Physical properties of layered homologous RE-B-C(N) compounds

Physical properties of a series of homologous RE-B-C(N) B₁₂ cluster compounds REB₁₇CN, REB₂₂C₂N, and were investigated. The structures of the compounds are layer-like along the c-axis, with rare earth and B₆ octahedral layers separated by B₁₂ icosahedral and C-B-C chain layers whose number increases successively from two B₁₂ layers for the REB₁₇CN compound to four for the REB_28.5C₄ compound. The rare earth atoms are configured in two triangular flat layers which are stacked on top of one another in AB stacking where the nearest-neighbor rare earth directions are the three atoms forming a triangle in the adjacent layer. The series of homologous compounds exhibit a spin glass transition with T_f shifting in correspondence with variations of the basal plane lattice constants, consistent with the magnetic interaction being effective in the basal planes. The isothermal remanent magnetization shows a stretched exponential decay . Exponents determined for the different homologous compounds were scaled as a function of T_r=T/T_f and found to follow the empirical dependency determined for typical spin glasses. It is indicated that a mixture of disorder originating from the partial occupancy of the rare earth sites and frustration of interactions due to the unique configuration is responsible for the manifestation of spin glass transitions in these homologous systems.     

Multianvil high-pressure/high-temperature preparation, crystal structure, and properties of the new oxoborates Dy4B6O14(OH)2 and Ho4B6O14(OH)2

This paper reports about two new hydrogen-containing rare-earth oxoborates RE₄B₆O₁₄(OH)₂ (RE=Dy, Ho) synthesized under high-pressure/high-temperature conditions from the corresponding rare-earth oxides, boron oxide, and water using a Walker-type multianvil equipment at 8 GPa and 880 °C. The single crystal structure determination of Dy₄B₆O₁₄(OH)₂ showed: Pbcn, a=1292.7(2), b=437.1(2), , Z=2, R1=0.0190, and wR2=0.0349 (all data). The isotypic holmium species revealed: Pbcn, a=1292.8(2), b=436.2(2), , Z=2, R1=0.0206, and wR2=0.0406 (all data). The compounds exhibit a new type of structure, which is built up from layers of condensed BO₄-tetrahedra. Between the layers, the rare-earth cations are coordinated by 7+2 oxygen atoms. Furthermore, we report about temperature-resolved in situ powder diffraction measurements, DTA/TG, and IR-spectroscopic investigations into RE₄B₆O₁₄(OH)₂ (RE=Dy, Ho).     

Effect of transition metal doping and carbon doping on thermoelectric properties of YB66 single crystals

We have been investigating the high-temperature thermoelectric properties of some novel rare earth borides with a structure containing B₁₂ icosahedra. Doping effects on the TE properties in such systems were investigated for the first time. A series of Nb-doped YB₆₆ and C-doped YB₆₆ single crystals were grown by the floating zone method. The Nb-doped compounds have approximate chemical formulas ranging from YNb_0.30B₆₆ to YNb_0.33B₆₆ while the C-doped compound has a formula of YB₆₆C_0.6. The effect of Nb-doping on the thermoelectric properties was not monotonic and appears to be complex. As a result of Nb-doping, the room temperature resistivity and the characteristic temperature T₀ were considerably reduced. At room temperature the power factor of the Nb-doped YB₆₆ sample with 89% site occupancy was three times greater than that of non-doped YB₆₆. However, in the important high-temperature region, the non-doped sample actually exhibited the highest power factor for . Furthermore, owing to a structural feature of YB₆₆, thermal conductivity actually increases with doping of transition metals. Taking into account all the thermoelectric properties, transition metal doping of YB₆₆ is therefore not suitable for our purposes. On the other hand, doping of carbon, which is assumed not to go into the same sites as the transition metals, yielded a lowering of the thermal conductivity. Furthermore, contrary to Nb-doping, carbon doping did not result in a reversal of the relative magnitude of resistivity at extremely high temperatures and therefore, an increase in the figure of merit of factor 2 was realized at 1000 K.     

