Search Results (47)

Due to the high impact of semiconductors with respect to many applications for electronics and energy transformation, the search for new compounds and a deep understanding of the structure–property relationship in such materials has a high priority. Electron-precise Zintl compounds of the composition A₃TrPn₂ (A = Li − Cs, Tr = Al − In, Pn = P, As) have been reported for 22 possible element combinations and show a large variety of different crystal structures comprising zero-, one-, two- and three-dimensional polyanionic substructures. From Li to Cs, the compounds systematically lower the complexity of the anionic structure. For an insight into possible crystal–structure band–structure relations for all compounds (experimentally known or predicted), their band structures, density of states and crystal orbital Hamilton populations were calculated on a basis of DFT/PBE0 and SVP/TZVP basis sets. All but three (Na₃AlP₂, Na₃GaP₂ and Na₃AlAs₂) compounds show direct or pseudo-direct band gaps. Indirect band gaps seem to be linked to one specific structure type, but only for Al and Ga compounds. Arsenides show smaller band gaps than phosphides due to weaker Tr-As bonds. The bonding situation was confirmed by a Mullikan analysis, and most states close to the Fermi level were assigned to non-bonding orbitals. Full article

Three novel binary barium arsenides, Ba₃As₄, Ba₅As₄, and Ba₁₆As₁₁, were synthesized and their crystal and electronic structures were investigated. Structural data collected via the single-crystal X-ray diffraction method indicate that the anionic substructures of all three novel compounds are composed of structural motifs based on the homoatomic As–As contacts, with [As₂]⁴⁻ dimers found in Ba₅As₄ and Ba₁₆As₁₁, and an [As₄]⁶⁻ tetramer found in Ba₃As₄. Ba₃As₄ and Ba₅As₄ crystallize in the orthorhombic crystal system—with the non-centrosymmetric space group Fdd2 (a = 15.3680(20) Å, b = 18.7550(30) Å, c = 6.2816(10) Å) for the former, and the centrosymmetric space group Cmce (a = 16.8820(30) Å, b = 8.5391(16) Å, and c = 8.6127(16) Å) for the latter—adopting Eu₃As₄ and Eu₅As₄ structure types, respectively. The heavily disordered Ba₁₆As₁₁ structure was solved in the tetragonal crystal system with the space group $P\overline{4}{2}_{1}m$ (a = 12.8944(12) Å and c = 11.8141(17) Å). The Zintl concept can be applied to each of these materials as follows: Ba₃As₄ = (Ba²⁺)₃[As₄]⁶⁻, Ba₅As₄ = (Ba²⁺)₅(As³⁻)₂[As₂]⁴⁻, and 2 × Ba₁₆As₁₁ = (Ba²⁺)₃₂(As³⁻) ≈ ₂₀[As₂]⁴⁻ ≈ 1, pointing to the charge-balanced nature of these compounds. Electronic structure calculations indicate narrow bandgap semiconducting behavior, with calculated bandgaps of 0.47 eV for Ba₃As₄, 0.34 eV for Ba₅As₄, and 0.33 eV for Ba₁₆As₁₁. Full article

Bi-based YbMg₂Bi_1.98 Zintl compounds represent promising thermoelectric materials. Precise composition and appropriate doping are of great importance for this complex semiconductor. Here, the influence of Zn substitution for Mg on the microstructure and thermoelectric properties of p-type YbMg_1.85−xZn_xBi_1.98 (x = 0, 0.05, 0.08, 0.13, 0.23) was investigated. Polycrystalline samples were prepared using induction melting and densified with spark plasma sintering. X-ray diffraction confirmed that the major phase of the samples possesses the trigonal CaAl₂Si₂-type crystal structure, and SEM/EDS indicated the presence of minor secondary phases. The electrical conductivity increases and the lattice thermal conductivity decreases with more Zn doping in YbMg_1.85−xZn_xBi_1.98, whereas the Seebeck coefficient has a large reduction. The band gap decreases with increasing Zn concentration and leads to bipolar conduction, resulting in an increase in the thermal conductivity at higher temperatures. Figure of merit ZT values of 0.51 and 0.49 were found for the samples with x = 0 and 0.05 at 773 K, respectively. The maximum amount of Zn doping is suggested to be less than x = 0.1. Full article

