Magnetic and transport properties of RCr0.3Ge2 (R=Tb, Dy, Ho and Er) compounds

The magnetic and transport properties of ternary rare-earth chromium germanides RCr_0.3Ge₂ (R=Y and Tb-Er) have been determined. X-ray and neutron diffraction studies indicate that these compounds have the CeNiSi₂-type structure (space group Cmcm) [1]. Magnetic measurements reveal the antiferromagnetic ordering below T_N equal to 18.5 K (R=Tb), 11.8 K (Dy), 5.8 K (Ho) and 3.4 K (Er). From the neutron diffraction data the magnetic structures have been determined. For TbCr_0.3Ge₂ and DyCr_0.3Ge₂ at low temperatures the magnetic ordering can be described by two vectors k₁=(,0,0) and k₂=(,0,), and k₁′=(,0,0) and k₂′=(,0,), respectively. In HoCr_0.3Ge₂ and ErCr_0.3Ge₂ the ordering can be described by one propagation vector equal to (,,0) and (0,0,0.4187(2)), respectively. In DyCr_0.3Ge₂ some change in the magnetic ordering is observed at T_t=5.1 K. In temperature range from T_t to T_N the magnetic ordering is given by one propagation vector k=(,0,0). YCr_0.3Ge₂ is a Pauli paramagnet down to 1.72 K which suggests that in the entire RCr_0.3Ge₂ series the Cr atoms do not carry magnetic moments. All compounds studied exhibit metallic character of the electrical conductivity. The temperature dependencies of the lattice parameters reveal strong magnetostriction effect at the respective Nèel temperatures.     

Preparation, crystal structure, and superconductive characteristics of new oxynitrides (Nb1−xMx)(N1−yOy) where M=Mg, Si, and x≈y

New niobium oxynitrides containing either magnesium or silicon were prepared at 1000 °C by ammonia nitridation of oxide precursors obtained via the citrate route. The products had rock-salt type crystal structures. Crystallinity was improved by annealing in 0.5 MPa N₂ and the final compositions were (Nb_0.95Mg_0.05)(N_0.92O_0.08) at 1500 °C and (Nb_0.87Si_0.09□_0.04)(N_0.87O_0.13) at 1200 °C. The magnesium and oxide ions partially co-substitute the niobium and nitride ions in the octahedral sites of the δ-NbN lattice, respectively. Silicon ions were also successfully doped together with oxide ions into the rock-salt type NbN lattice. The Si doped product exhibited relatively large displacement at the octahedral sites and was accompanied by a small amount of cation vacancies. Superconductivity was improved by annealing to obtain critical temperatures/volume fractions of T_c=17.6 K/100% for Mg- and T_c=16.2 K/95% for the Si-doped niobium oxynitrides.     

Structural and magnetic properties of RMn2−xFexD6 compounds (R=Y, Er; x≤0.2) synthesized under high deuterium pressure

RMn_2−xFe_xD₆ compounds were obtained by applying a deuterium pressure of several kbar to RMn_2−xFe_x compounds for x≤0.2 and R=Y, Er. These compounds are isostructural to RMn₂D₆ compounds and crystallize in a K₂PtCl₆ type structure with a random substitution of R and half the Mn atoms in the same 8c site whereas the other Mn atoms are located on the 4a site and surrounded by six D atoms (24e site). According to neutron powder diffraction analysis the Fe atoms are preferentially substituted on the 4a site. YMn_2−xFe_xD₆ compounds are paramagnetic and their molar susceptibility follows a modified Curie-Weiss law. ErMn_2−xFe_xD₆ compounds display a ferromagnetic behavior at 2 K, but their saturation magnetization (M_S∼4.0 μ_B/f.u.) is half that of their parent compounds (M_S∼8.0 μ_B/f.u.). The neutron diffraction patterns of ErMn_1.8Fe_0.2D₆ display below 13 K both ferromagnetic and antiferromagnetic short range order, which can be related to a disordered distribution of Er moments. The paramagnetic temperatures of ErMn_2−xFe_xD₆ compounds are negative and decrease versus the Fe content whereas they are positive and increase for their parent compounds.     