Crystal structures, physical properties and NMR experiments on the ternary rare-earth metal silicide boride compounds RE5Si2B8 (RE=Y, Sm, Gd, Tb, Dy, Ho)

The ternary rare-earth metal silicide borides RE₅Si₂B₈ (RE=Y, Sm, Gd, Tb, Dy Ho) were prepared by arc melting the elemental components and subsequent annealing up to . The crystal structure was determined for each term of the series from single-crystal X-ray data: tetragonal symmetry, space group P4/mbm, Z=2; unit cell parameters a=7.2616(3), and a=7.1830(2), for Sm₅Si₂B₈ and Ho₅Si₂B₈, respectively. The structure is a new type and can be structurally described as an intergrowth of ThB₄-like and U₃Si₂-like slabs of composition REB₄ and RE₃Si₂, respectively, alternating along the c direction. The boron and silicon substructures are wholly independent and well ordered. The magnetic properties are as follows: Y₅Si₂B₈ is a Pauli-type paramagnet above 1.8 K, Gd₅Si₂B₈ undergoes a weak (canted) ferromagnetic-like order at 70 K followed by a colinear antiferromagnetic spin alignment at 44 K. Tb₅Si₂B₈ and Dy₅Si₂B₈ order antiferromagnetically at a Néel temperature of T_N=45 and 28 K, respectively. In the paramagnetic regime, the effective moments are in good accord with the theoretical RE³⁺ free ion moments. The temperature dependence of the electrical resistivities for the Y, Gd, Tb, and Dy containing samples corroborates with the metallic state of the nonmagnetic (Y) and the magnetically ordered compounds. ¹¹B, ²⁹Si and ⁸⁹Y nuclear magnetic resonance (NMR) spectroscopy on nonmagnetic Y₅Si₂B₈ shows different signals, which correspond to the expected number of distinct crystallographic sites in the structure. ¹¹B NMR on Y₅Si₂B₈ indicates that the local magnetic susceptibilities are substantially different from the ones observed in the related compound YB₄.     

New members of ternary rare-earth metal boride carbides containing finite boron-carbon chains: RE25B14C26 (RE=Pr, Nd) and Nd25B12C28

New ternary rare-earth metal boride carbides RE₂₅B₁₄C₂₆ (RE=Pr, Nd) and Nd₂₅B₁₂C₂₈ were synthesized by co-melting the elements. Nd₂₅B₁₂C₂₈ is stable up to 1440 K. RE₂₅B₁₄C₂₆ (RE=Pr, Nd) exist above 1270 K. The crystal structures were investigated by means of single-crystal X-ray diffraction. Nd₂₅B₁₂C₂₈: space group P, a=8.3209(7) Å, b=8.3231(6) Å, c=29.888(2) Å, α=83.730(9)°, β=83.294(9)°, γ=89.764(9)°. Pr₂₅B₁₄C₂₆: space group P2₁/c, a=8.4243(5) Å, b=8.4095(6) Å, c=30.828(1) Å, β=105.879(4)°, V=2100.6(2) Å³, (R1=0.048 (wR2=0.088) from 2961 reflections with I_o>2σ(I_o)); for Nd₂₅B₁₄C₂₆ space group P2₁/c, Z=2, a=8.3404(6) Å, b=8.3096(6) Å, c=30.599(2) Å, β=106.065(1)°. Their structures consist of a three-dimensional framework of rare-earth metal atoms resulting from the stacking of slightly corrugated and distorted square nets, leading to cavities filled with cumulene-like molecules [B₂C₄]⁶⁻ and [B₃C₃]⁷⁻, nearly linear [BC₂]⁵⁻ and bent [BC₂]⁷⁻ units and isolated carbon atoms. Structural and theoretical analysis suggests the ionic formulation for RE₂₅B₁₄C₂₆: (RE³⁺)₂₅[B₂C₄]⁶⁻([B₃C₃]⁷⁻)₂([BC₂]⁵⁻)₄([BC₂]⁷⁻)₂(C⁴⁻)₄·5e⁻ and for Nd₂₅B₁₂C₂₈: (Nd³⁺)₂₅([B₂C₄]⁶⁻)₃([BC₂]⁵⁻)₄([BC₂]⁷⁻)₂(C⁴⁻)₄·7e⁻. Accordingly, extended Hückel tight-binding calculations indicate that the compounds are metallic in character.     

Novel rare-earth borosilicide RE1−xB12Si3.3−δ (RE=Y, Gd-Lu) (0?x?0.5, δ≈0.3): synthesis, crystal growth, structure analysis and properties

New rare-earth boron-rich compounds with the formula of RE_1−xB₁₂Si_3.3−δ (RE=Y, Gd-Lu) (0?x?0.5,δ≈0.3) have been synthesized. They belong to a new type of rhombohedral structure with the space group of R-3m (No. 166) and z=9. The lattice constants were measured from powder XRD data. Crystal structure solved from powder XRD data for Tb_0.68B₁₂Si₃ as a representative has been compared with that of YB_17.6Si_4.6 (or Y_0.68B₁₂Si_3.01), whose structure was solved from single-crystal reflection data. The structure model is confirmed by high-resolution transmission microscope analysis. The vibrational modes of the new crystals were measured by Raman spectroscopy. Temperature dependence of magnetic susceptibility which was measured for RE_1−xB₁₂Si_3.3−δ single crystals by SQUID revealed that they are paramagnetic materials down to 2.0 K.     

