi₃TeI illustrates the influence of structural modifications on topological properties