Phase formation, crystal structures and magnetic properties of perovskite-type phases in the system La2Co1+z(MgxTi1−x)1−zO6

Perovskite-type cobaltates in the system La₂Co_1+z(Mg_xTi_1−x)_1−zO₆ were studied for z=0≤x≤0.6 and 0≤x<0.9, using X-ray and neutron powder diffraction, electron diffraction (ED), magnetic susceptibility measurements and X-ray absorption near-edge structure (XANES) spectroscopy. The samples were synthesised using the citrate route in air at 1350 °C. The space group symmetry of the structure changes from P2₁/n via Pbnm to R3¯c with both increasing Mg content and increasing Co content. The La₂Co(Mg_xTi_1−x)O₆ (z=0) compounds show anti-ferromagnetic couplings of the magnetic moments for the Co below 15 K for x=0, 0.1 and 0.2. XANES spectra show for the compositions 0≤x≤0.5 a linear decrease in the L₃/(L₃+L₂) Co-L_2,3 edge branching ratio with x, in agreement with a decrease of the average Co ion spin-state, from a high-spin to a lower-spin-state, with decreasing nominal Co²⁺ ion content.     

Synthesis, structure, and magnetic properties of SrMn1−xGaxO3−δ (x=0-0.5) perovskites

We report the synthesis of SrMn_1−xGa_xO_3−δ perovskite compounds and describe the dependence of their phase stability and structural and physical properties over extended cation and oxygen composition ranges. Using special synthesis techniques derived from thermogravimetric measurements, we have extended the solubility limit of random substitution of Ga³⁺ for Mn in the cubic perovskite phase to x=0.5. In the cubic perovskite phase the maximum oxygen content is close to 3−x/2, which corresponds to 100% Mn⁴⁺. Maximally oxygenated solid solution compounds are found to order antiferromagnetically for x=0-0.4, with the transition temperature linearly decreasing as Ga content increases. Increasing the Ga content introduces frustration into the magnetic system and a spin-glass state is observed for SrMn_0.5Ga_0.5O_2.67(3) below 12 K. These properties are markedly different from the long-range antiferromagnetic order below 180 K observed for the layer-ordered compound Sr₂MnGaO_5.50 with nominally identical chemical composition.     

Preparation and structural study from neutron diffraction data of R2MoO6 (R=Dy, Ho, Er, Tm, Yb, Y)

The title compounds have been prepared as polycrystalline powders by thermal treatments of stoichiometric mixtures of R₂O₃ and MoO₃ in air. The room-temperature crystal structure for all the series has been refined from high-resolution neutron powder diffraction data. All the phases are isostructural (space group C2/c, Z=8) with the polymorph α-R₂MoO₆, typified by Sm₂MoO₆. The structure contains four zigzag, one-dimensional MoO₅ polyhedral rows per unit cell, running through the RO₈ polyhedral framework along the [001] direction. MoO₅ form discrete units (i.e. do not share common oxygen), with Mo-O distances ranging from 1.77 to 2.24 Å, although the oxygen coordination can be extended to distances of about 3.1 Å, giving rise to strongly distorted MoO₈ scalenohedra. Thus, MoO₈ and RO₈ polyhedra are fully ordered in R₂MoO₆ compounds, which in fact can be considered as superstructures of fluorite (M₃O₆), containing 24 MO₂ fluorite units per unit cell, with unit-cell parameters related to that of cubic fluorite ( Å). A bond valence study demonstrates that the present crystal structure is especially stable for small rare-earth cations, and becomes more unstable when the R³⁺ size increases, thus explaining the observed preference of the large rare-earth molybdates for polymorphs β and γ with the same stoichiometry.     

Crystal structures of the La3AgSnSe7 and R3Ag1−δSnS7 (R=La, Ce; δ=0.18−0.19) compounds