Bonding and doping of simple icosahedral-boride semiconductors

A simple model of the bonding and doping of a series of icosahedral-boride insulators is presented. Icosahedral borides contain clusters of boron atoms that occupy the 12 vertices of icosahedra. This particular series of icosahedral borides share both the stoichiometry B₁₂X₂, where X denotes a group V element (P or As), and a common lattice structure. The inter-icosahedral bonding of these icosahedral borides is contrasted with that of B₁₂O₂ and with that of α-rhombohedral boron. Knowledge of the various types of inter-icosahedral bonding is used as a basis to address effects of inter-icosahedral atomic substitutions. The inter-icosahedral bonding is maintained when an atom of a group V element is replaced with an atom of a group IV element, thereby producing a p-type dopant. However, changes of inter-icosahedral bonding occur upon replacing an atom of a group V element with an atom of a group VI element or with a vacancy. As a result, these substitutions do not produce effective n-type dopants. Moreover, partial substitution of boron atoms for atoms of group V elements generally renders these materials p-type semiconductors.     

Mn5Si3-type host-interstitial boron rare-earth metal silicide compounds RE5Si3: Crystal structures, physical properties and theoretical considerations

A series of binary rare-earth metal silicides RE₅Si₃ and ternary boron-interstitial phases RE₅Si₃B_x (RE=Gd, Dy, Ho, Lu, and Y) adopting the Mn₅Si₃-type structure, have been prepared from the elemental components by arc melting. Boron “stuffed” phases were subsequently heated at 1750 K within a high-frequency furnace. Crystal structures were determined for both binary and ternary series of compounds from single-crystal X-ray data: hexagonal symmetry, space group P6₃/mcm, Z=2. Boron insertion in the host binary silicides results in a very small decrease of the unit cell parameters with respect to those of the binaries. According to X-ray data, partial or nearly full boron occupancy of the interstitial octahedral sites in the range 0.6-1 is found. The magnetic properties of these compounds were characterized by the onset of magnetic ordering below 100 K. Boron insertion induces a modification of the transition temperature and θ_p values in most of the antiferromagnetic binary silicides, with the exception of the ternary phase Er₅Si₃B_x which was found to undergo a ferromagnetic transition at 14 K. The electrical resistivities for all binary silicides and ternary boron-interstitial phases resemble the temperature dependence of metals, with characteristic changes of slope in the resistivity curves due to the reduced electron scattering in the magnetically ordered states. Zintl-Klemm concept would predict a limiting composition RE₅Si₃B_0.6 for a valence compound and should then preclude the stoichiometric formula RE₅Si₃B. Density functional theory calculations carried out on some RE₅Si₃Z_x systems for different interstitial heteroatoms Z and different x contents from 0 to 1 give some support to this statement.     

New examples of ternary rare-earth metal boride carbides containing finite boron-carbon chains: The crystal and electronic structure of RE15B6C20 (RE=Pr, Nd)

The ternary rare-earth metal boride carbides RE₁₅B₆C₂₀ (RE=Pr, Nd) were synthesized by co-melting the elements. They exist above 1270 K. Their crystal structures were determined from single-crystal X-ray diffraction data. Both crystallize in the space group P1¯, Z=1, a=8.3431(8) Å, b=9.2492(9) Å, c=8.3581(8) Å, α=84.72(1)°, β=89.68(1)°, γ =84.23(1)° (R1=0.041 (wR2=0.10) for 3291 reflections with I_o>2σ(I_o)) for Pr₁₅B₆C₂₀, and a=8.284(1) Å, b=9.228(1) Å, c=8.309(1) Å, α=84.74(1)°, β=89.68(1)°, γ=84.17(2)° (R1=0.033 (wR2=0.049) for 2970 reflections with I_o>2σ(I_o)) for Nd₁₅B₆C₂₀. Their structure consists of a three-dimensional framework of rare-earth metal atoms resulting from the stacking of slightly corrugated and distorted square nets, leading to cavities filled with unprecedented B₂C₄ finite chains, disordered C₃ entities and isolated carbon atoms, respectively. Structural and theoretical analyses suggest the ionic formulation (RE³⁺)₁₅([B₂C₄]⁶⁻)₃([C₃]⁴⁻)₂(C⁴⁻)₂·11ē. Accordingly, density functional theory calculations indicate that the compounds are metallic. Both structural arguments as well as energy calculations on different boron vs. carbon distributions in the B₂C₄ chains support the presence of a CBCCBC unit. Pr₁₅B₆C₁₈ exhibits antiferromagnetic order at T_N=7.9 K, followed by a meta-magnetic transition above a critical external field B>0.03 T. On the other hand, Nd₁₅B₆C₁₈ is a ferromagnet below T_C≈40 K.     

