Thermoelectric properties of homologous p- and n-type boron-rich borides

Thermoelectric properties of a series of layered homologous rare-earth boron carbonitrides: HoB₁₇CN, REB₂₂C₂N (RE=Y,Er,Lu), and YB_28.5C₄, were investigated. Samples for measurements were prepared in the form of hot pressed or isostatically pressed and annealed single phase polycrystalline powder. This series of compounds has structures where B₆ octahedral and rare-earth atomic layers reside between an increasing number of B₁₂ icosahedral and C-B-C chain layers, and has structural analogy to boron carbide. Interestingly, a variation from p-type thermoelectric behavior for YB_28.5C₄ to n-type for REB₂₂C₂N and HoB₁₇CN was observed. This is the first non-doped compound among the boron-rich borides in which n-type thermoelectric behavior has been observed. Similar to other boron cluster compounds low values of the thermal conductivity κ were found. The origins of the low κ in such compounds has not been fully explained, but comparison among the homologous series shows that the thermal conductivity appears to increase as the number of boron cluster layers increases. This result indicates that the heavy rare-earth atoms residing in the boron matrix may play a role in depressing thermal conductivity in addition to other features common to boron cluster compounds. Although the absolute values of the determined figures of merit ZT are not large for hot pressed samples, the Seebeck coefficients and power factors for both n-type and p-type in this series show an increase at temperatures exceeding 1000 K.     

Interpenetration of a 3D Icosahedral M@Ni12 (M=Al,Ga) Framework with Porphyrin‐Reminiscent Boron Layers in MNi9B8

Two ternary borides MNi₉B₈ (M=Al, Ga) were synthesized by thermal treatment of mixtures of the elements. Single‐crystal X‐ray diffraction data reveal AlNi₉B₈ and GaNi₉B₈ crystallizing in a new type of structure within the space group Cmcm and the lattice parameters a=7.0896(3) Å, b=8.1181(3) Å, c=10.6497(4) Å and a=7.0897(5) Å, b=8.1579(4) Å, c=10.6648(7) Å, respectively. The boron atoms build up two‐dimensional layers, which consist of puckered [B₁₆] rings with two tailing B atoms, whereas the M atoms reside in distorted vertices‐condensed [Ni₁₂] icosahedra, which form a three‐dimensional framework interpenetrated by boron porphyrin‐reminiscent layers. An unusual local arrangement resembling a giant metallo‐porphyrin entity is formed by the [B₁₆] rings, which, due to their large annular size of approximately 8 Å, chelate four of the twelve icosahedral Ni atoms. An analysis of the chemical bonding by means of the electron localizability approach reveals strong covalent B?B interactions and weak Ni?Ni interactions. Multi‐center dative B?Ni interaction occurs between the Al–Ni framework and the boron layers. In agreement with the chemical bonding analysis and band structure calculations, AlNi₉B₈ is a Pauli‐paramagnetic metal.     

Sur le diborure et le tétraborure de magnésium. Considérations cristallochimiques sur les tétraborures

MgB₂ and MgB₄ have been prepared at high temperature in sealed molybdenum vessels from mixtures of the elements. The utilization of an excess of metal in the vessel generates a pressure of magnesium vapor which inhibits thermal decomposition of the compounds during the synthesis. The structure of MgB₄ has been established from single crystal data collected on an automatic diffractometer. MgB₄ is orthorhombic, space group Pnam with a = 5.464; b = 7.472; c = 4.428Å and Z = 4. The structure of MgB₄, is based on chains of boron pentagonal pyramids in which the averaged BB bond is 1.787 Å. Interchain BB bonds of 1.730 Å are responsible for the three-dimensional boron framework. The Mg atoms, located in tunnels, form zigzag chains. The structure of MgB₄ is compared to those of ThB₄ and CrB₄; it is concluded that the size of the metal atom plays an important role in the nature of the boron framework which exists. In MgB₄, the fundamental unit of the boron skeleton is a pentagonal pyramid; a new feature, in boron-rich borides where this type of coordination polyhedron was previously found only in B₁₂ icosahedra.     