The crystal structures of new quaternary compounds La₃AgSnSe₇ (space group P6₃, Pearson symbol hP24, a=1.0805(4) nm, c=0.6245(1) nm, R1=0.0315), La₃Ag_0.82SnS₇ (space group P6₃, Pearson symbol hP23.64, a=1.0399(1) nm, c=0.6016(1) nm, R1=0.0149) and Ce₃Ag_0.81SnS₇ (space group P6₃, Pearson symbol hP23.62, a=1.0300(1) nm, c=0.6002(1) nm, R1=0.0151) were determined by means of X-ray single crystal diffraction. Structural investigations of the R₃Ag_1−δSnS₇ (R=La, Ce; δ=0.18-0.19(1)) compounds at 450 and 530 K were performed. Low temperature data (12 K) for Ce₃Ag_0.81SnS₇ were also collected. The nearest neighbours of the La(Ce), Ag and Sn atoms are exclusively Se(S) atoms. The latter form distorted trigonal prisms around the La(Ce) atoms, and distorted tetrahedrons around the Sn atoms. The Ag (Ag1) atoms have triangular surroundings: they are located very close to the planes built of three Se(S) atoms. The Ag2 atoms in the structures of the La₃Ag_0.82SnS₇, Ce₃Ag_0.81SnS₇ compounds are located practically in the centres of trigonal antiprisms. The pseudo-potentials determined through the Ag atoms show relatively low barrier between two nearest positions which decreases when temperature rises.     

X-ray and neutron powder diffraction study of the order-disorder transition in Eu2IrH5 and the mixed crystal compounds Eu2−xAxIrH5 (A=Ca, Sr; x=1.0, 1.5)

The title compounds and their deuterides have been prepared by solid-state and solid-gas reactions from the elements and investigated by X-ray and neutron powder diffraction as a function of temperature. At room temperature they crystallize with an anion-deficient cubic K₂PtCl₆-type structure (space group ) in which five hydrogen (deuterium) atoms surround iridium randomly on six octahedral sites with average bond distances of Ir-D=169-171 pm. At low temperature they undergo a tetragonal deformation (space group I4/mmm) to the partially ordered Sr₂IrD₅ (T=4.2K)-type structure in which four hydrogen (deuterium) atoms occupy planar sites with full occupancy (Ir-D=166-170 pm) and two hydrogen (deuterium) atoms axial sites (Ir-D=174-181 pm) with ∼50% occupancy, i.e., the data are consistent with a mixture of square-pyramidal [IrD₅]⁴⁻ complexes pointing in two opposite directions. The transitions occur at ∼240 K (Eu_0.5Ca_1.5IrD₅, Eu_0.5Sr_1.5IrD₅), ∼210 K (EuSrIrD₅), ∼200 K (EuCaIrD₅, Eu₂IrD₅), and are presumably of first order.     

Magnetostructural correlations in the antiferromagnetic Co2−x Cux(OH)AsO4 (x=0 and 0.3) phases

The Co_2−xCu_x(OH)AsO₄ (x=0 and 0.3) compounds have been synthesized under mild hydrothermal conditions and characterized by X-ray single-crystal diffraction and spectroscopic data. The hydroxi-arsenate phases crystallize in the Pnnm orthorhombic space group with Z=4 and the unit-cell parameters are a=8.277(2) Å, b=8.559(2) Å, c=6.039(1) Å and a=8.316(1) Å, b=8.523(2) Å, c=6.047(1) Å for x=0 and 0.3, respectively. The crystal structure consists of a three-dimensional framework in which M(1)O₅-trigonal bipyramid dimers and M(2)O₆-octahedral chains (M=Co and Cu) are present. Co₂(OH)AsO₄ shows an anomalous three-dimensional antiferromagnetic ordering influenced by the magnetic field below 21 K within the presence of a ferromagnetic component below the ordering temperature. When Co²⁺ is partially substituted by Cu²⁺ions, Co_1.7Cu_0.3(OH)AsO₄, the ferromagnetic component observed in Co₂(OH)AsO₄ disappears and the antiferromagnetic order is maintained in the entire temperature range. Heat capacity measurements show an unusual magnetic field dependence of the antiferromagnetic transitions. This λ-type anomaly associated to the three-dimensional antiferromagnetic ordering grows with the magnetic field and becomes better defined as observed in the non-substituted phase. These results are attributed to the presence of the unpaired electron in the dx²−y² orbital and the absence of overlap between neighbour ions.     