The compounds AELi₂Ge (AE = Ca, Sr and Ba) were synthesized, and their structures were determined as a part of the exploratory work in the Li-rich regions of the respective ternary systems. The three compounds are isostructural, and their crystal structure is analogous with the orthorhombic structure of BaLi₂Si and KLi₂As (space group Pmmn). The atomic arrangement can be viewed as an intergrowth of corrugated AEGe layers, alternated with slabs of Li atoms, suggestive of the possible application of these phases as electrode materials for lithium-ion batteries. Both experimental electronic density and calculated electronic structure suggest the existence of Li–Li and Li–Ge interactions with largely covalent character. Despite that, the valence electrons can be partitioned as (AE²⁺)(Li⁺)₂(Ge^4–), i.e., the title compounds can be viewed as valence-precise Zintl phases. The band structure calculations for BaLi₂Ge show that a bona fide energy gap in the band structure does not exist and that the expected poor metallic behavior is originated from the AEGe sub-lattice and related to hybridization of Ba5d and Ge3p states in the valence band in proximity of the Fermi level. In addition, electrochemical measurements indicate that Li atoms can be intercalated into CaGe with a maximum capacity of 446 mAh/g, close to the theoretical value of 480 mAh/g of CaLi₂Ge, which reveals the possibility of this Li-rich compound to be used as an electrode in Li-ion batteries. Full article

A novel binary compound within the Ba–Sb phase diagram, Ba₅Sb₈, was synthesized by combining elements with an excess of Sb in an alumina crucible. Structural elucidation was performed using single-crystal X-ray diffraction. This compound crystallizes in the orthorhombic space group Fdd2 with unit cell parameters of a = 15.6568(13) Å, b = 35.240(3) Å, c = 6.8189(6) Å, adopting its own structure type. The most distinctive features of the structure are the eight-membered [Sb₈]¹⁰⁻ polyanionic fragments which have no known precedents among antimonides. They are separated by five Ba²⁺ cations, which afford the charge balance and enable adherence to the Zintl–Klemm formalism. Ba₅Sb₈ is the highest known member of the homologous series within the family of barium antimonides Ba_nSb_2n−2 (n ≥ 2), all of which boast anionic substructures with oligomeric moieties of pnictogen atoms with varied lengths and topologies. Electronic structure calculations indicate an indirect narrow bandgap of ca. 0.45 eV, which corroborates the valence-precise chemical bonding in Ba₅Sb₈. Full article

Reported herein are the synthesis and crystal chemistry analysis of the Zintl phase Sr₂₁Cd₄Sb₁₈. Single crystals of this compound were grown using the Sn-flux method, and structural characterization was carried out using single-crystal X-ray diffraction. Crystal data: Monoclinic space group C2/m (No. 12, Z = 4); a = 18.2536(6) Å, b = 17.4018(5) Å, and c = 17.8979(6) Å, β = 92.024(1)°. The structure is based on edge- and corner-shared CdSb₄ tetrahedra, which ultimately form octameric [Cd₈Sb₂₂] fragments, where two symmetry-equivalent subunits are connected via a homoatomic Sb–Sb interaction. The electronic band structure calculations contained herein reveal the emergence of a direct gap between the valence and the conduction bands. Full article

Reported are the synthesis and structural characterization of a series of quaternary lithium-alkaline earth metal alumo-silicides and alumo-germanides with the base formula A₂LiAlTt₂ (A = Ca, Sr, Ba; Tt = Si, Ge). To synthesize each compound, a mixture of the elements with the desired stoichiometric ratio was loaded into a niobium tube, arc welded shut, enclosed in a silica tube under vacuum, and heated in a tube furnace. Each sample was analyzed by powder and single-crystal X-ray diffraction, and the crystal structure of each compound was confirmed and refined from single-crystal X-ray diffraction data. The structures, despite the identical chemical formulae, are different, largely dependent on the nature of the alkaline earth metal. The differing cation determines the structure type—the calcium compounds are part of the TiNiSi family with the Pnma space group, the strontium compounds are isostructural with Na₂LiAlP₂ with the Cmce space group, and the barium compounds crystallize with the PbFCl structure type in the P4/nmm space group. The anion (silicon or germanium) only impacts the size of the unit cell, with the silicides having smaller unit cell volumes than the germanides. Full article

This work details the synthesis and the crystal structures of the ternary compounds NaSrSb, NaBaSb and NaEuSb. They are isostructural and adopt the hexagonal ZrNiAl-type structure (space group P$\overline{6}$2m; Pearson code hP9). The structure determination in all three cases was performed using single-crystal X-ray diffraction methods. The structure features isolated Sb^3– anions arranged in layers stacked along the crystallographic c-axis. In the interstices, alkali and alkaline-earth metal cations are found in tetrahedral and square pyramidal coordination environments, respectively. The formal partitioning of the valence electrons adheres to the valence rules, i.e., Na⁺Sr²⁺Sb^3–, Na⁺Ba²⁺Sb^3– and Na⁺Eu²⁺Sb^3– can be considered as Zintl phases with intrinsic semiconductor behavior. Electronic band structure calculations conducted for NaBaSb are consistent with this notion and show a direct gap of approx. 0.9 eV. Additionally, the calculations hint at possible inverted Dirac cones, a feature that is reminiscent of topological quantum materials. Full article