Electron Deformation Density in Rhombohedral α‐Boron

α‐boron is the most simple structure of all boron modifications having one B₁₂ icosahedron per (rhombohedral) unit cell. The conventional, free atom crystal structure refinement with R = 6.2% indicates considerable charge redistribution into covalent bonding. In a high order‐low order and multipole refinement the R value could be reduced to 1.19%. The ensuing deformation difference density maps reveal bonding between the boron atoms, in the icosahedra and between the icosahedra.     

From an Icosahedron to a Plane: Flattening Dodecaiodo‐dodecaborate by Successive Stripping of Iodine

It has been shown by electrospray ionization–ion‐trap mass spectrometry that B₁₂I₁₂^2? converts to an intact B₁₂ cluster as a result of successive stripping of single iodine radicals or ions. Herein, the structure and stability of all intermediate B₁₂I_n^? species (n=11 to 1) determined by means of first‐principles calculations are reported. The initial predominant loss of an iodine radical occurs most probably via the triplet state of B₁₂I₁₂^2?, and the reaction path for loss of an iodide ion from the singlet state crosses that from the triplet state. Experimentally, the boron clusters resulting from B₁₂I₁₂^2? through loss of either iodide or iodine occur at the same excitation energy in the ion trap. It is shown that the icosahedral B₁₂ unit commonly observed in dodecaborate compounds is destabilized while losing iodine. The boron framework opens to nonicosahedral structures with five to seven iodine atoms left. The temperature of the ions has a considerable influence on the relative stability near the opening of the clusters. The most stable structures with five to seven iodine atoms are neither planar nor icosahedral.     

Crystal structure of tetragonal boron related to α-AlB12

Single crystals of the so-called β-tetragonal (or tetragonal II or III) boron modification have been obtained from boron deposits prepared by hydrogen reduction of BBr₃ on tantalum filaments at 1200°C. Chemical analysis of the samples shows that this phase can be regarded as a true modification of pure elemental boron in contrast to α-tetragonal phases which require small amounts of foreign atoms to stabilize their boron framework.The lattice parameters (a = 10.14(1)Å; c = 14.17(1)Å) were obtained and refined from single crystal data. The unit cell contains four chemical units, B₂₁ · 2B₁₂ · B_2.5 resulting in d_c = 2.34 g cm^?3 (d_m = 2.36(2) g cm^?3). The systematic extinctions are compatible with space group P4₁ or P4₃.The structure was determined from 1009 independent reflexions using a model derived from the recently solved structure of α - AlB₁₂ (a = 10.161Å; c = 14.283Å; space group P4₁2₁2 or P4₃2₁2). The final R value (unweighted data) is 9.6%.Basically, the structure of this tetragonal form of boron consists of the same three-dimensional boron skeleton, built upon simple and twinned icosahedra, as that of α-AlB₁₂. However, the defective twinned icosahedral B₁₉ units in α-AlB₁₂ are now completed (B₂₁ units) in the related tetragonal boron. A number of interstitial sites, located at positions different from those occupied by aluminum in α-AlB₁₂, are totally or partially filled by boron atoms and very probably increase the stability of the boron framework.     