Structural characterization and physical properties of the system La1.33LixCrxTi2−xO6

New phases which arise from partial substitution of Ti⁴⁺ by Cr³⁺ and Li⁺ of the compound La_2/3TiO₃ have been obtained, giving rise to the series La_1.33Li_xCr_xTi_2−xO₆ (x=0.66, 0.55 and 0.44). These phases adopt a perovskite-type structure as deduced from their structural characterization. Rietveld's analyses of neutron diffraction data show that it is orthorhombic (S.G. Pbnm) with ordered domains. Conductivity has been examined by complex impedance spectroscopy and it increases with increasing lithium and chromium content. These materials behave as mixed conductors with low activation energies. Magnetic susceptibility variation with temperature shows antiferromagnetic interactions at the lowest temperatures.     

Structural and magnetic study of RFe0.5V0.5O3 (R=Y, Eu, Nd, La) perovskite compounds

B-site disordered RFe_0.5V_0.5O₃ compounds, with R=La, Nd, Eu and Y, have been prepared by solid-state reaction technique and their structures and magnetic properties have been investigated through X-ray powder diffraction, time-of-flight neutron powder diffraction and magnetization measurements at temperatures ranging from 5 to 700 K. The four compounds can be described as distorted perovskites with space group symmetry Pbnm and a⁺b⁻b⁻ tilt system. The studied compounds also show antiferromagnetic ordering with Neel temperatures of 299, 304, 304, and 335 K respectively. The magnetic structures of R=La, Nd and Y compounds were determined from the neutron powder diffraction as G_z with observed magnetic moments of 2.55, 2.54 and 2.69μ_B at 30, 40 and 40 K, respectively.     

Synthesis and characterization of Sr0.75Y0.25Co1−xMxO2.625+δ (M=Ga, 0.125?x?0.500 and M=Fe, 0.125?x?0.875)

The effect of replacing Co³⁺ by Ga³⁺ and Fe³⁺ in the perovskite-related tetragonal phase Sr_0.75Y_0.25CoO_2.625 with unit cell parameters: a=2a_p, and c=4a_p (314 phase) has been investigated. The 314 phase is formed by Sr_0.75Y_0.25Co_1−xM_xO_2.625+δ, with x?0.375 for M=Ga and x?0.625 for M=Fe. High-resolution transmission electron microscopy and electron diffraction revealed frequent microtwinning in the iron-containing compounds, in contrast to the Ga-substituted 314 phases. Diffraction experiments and electron microscope images indicated that at higher Fe contents, 0.75?x?0.875, a disordered cubic perovskite structure forms. The crystal structures of Sr_0.75Y_0.25Co_0.75Ga_0.25O_2.625 and Sr_0.75Y_0.25Co_0.5Fe_0.5O_2.625+δ were refined using neutron powder diffraction data. It was found that the oxygen content of Sr_0.75Y_0.25Co_0.5Fe_0.5O_2.625+δ is higher than in Fe-free 314 phase, so that δ corresponds to 0.076, whereas δ=0 in Sr_0.75Y_0.25Co_0.75Ga_0.25O_2.625+δ. Magnetization measurements on the unsubstituted Sr_0.7Y_0.3CoO_2.62 and Ga-substituted Sr_0.75Y_0.25Co_0.75Ga_0.25O_2.625 compounds indicate the presence of a ferromagnetic-like contribution to the measured magnetization at 320 and 225 K, respectively, while replacing Co by Fe leads to the suppression of this contribution. A neutron diffraction study shows that the Sr_0.75Y_0.25Co_0.5Fe_0.5O_2.625+δ compound is G-type antiferromagnetic at room temperature, whereas Sr_0.75Y_0.25Co_0.75Ga_0.25O_2.625 does not exhibit magnetic ordering at room temperature.     

Structural and magnetic properties of Nd18Li8Co4−xFexO39−y and Nd18Li8Co4−xTixO39−y

Nd₁₈Li₈Co₃FeO_39−y, Nd₁₈Li₈CoFe₃O_39−y and Nd₁₈Li₈Co₃TiO_39−y have been synthesised and characterised by neutron powder diffraction, magnetometry and Mössbauer spectroscopy. Their cubic structure (Pm3?n, a∼11.9 Å) is based on intersecting <1 1 1> chains comprised of alternating octahedral and trigonal-prismatic coordination sites. These chains lie within hexagonal-prismatic cavities formed by a Nd-O framework. Each compound has an incomplete oxide sublattice (y∼1), with vacancies located around the octahedral sites that lie at the points of chain intersection. These sites are fully occupied by a disordered arrangement of transition-metal cations but only 75% of the remaining octahedral sites are occupied. The trigonal-prismatic sites are fully occupied by lithium except in the case of Nd₁₈Li₈CoFe₃O_39−y where some iron is present. Antiferromagnetic interactions are present on the Nd sublattice in each composition, but a spin glass forms below 5 K when a high concentration of spins is also present on the octahedral sites.     