Synthesis of Boron-Containing Siloxanes by Reaction of Hydroxy-<Emphasis Type="Italic">closo</Emphasis>-Decaborates with Dihalosilanes

A new approach to the preparation of boron-containing siloxanes based on cluster boron anions has been suggested. This approach consists in the reaction of hydroxy derivatives of closo-decaborate anion [B₁₀H_10–n(OH)_n]^2– (n = 1, 2) with dihalosilanes SiR₂Cl₂ (R = CH₃, _t- C₄H₉, C₆H₅). The obtained compounds have been characterized by IR, ESI/MS, and NMR (1H, 11B, 13C) spectroscopy.     

Theoretical prediction of a new noncluster lithium boride Li<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>B<Subscript>9</Subscript> with one-dimensional ionic and two-dimensional electronic conductivity in mutually perpendicular directions

Based on the features of the structure of B₅H₁₁ and other known boranes, the possibility of the existence of a new structure type—LiB₉ (hexagonal, space group P6₃ cm, a = 0.565 nm, c = 0.504 nm, Z = 2, d = 2.49 g/cm³)—was predicted. The basal plane contains perforated deltahedral layers of boron atoms with delocalized electrons combined into a framework by fixed 3c2e bonds. Discrete, almost cylindrical channels accommodating Li⁺ cations are perpendicular to the layers. Thermal or electrochemical removal of part of lithium should be favorable for the appearance or buildup of the cationic conductivity with the possible intermediate formation of lithium incommensurate phase. Valence-scheme analysis of boride layers revealed low-barrier hole bipolaron conductivity within the layers and considerable hindrance to interlayer electron transport.     

Synthesis, characterization and thermal behaviour of new copper and rare-earth metal tungstates

Three series of new copper and rare-earth metal tungstates with the formulas: CuRE ₂W₂O₁₀ (RE = Nd, Sm, Eu) and Cu₃ RE ₂W₄O₁₈ (RE = Sm, Eu or RE = Dy, Ho, Er) were synthesized by the solid-state reaction method. The CuRE ₂W₂O₁₀ and Cu₃ RE ₂W₄O₁₈ (RE = Dy, Ho, Er) compounds crystallize in the monoclinic system. The Cu₃ RE ₂W₄O₁₈ phases with the large rare-earth ions crystallize in the triclinic system. All obtained compounds melt incongruently below 1273 K. The anion lattice of the Cu₃ RE ₂W₄O₁₈ phases is built from isolated groups of octahedra (W₄O₁₆)^8?, while CuRE ₂W₂O₁₀ from WO₆ octahedra forming structural elements [(W₂O₉)^6?]^∞. The EPR spectra of analyzed compounds consisted of an intense line originating generally from the rare-earth ions and a weak, narrow line from Cu²⁺ separate centers appearing only on the surface of the grains. The absence of bulk copper in the EPR spectrum is probably due to a very short relaxation time of the Cu²⁺ subsystem.     

Synthesis, chemical bonding and physical properties of RERhB4 (RE=Y, Dy-Lu)

The compounds of rare-earth metals with rhodium and boron RERhB₄ (RE=Y, Dy-Lu) crystallize with the orthorhombic structure type YCrB₄ (space group Pbam, Pearson symbol oP24). The crystal structures of the compounds with RE=Y, Er, Tm and Yb were refined by using single-crystal diffraction data. Analysis of chemical bonding for YRhB₄ and YbRhB₄ was performed by electron localizability indicator and by calculation of quantum chemical charges (quantum theory of atoms in molecules). Boron and rhodium form the 3-D polyanion containing planar nets of three-bonded boron atoms interconnected by rhodium along [001]. The interaction of the RE species with the rhodium-boron polyanion is predominantly ionic. Magnetic susceptibility data of TmRhB₄ and YbRhB₄ showed that the RE species are in 4f¹² (Tm) and 4f¹³ (Yb) electronic states, respectively. In the low-temperature region, the specific heat revealed a Schottky anomaly for TmRhB₄ while an antiferromagnetic transition is observed at 3.5 K for YbRhB₄. X-ray absorption measurement at the Yb L_III edge for YbRhB₄ reveals the 4f¹³ state of ytterbium.     