An unexpectedly stable Y2B5 compound with the fractional stoichiometry under ambient pressure

Boron-rich yttrium borides are an exceptional group of compounds not only with excellent mechanical properties, but also with particular superconducting and thermoelectric properties. Although the Y–B compounds with integral components have been extensively investigated experimentally and theoretically, the yttrium borides with the fractional stoichiometries are rarely observed. Herein, utilizing a combination of the CALYPSO method for crystal structure prediction and first-principles calculations, we made an investigation on a broad range of stoichiometries of yttrium borides. An extraordinary stable Y₂B₅ compound possessing the fractional stoichiometry with the monoclinic P12₁/c1 phase is firstly uncovered. Structurally, the P12₁/c1-Y₂B₅ crystalline consists of the distorted B₆ octahedrons and seven-member B rings. Remarkably, the B–B covalent network following the increment of the boron content in six concerned yttrium borides undergoes an increasing dimension, quasi one-dimensional chain → two-dimensional B ring → a combination of two-dimensional B ring and three-dimensional B₆ octahedron → three-dimensional B₂₄ cage. According to a microscopic hardness model, P12₁/c1–Y₂B₅ is considered as an incompressible and hard material with the hardness of 18.83 GPa. More importantly, Fm-3 m-YB₁₂ can be classified into an ultra-incompressible material with the appreciable Vickers hardness of 33.16 GPa. The present consequences can provide important insights for understanding the complex crystal structures of boron-rich yttrium borides and stimulate further experimental synthesis of novel multifunctional materials with the fractional compositions.     

Deformation of the valence electron distributions in the icosahedral boron structure of α-AlB12

X-ray difference electron densities in α-AlB₁₂ were examined. Broad peaks presumably characteristic of highly delocalized three-center bonds were observed at the centers of the triangular faces of the B₁₂ icosahedron. Another kind of prominent peak on the external BB bonds of the icosahedron indicated that the inter-icosahedral BB bonds are two-center bonds. An averaging over the icosahedral symmetry significantly reduced the random noises in the difference densities. Theoretical difference electron densities for a B₁₂H^?2₁₂ molecular ion, calculated by the $\text{CNDO}\text{2}$ molecular orbital method, were qualitatively in good agreement with the observed densities mentioned above. Residual peaks were observed on all the five edges of the open pentagonal face of the B₁₉ unit; they had broad tails extending toward the aluminum triplet sites. The observed difference densities around the “single” B(23) atom suggested that the atom belongs to the B₁₉ unit rather than being single.     

Crystal structure of α-AlB12

The crystal structure of α-AlB₁₂ (tetragonal; a = 10.158(2) Å, c = 14.270(5) Å, space group P4₁2₁2 or P4₃2₁2) has been determined by the single-crystal X-ray diffraction method. It was solved by the Fourier technique initially based on a partial B₁₂ icosahedral structure, which was inferred from crystal chemical considerations. Refinement was made with the aid of a full-matrix least-squares program leading to a final R value of 3.0%. The structure is based on a three-dimensional framework consisting of B₁₂ icosahedra, B₁₉ units, and single B atoms; the B₁₉ unit is a twinned icosahedron with a triangular composition plane and a vacant apex on each side. The chemical unit is Al_3.2·2B₁₂·B·B₁₉ and its number in the unit cell is 4. The Al atoms are distributed statistically over five sites in the boron framework. The occupancies of the sites are 72, 49, 24, 15, and 2%, respectively.     

First-principles study of the crystal and electronic structures of α-tetragonal boron

The crystal and electronic structures of α-tetragonal (α-t) boron were investigated by first-principles calculation. Application of a simple model assuming 50 atoms in the unit cell indicated that α-t boron had a metallic density of state, thus contradicting the experimental fact that it is a p-type semiconductor. The presence of an additional two interstitial boron atoms at the 4c site made α-t boron semiconductive and the most stable. The cohesive energy per atom was as high as those of α- and β-rhombohedral boron, suggesting that α-t boron is produced more easily than was previously thought. The experimentally obtained α-t boron in nanobelt form had about two interstitial atoms at the 8i sites. We consider that the shallow potential at 8i sites generates low-energy phonon modes, which increase the entropy and consequently decrease the free energy at high temperatures. Calculation of the electronic band structure showed that the highest valence band had a larger dispersion from Γ to Z than from Γ to X; this indicated a strong anisotropy in hole conduction.     