This work details the synthesis and the crystal structures of the quaternary Zintl phases Na₂CaCdSb₂, Na₂SrCdSb₂ and Na₂EuCdSb₂. They are isostructural and their noncentrosymmetric structure is with the space group Pmc2₁ (Pearson code oP12). All structural work is carried out via single-crystal X-ray diffraction methods. The structure features [CdSb₂]^4– layers of corner-shared CdSb₄ tetrahedra, which are stacked along the b-crystallographic axis and are separated by cations. The results from the structure refinements suggest that in addition to full cation ordering, which is typical for this structure, there also exists a possibility for an accommodation of a small degree of cation disorder. Full article

The calcium- and strontium- alumo-germanides Sr_xCa_1–xAl₂Ge₂ (x ≈ 0.4) and SrAl₂Ge₂ have been synthesized and structurally characterized. Additionally, a binary calcium germanide CaGe has also been identified as a byproduct. All three crystal structures have been established from single-crystal X-ray diffraction methods and refined with high accuracy and precision. The binary CaGe crystallizes with a CrB-type structure in the orthorhombic space group Cmcm (no. 63; Z = 4; Pearson symbol oC8), where the germanium atoms are interconnected into infinite zigzag chains, formally [Ge]²⁻. The calcium atoms are arranged in monocapped trigonal prisms, centered by Ge atoms. Sr_xCa_1−xAl₂Ge₂ (x ≈ 0.4) and SrAl₂Ge₂ have been confirmed to crystallize with a CaAl₂Si₂-type structure in the trigonal space group P$\overline{3}$m1 (no. 164; Z = 1; Pearson symbol hP5), where the germanium and aluminum atoms form puckered double-layers, formally [Al₂Ge₂]²⁻. The calcium atoms are located between the layers and reside inside distorted octahedra of Ge atoms. All presented structures have a valence electron count satisfying the octet rules (e.g., Ca²⁺Ge²⁻ and Ca²⁺[Al₂Ge₂]²⁻) and can be regarded as Zintl phases. Full article

Crystals of a new ternary compound in the Ca-In-As family, Ca₃InAs₃, have been successfully synthesized via flux growth techniques. This is only the third known compound between the respective elements. As elucidated by single-crystal X-ray diffraction measurements, Ca₃InAs₃ crystallizes in the orthorhombic space group Pnma (No. 62, Pearson symbol oP28) with unit cell parameters a = 12.296(2) Å, b = 4.2553(7) Å, and c = 13.735(2) Å. The smallest building motifs of the structure are InAs₄ tetrahedra, which are connected to one another by shared As corners, forming infinite [InAs₂As_2/2] chains. The latter propagate along the crystallographic b-axis. The As-In-As bond angles within the InAs₄ tetrahedra deviate from the ideal 109.5° value and range from 98.12(2)° to 116.53(2)°, attesting to a small distortion from the regular tetrahedral geometry. Electronic structure calculations indicate the opening of a bandgap, consistent with the expected (Ca²⁺)₃(In³⁺)(As^3–)₃ formula breakdown based on conventional oxidation numbers. The calculations also show that the Ca–As interactions are an intermediate between covalent and ionic, while providing evidence of strong covalent features of the In–As interactions. Weak s-p hybridization of In states was observed, supporting the experimentally found deviation of the InAs₄ moiety from the ideal tetrahedral symmetry. Full article

Alkali metal thallides have been known since the report of E. Zintl on NaTl in 1932. Subsequently, binary and ternary thallides of alkali metals have been characterized. At an alkali metal proportion of approximately 33% (A:Tl~1:2, A = alkali metal), three different unique type structures are reported: K₄₉Tl₁₀₈, Rb₁₇Tl₄₁ and A₁₅Tl₂₇ (A = Rb, Cs). Whereas Rb₁₇Tl₄₁ and K₄₉Tl₁₀₈ feature a three-dimensional sublattice of Tl atoms, the A₁₅Tl₂₇ structure type includes isolated Tl₁₁ clusters as well as two-dimensional Tl-layers. This unique arrangement is only known so far when the heavier alkali metals Rb and Cs are included. In our contribution, we present single-crystal X-ray structure analyses of new ternary and quaternary compounds of the A₁₅Tl₂₇ type structure, which include different amounts of potassium. The crystal structures allow for the discussion of the favored alkali metal for each of the four Wyckoff positions and clearly demonstrate alkali metal dependent site preferences. Thereby, the compound Cs_2.27K_12.73Tl₂₇ unambiguously proves the possibility of a potassium-rich A₁₅Tl₂₇ phase, even though a small amount of cesium appears to be needed for the stabilization of the latter structure type. Furthermore, we also present two compounds that show an embedding of Tl instead of alkali metal into the two-dimensional substructure, being equivalent to the formal oxidation of the latter. Cs_14.53Tl_28.4 represents the binary compound with the so far largest proportion of incorporated Tl in the structure type A₁₅Tl₂₇. Full article