Structural phase transition and magnetic properties of the Sr2FeRe1−xBx′O6 (B′=Nb, Ta; x=0, 0.1) double perovskites

The double perovskites, Sr₂FeReO₆ and Sr₂FeRe_0.9M_0.1O₆ (M=Nb, Ta) have been obtained by soft synthesis methods which yield homogeneous particles of micrometric grain size. The materials have been studied by X-ray and neutron powder diffraction, scanning electron microscopy and magnetic measurements. Rietveld refinements show that the compounds adopt a tetragonal I4/mmm structure at high temperatures and monoclinic P2₁/n below the transition temperature. The magnetic structures were determined by neutron powder diffraction at 100 and 300 K for the Sr₂FeReO₆, Sr₂FeRe_0.9Nb_0.1O₆ and Sr₂FeRe_0.9Ta_0.1O₆ phases, respectively. Evidence for a ferrimagnetic coupling between the Fe³⁺ and Re⁵⁺ sublattices has been observed. Magnetic measurements yield magnetic moments lower than the theoretical ones being in accord with the antisite disorder of 25% in the B-B′ positions.     

Neutron diffraction and magnetic study of the Nd0.7Pb0.3Mn1−xFexO3 (0?x?0.1) perovskites

The effect of Fe doping on the ferromagnetic Nd_0.7Pb_0.3Mn_1−xFe_xO₃ (x=0, 0.025, 0.05, 0.075, 0.1) phases has been studied in order to analyze the double-exchange interaction. The structural and magnetic study has been carried out by neutron powder diffraction and susceptibility measurements between 1.7 and 300 K. The substitution of Fe at the Mn site results in reductions in both the Curie temperature T_c and the magnetic moment per Mn ion without appreciable differences in the crystal structures. All the compounds crystallize in Pnma space group. The thermal evolution of the lattice parameters of the Nd_0.7Pb_0.3Mn_1−xFe_xO₃ (x=0.025, 0.05, 0.075) compounds shows discontinuities in volume and lattice parameters close to the magnetic transition temperature. Increasing amounts of Fe³⁺ reduces the double exchange interactions and no magnetic contribution for x=0.1 is observed. The magnetic structures of Nd_0.7Pb_0.3Mn_1−xFe_xO₃ (x=0, 0.025, 0.05, 0.075) compounds show that the Nd and Mn ions are ferromagnetically ordered.     

Crystal structure of Bi1−xTbxFeO3 from high-resolution neutron diffraction

Samples of Bi_1−xTb_xFeO₃, with x=0.05, 0.10, 0.15, 0.20 and 0.25, have been synthesised by solid state reaction. The crystal structures of the perovskite phases, characterised via Rietveld analysis of high resolution powder neutron diffraction data, reveal a structural transition from the R3c symmetry of the parent phase BiFeO₃ to orthorhombic Pnma symmetry, which is complete for x=0.20. The x=0.10 and 0.15 samples are bi-phasic. The transition from a rhombohedral to orthorhombic unit cell is suggested to be driven by the dilution of the stereochemistry of the Bi³⁺ lone pair at the A-site. The G-type antiferromagnetic spin structure, the size of the ordered magnetic moment (∼3.8 μ_B) and the T_N (∼375 °C) are relatively insensitive to increasing Tb concentrations at the A-site.     