Die Boride HfCo3B2 und ZrCo3B2 als ternärer CaCu5-Typ

The ternary bodides HfCo₃B₂ and ZrCo₃B₂ crystallize with the hexagonal D 2_d structure type, space group P 6/mmm,a=4.840,c=3.036Å,c/a=0.627 anda=4.863,c=3.043Å,c/a=0.625, respectively. The boron atoms are located inside a trigonal prism that is familiar from other metal-rich borides. The ternary CaCu₅ type has good space filling at a theoretical axial ratioc/a=0.75 and a theoretical radius ratio of 1.5 ∶ 1 ∶ 0.81.     

Stability of lithium in α-rhombohedral boron

The stability of lithium atoms in α-rhombohedral boron was studied by the density functional theory and Car-Parrinello molecular dynamics (MD) simulations. At a low Li concentration (1.03 at%), a Li atom at the center of the icosahedral B₁₂ site (the I-site) was found to be metastable, and the potential barrier was estimated at 775±25 K (=67±25 meV). Over 800 K, Li atoms began to escape from the B₁₂ cage and settled at the tetrahedral site (the T-site) or at the octahedral site (the O-site). Li at the T-site was also metastable below 1400 K, and Li at the O-site was energetically the most favorable. At a high Li concentration (7.69 at%), the I-site changed to an unstable saddle point. The T-site was still metastable, and the O-site was the most stable. Regardless of concentration, MD simulations showed that Li atoms at the O-site never jumped to other sites below 1400 K. The migration of Li would be very slow below this temperature.     

Zur Darstellung und Struktur von tetragonalem und rhombischem (Al,Be)B12 vom Typ des β-AlB12

Preparation and Structure of Tetragonal and Orthorhombic (Al, Be)B₁₂ of the β-AlB₁₂ Type Details are given about the preparation and structural characterisation of tetragonal AlBeB₂₄ which may also be named tetragonal β-(Al, Be)B₁₂ due to its relation to β-AlB₁₂. Now an orthorhombic form of the β-(Al, Be)B₁₂ has been obtained which is considered as an intermediate between tetragonal β-(Al, Be)B₁₂ and β-AlB₁₂. It has the same unit cell as the A-phase of β-AlB₁₂. The formation of tetragonal or orthorhombic β-(Al, Be)B₁₂ depends on a statistic or an ordered dislocation of aluminium atoms from the 2a-positions of the basic tetragonal lattice. The degree of ordering may be regarded in connection with a partial substitution of aluminium resp. beryllium atoms on or near icosahedral boron places.     

Synthesis and crystal structure of cyclopentadienyl(1,4-dimethyl-1,4-dibora-2,5-cyclohexadiene)cobalt

The synthesis of the organometallic derivative cyclopentadienyl(1,4-dimethyl-1,4-diboracyclohexa-2,5-diene)cobalt is described. This complex, [(CH₃BC₄H₄BCH₃)Co(η-C₅H₅)], forms red-oranged monoclinic crystals, space group P2₁/a with Z = 4 in a unit cell of dimensions a 11.362(7), b, 7.467(7), c 13.290(12) Å, β 103.76(6)°. The structure has been elucidated by heavy-atom methods from 1732 reflections (I > 2σ(I)) measured on a Syntex P2₁ four-circle diffractometer and refined to R = 0.055. In the coordination complex all six atoms of the cyclohexadiene ring are within bonding distance of the metal atom, but the two boron atoms bend away from the metal atom, and the ring elongates slightly in the B---B direction. As a standard of comparison the known geometry of the free ligand [1,4-difluoro-1,4-dibora-2,3,5,6-tetramethylcyclohexa-2,5-diene] is used. The terminal methyl groups on the boron atoms, by contrast, bend slightly back towards the metal atom. The cyclopentadienyl ring remains planar but is positionally disordered.     