The tight atomic packing generally exhibited by alloys and intermetallics can create the impression of their being composed of hard spheres arranged to maximize their density. As such, the atomic size factor has historically been central to explanations of the structural chemistry of these systems. However, the role atomic size plays structurally has traditionally been inferred from empirical considerations. The recently developed DFT-Chemical Pressure (CP) analysis has opened a path to investigating these effects with theory. In this article, we provide a step-by-step tutorial on the DFT-CP method for non-specialists, along with advances in the approach that broaden its applicability. A new version of the CP software package is introduced, which features an interactive system that guides the user in preparing the necessary electronic structure data and generating the CP scheme, with the results being readily visualized with a web browser (and easily incorporated into websites). For demonstration purposes, we investigate the origins of the crystal structure of K₃Au₅Tl, which represents an intergrowth of Laves and Zintl phase domains. Here, CP analysis reveals that the intergrowth is supported by complementary CP features of NaTl-type KTl and MgCu₂-type KAu₂ phases. In this way, K₃Au₅Tl exemplifies how CP effects can drive the merging for geometrical motifs derived from different families of intermetallics through a mechanism referred to as epitaxial stabilization. Full article

The crystal structures of three new ternary compounds, NaCd_0.92Sn_1.08 (I), Na(Cd_0.28Sn_0.72)₂ (II), and Na₂CdSn₅ (III) synthesized in a sodium-cadmium-tin system were determined by single-crystal X-ray analysis to be the following: (I) LiGeZn-type structure (hexagonal, a = 4.9326(1) Å, c = 10.8508(3) Å, space group P-6m2); (II) CaIn₂-type structure (hexagonal, a = 4.8458(2) Å, c = 7.7569(3) Å, P6₃/mmc); and (III) isotype with tI-Na₂ZnSn₅ (tetragonal, a = 6.4248(1) Å, c = 22.7993(5) Å, I-42d). Each compound has a three-dimensional framework structure mainly composed of four-fold coordinated Cd and Sn atoms with Na atoms located in the framework space. Elucidation of the electrical properties of the polycrystalline samples indicated that compounds (I) and (II) are polar intermetallics with metallic conductivity, and compound (III) is a semiconducting Zintl compound. These properties were consistent with the electronic structures calculated using the ordered structure models of the compounds. Full article

The Ga-rich gallides of the alkali metals present an interesting, yet still scarcely investigated case of polyanionic cluster compounds with subtle variations in the character of their chemical bonding. In the present work, the Ga richest phases K${}_3$Ga${}_{13}$, RbGa${}_7$, and CsGa${}_7$, which are formally electron-precise Zintl/Wade cluster compounds, are systematically studied with respect to a partial substitution of Ga by In and Hg. The pure hepta-gallides AGa${}_7$ (A = Rb/Cs; $R\overline{3}m$), which were formerly obtained from Ga-rich melts in powder form only, were crystallized from Hg-rich melts. Herein, up to 9.9/13.6% (Rb/Cs) of Ga could be substituted by In, which partly takes the four-bonded [M${}_2$] dumbbells connecting layers of Ga-icosahedra. Even though the structures are electron precise, the pseudo band gap does not coincide with the Fermi level. In the most Ga-rich potassium compound K${}_3$Ga${}_{13}$ ($Cmcm$) only 1.2% of In and 2.7% of Hg could be incorporated. Although Rb${}_3$Ga${}_{13}$ remains unknown, ternary variants containing 5.2 to 8.2% In could be obtained; this structure is also stabilized by a small Hg-proportion. The likewise closed-shell 3D polyanion consists of all-exo-bonded Ga-icosahedra and closo [Ga${}_{11}$] clusters, which are connected by two tetrahedrally four-bonded Ga${}^{-}$ and a trigonal-planar three-bonded Ga${}^0$. The aspects of the electronic structures and the site-specific Ga↦Hg/In substitution in the polyanion (“coloring”) are discussed for the title compounds and other mixed Ga/In trielides. Full article