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.     

Carbon in boron carbide: The crystal structure of B<Subscript>11.4</Subscript>C<Subscript>3.6</Subscript>

A single crystal of boron carbide obtained from a self-propagating high-temperature synthesis (SHS) product was studied by X-ray crystallography: B_11.4C_3.6, a = 5.594(2) Å, c = 11.977(7) Å, V = 324.6(7) ų, space group R3m, Z = 3, ρ_calcd = 2.56 g/cm³, R = 0.048. The content of carbon in the single crystal was estimated at ～24 at % from analysis of the unit cell parameters, bond lengths, and the volume of B_{12 ? x} C _x icosahedra, which demonstrated the possibility of obtaining by SHS carbon-rich boron carbide crystals due to the substitution of carbon atoms for boron atoms in icosahedra. Comparison of the X-ray crystallographic data for single crystals of boron carbide with the results of quantum-chemical calculations (an ab initio method (the 3–21G basis set) with geometry optimization) showed that the C-B-C group in a crystal has a nonlinear structure.     

Enthalpies of formation of rare earth orthovanadates, REVO4

Rare earth orthovanadates, REVO₄, having the zircon structure, form a series of materials interesting for magnetic, optical, sensor, and electronic applications. Enthalpies of formation of REVO₄ compounds (RE=Sc, Y, Ce-Nd, Sm-Tm, Lu) were determined by oxide melt solution calorimetry in lead borate (2PbO·2B₂O₃) solvent at 1075 K. The enthalpies of formation from oxide components become more negative with increasing RE ionic radius. This trend is similar to that obtained for the rare earth phosphates.     

Der Dreistoff Yttrium—Bor—Kohlenstoff

Zusammenfassung Der Dreistoff Y–B–C wird an Hand von abgeschreckten Schmelzproben röntgenographisch untersucht. Folgende Phasen wurden hinsichtlich ihrer Kristallstruktur charakterisiert: Y₁₅C₁₉ (-Phase), YB₂C₂, YB₂C und YBC.

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The ternary system yttrium—boron—carbon

The ternary system yttrium–boron–carbon has been investigated by means of X-rays. The crystal structure of the phases Y₁₅C₁₉ (-phase), YB₂C₂, YB₂C, and YBC has been derived.

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Mit 4 Abbildungen     

Thermal properties of rare earth dodecaborides

We have measured heat capacity and thermal expansion of rare earth dodecaborides REB₁₂ (RE=Y, Tb-Tm, Lu). YB₁₂ and LuB₁₂ are diamagnetics whereas the other dodecaborides are ordered antiferromagnetically. The amplitude of the heat capacity discontinuity at the Néel temperature and the shape of the heat capacity variation in the critical region for all these antiferromagnetics are characteristics for amplitude-modulated magnetic structures. In the ordered state TbB₁₂ reveals two first-order phase transitions, most likely due to magnetic structure changes. The heat capacity of ErB₁₂ just below the Néel point shows an anomaly of unclear origin. From the Schottky contribution to the heat capacity we have determined crystal field parameters. They are completely different than that is estimated from Point Charge Model.     

Structural analysis of boron compounds using 11B-3QMAS solid state NMR

The structural analysis of three boron compounds, boron carbide (B₄C), silicon tetraboride (SiB₄) and hexagonal-boron nitride (h-BN), were performed using 2D ¹¹B-triple quantum MAS (3QMAS) solid state NMR capable of averaging the second-order quadrupolar interaction of ¹¹B that cause line broadening, splitting and low frequency shift of the central transition (−1/2, 1/2). The coordination number around the boron atom and structural symmetry of each boron compound is discussed by means of the isotropic chemical shift Δσ and the quadrupolar coupling constant C_Q calculated from 3QMAS spectra. Δσ of SiB₄ is quite larger than that of B₄C, which is thought to be caused by its structural distortion and distribution. Δσ of h-BN was found to be higher frequency shift obviously than that of B₄C and SiB₄ because of the difference of the boron coordination number, three-coordinated in h-BN and six-coordinated in B₄C and SiB₄. h-BN has very large C_Q compared to other two boron compounds since the h-BN forming a two-dimensional network has less structural symmetry than B₄C and SiB₄.