A novel boron-rich quaternary scandium borocarbosilicide Sc3.67−xB41.4−y−zC0.67+zSi0.33−w

A novel quaternary scandium borocarbosilicide Sc_3.67−xB_41.4−y−zC_0.67+zSi_0.33−w was found. Single crystallites were obtained as an intergrowth phase in the float-zoned single crystal of Sc_0.83−xB_10.0−yC_0.17+ySi_0.083−z that has a face-centered cubic crystal structure. Single crystal structure analysis revealed that the compound has a hexagonal structure with lattice constants a = b = 1.43055(8) nm and c = 2.37477(13) nm and space group (No. 187). The crystal composition calculated from the structure analysis for the crystal with x = 0.52, y = 1.42, z = 1.17, and w = 0.02 was ScB_12.3C_0.58Si_0.10 and that agreed rather well with the composition of ScB_11.5C_0.61Si_0.04 measured by EPMA. In the crystal structure that is a new structure type of boron-rich borides, there are 79 structurally independent atomic sites, 69 boron and/or carbon sites, two silicon sites and eight scandium sites. Boron and carbon form seven structurally independent B₁₂ icosahedra, one B₉ polyhedron, one B₁₀ polyhedron, one irregularly shaped B₁₆ polyhedron in which only 10.7 boron atoms are available because of partial occupancies and 10 bridging sites. All polyhedron units and bridging site atoms interconnect each other forming a three-dimensional boron framework structure. Sc atoms reside in the open spaces in the boron framework structure.     

The Planar CoB18− Cluster as a Motif for Metallo‐Borophenes

Monolayer‐boron (borophene) has been predicted with various atomic arrangements consisting of a triangular boron lattice with hexagonal vacancies. Its viability was confirmed by the observation of a planar hexagonal B₃₆ cluster with a central six‐membered ring. Here we report a planar boron cluster doped with a transition‐metal atom in the boron network (CoB₁₈^?), suggesting the prospect of forming stable hetero‐borophenes. The CoB₁₈^? cluster was characterized by photoelectron spectroscopy and quantum chemistry calculations, showing that its most stable structure is planar with the Co atom as an integral part of a triangular boron lattice. Chemical bonding analyses show that the planar CoB₁₈^? is aromatic with ten π‐electrons and the Co atom has strong covalent interactions with the surrounding boron atoms. The current result suggests that transition metals can be doped into the planes of borophenes to create metallo‐borophenes, opening vast opportunities to design hetero‐borophenes with tunable chemical, magnetic, and optical properties.     

Symmetry and topology code of cluster self-assembly of icosahedral framework structures of boron and borides

The topological types of suprapolyhedral clusters composed of i-B₁₂ icosahedra have been modeled. The models of icosahedral supraclusters have been used in analysis of the crystal structures of boron and borides (the TOPOS program package). To identify nanocluster precursors in crystal structures, there have been used special algorithms for partitioning structural graphs into disjoint substructures and constructing a basis 3D network of the structure as a graph with the nodes corresponding to the positions of the centroids of the cluster precursors. The cluster self-assembly have been modeled for 25 types of icosahedral framework structures of boron—B₁₂-hR ₁₂, (B₁₂)2(B₂)₂-oP ₂₈, (B₁₂)₄B₂-P50, B₁₉₆-tP ₁₉₆, and B₃₃₃-hR ₃₃₃; binary borides—(B₁₂)O₂-hR ₁₄, (B₁₂)P₂-hR ₁₄, and (B₁₂)(CBC)-hR ₁₅; templated metal borides—Na₂(B₁₂)₂B₆-oI ₆₄, Mg₂(B₁₂)B₂-oI ₆₈, Tb(B₁₂)(B₄)-mI ₆₀, Al₄(B₁₂)₄B₈-oC ₈₈, (B₁₂)₄(Si4)₄-oI ₆₄, (B₁₂)₄B₂Be₄-tP ₅₈, Ti₂(B₁₂)₄B₂-tP ₅₂, Sc₁₂B₁₈₀-tP ₁₉₂, Cu₄Sc₁₂B₁₈₀-tP ₁₉₂, Si_1.5Sc₉B₁₇₈-tP ₂₁₆, Mg₂₈B₃₆₀-oP ₃₈₈, Al₂₈B₃₅₂-oP ₃₈₄, Si₂₈B₃₅₂-oP ₃₀₆, Y₂₄(B₁₅₆)₈(B₃₉)₈-cF1944, Sc₁₀B₃₁₅-hR339, and Li₂₄B₃₁₅-hR336. The symmetry and topology code of the crystal structure self-assembly from nanocluster precursors in the form of primary chain → microlayer → microframework has been completely restored. Frequency analysis of various topological and symmetry pathways for the formation and evolution of cluster precursors makes it possible to elucidate crystal-formation trends in inorganic systems at the microscopic level.     