Neutron powder diffraction study of the magnetic and crystal structures of SrFe2(PO4)2

The crystal and magnetic structures of SrFe²⁺₂(PO₄)₂ have been determined by neutron powder diffraction data at low temperatures (space group P2₁/c (no. 14); Z=4; a=9.35417(13) Å, b=6.83808(10) Å, c=10.51899(15) Å, and β=109.5147(7)° at 15 K). Two magnetic phase transitions were found at T₁=7.4 K (first-order phase transition) and T₂=11.4 K (second-order phase transition). The transition at T₂ was hardly detectable by dc and ac magnetization measurements, and a small anomaly was observed by specific heat measurements. At T₁, strong anomalies were found by dc and ac magnetization and specific heat. The structure of SrFe₂(PO₄)₂ consists of linear four-spin cluster units, Fe2-Fe1-Fe1-Fe2. Below T₁, the propagation vector of the magnetic structure is k=[0,0,0]. The magnetic moments of the inner Fe1-Fe1 atoms of the four-spin cluster unit are ferromagnetically coupled. The magnetic moment of the outer Fe2 atom is also ferromagnetically coupled with that of the Fe1 atom but with spin canting. The four-spin cluster units form ferromagnetic layers parallel to the [−101] plane, while these layers are stacked antiferromagnetically in the [−101] direction. Spin canting of the outer Fe2 atoms provides a weak ferromagnetic moment of about 1 μ_B along the b-axis. The refined magnetic moments at 3.5 K are 4.09 μ_B for Fe1 and 4.07 μ_B for Fe2. Between T₁ and T₂, a few weak magnetic reflections were observed probably due to incommensurate magnetic order.     

Magnetic and thermodynamic properties of NdT2Ge2 (T=Pd, Ag) compounds

Physical properties of NdPd₂Ge₂ and NdAg₂Ge₂, crystallizing with the tetragonal ThCr₂Si₂-type crystal structure, were investigated by means of magnetic, calorimetric, electrical transport as well as by neutron diffraction measurements. The specific heat studies and neutron diffraction measurements were performed down to 0.30 K and 0.47 K, respectively. Both compounds exhibit antiferromagnetic ordering below T_N equal to 1.5 K for NdPd₂Ge₂ and 1.8 K for NdAg₂Ge₂. Neutron diffraction data for the latter germanide indicate antiferromagnetic collinear structure described by the propagation vector k=(0.5, 0, 0.5). The Nd magnetic moments equal to 2.24(5) μ_B at 0.47 K are aligned along the a-axis and have the +− sequence within the crystal unit cell. For NdPd₂Ge₂ only very small Bragg peaks of magnetic origin were observed in the neutron diffraction patterns measured below T_N, thus hampering determination of the magnetic structure. Both compounds exhibit metallic-like electrical conduction. From the specific heat data the crystal electric field (CEF) levels schemes were determined. Difference between the overall CEF splitting in the two compounds is correlated with their structural parameters.     

Crystal structure and physical properties of quaternary clathrates Ba8ZnxGe46−x−ySiy, Ba8(Zn,Cu)xGe46−x and Ba8(Zn,Pd)xGe46−x

Three series of vacancy-free quaternary clathrates of type I, Ba₈Zn_xGe_46−x−ySi_y, Ba₈(Zn,Cu)_xGe_46−x, and Ba₈(Zn,Pd)_xGe_46−x, have been prepared by reactions of elemental ingots in vacuum sealed quartz at 800 °C. In all cases cubic primitive symmetry (space group Pm3?n, a∼1.1 nm) was confirmed for the clathrate phase by X-ray powder diffraction and X-ray single crystal analyses. The lattice parameters show a linear increase with increase in Ge for Ba₈Zn_xGe_46−x−ySi_y. M atoms (Zn, Pd, Cu) preferably occupy the 6d site in random mixtures. No defects were observed for the 6d site. Site preference of Ge and Si in Ba₈Zn_xGe_46−x−ySi_y has been elucidated from X-ray refinement: Ge atoms linearly substitute Si in the 24k site whilst a significant deviation from linearity is observed for occupation of the 16i site. A connectivity scheme for the phase equilibria in the “Ba₈Ge₄₆” corner at 800 °C has been derived and a three-dimensional isothermal section at 800 °C is presented for the Ba-Pd-Zn-Ge system. Studies of transport properties carried out for Ba₈{Cu,Pd,Zn}_xGe_46−x and Ba₈Zn_xSi_yGe_46−x−y evidenced predominantly electrons as charge carriers and the closeness of the systems to a metal-to-insulator transition, fine-tuned by substitution and mechanical processing of starting material Ba₈Ge₄₃. A promising figure of merit, ZT ∼0.45 at 750 K, has been derived for Ba₈Zn_7.4Ge_19.8Si_18.8, where pricey germanium is exchanged by reasonably cheap silicon.     

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.