First-principles study of the electronic structures of α-rhombohedral boron codoped with lithium and oxygen

α-Rhombohedral (α-rh) boron, which is the most stable of boron's polymorphs at low temperatures, has p-type semiconductive properties. There have been some attempts to dope the interstitial sites with alkali atoms to create metallic or n-type semiconductive α-rh boron, but this has yet to be achieved. In a previous work, we proposed the codoping of α-rh boron with Li and P or As, and revealed from first principles calculations that B₁₂PLi and B₁₂AsLi could be synthesized and become narrow-gap semiconductors. The band structure suggested that the mobility of electrons might be greater than that of holes. In this paper, based on these prospective results, we selected a new combination of dopants, Li and O, and theoretically studied such compounds as B₁₂OLi and B₁₂O₂Li. The results showed that both of these materials are metallic, while the reaction energies of the Li insertion into B₁₂O and B₁₂O₂ are lower (more unstable) than with B₁₂PLi and B₁₂AsLi. It was proved that the differences in the electronic structures are caused by the dangling bonds of the dopant atoms, O, P and As.     

A Novel Boron-Rich Scandium Borocarbide; Sc4.5−xB57−y+zC3.5−z(x=0.27, y=1.1, z=0.2)

A novel ternary boron-rich scandium borocarbide Sc_4.5−xB_57−y+zC_3.5−z (x=0.27, y=1.1, z=0.2) was found. Single crystals were obtained by the floating zone method by adding a small amount of Si. Single-crystal structure analysis revealed that the compound has an orthorhombic structure with lattice constants of a=1.73040(6), b=1.60738(6) and c=1.44829(6) nm and space group Pbam (No. 55). The crystal composition ScB_13.3C_0.78Si_0.008 calculated from the structure analysis agreed with the measured composition of ScB_12.9C_0.72Si_0.004. The orthorhombic crystal structure is a new structure type of boron-rich borides and there are six structurally independent B₁₂ icosahedra I1—I6, one B₈/B₉ polyhedron and nine bridging sites all which interconnect each other to form a three-dimensional boron framework. The main structural feature of the boron framework structure can be understood as a layer structure where two kinds of boron icosahedron network layer L1 and L2 stack each other along the c-axis. There are seven structurally independent Sc sites in the open spaces between the boron icosahedron network layers.     

Die Elektronen-Lokalisierungs-Funktion in closo-Bor-Clustern

The Electron Localization Function in closo Boron Clusters The structure and the electron density in the closo boron clusters B₄X₄ (X = H, Cl, Br, I), B₆X₆^2? (X = H, Cl, Br, I) and B₁₂H₁₂^2? were determined by pseudopotential Hartree-Fock calculations. The Electron Localization Function (ELF) was used to interpret the bonding characteristics. The regions of high ELF values in all cases have the form of the dual polyhedron of the boron cage. They show perfectly the 3 center 2 electron bonds. The comparison between Hartree-Fock and Extended Hückel calculations point out that semiempirical calculations can also be a good basis for ELF interpretations.