Synthesis, crystal and magnetic structure of a novel brownmillerite-type compound Ca2Co1.6Ga0.4O5

The novel compound Ca₂Co_1.6Ga_0.4O₅ with brownmillerite (BM) structure has been prepared from citrates at 950 °C. The crystal structure of Ca₂Co_1.6Ga_0.4O₅ was refined, from neutron powder diffraction (NPD) data, in space group Pnma, , , , χ²=1.798, , R_wp=0.0378 and R_p=0.0292. On the basis of the NPD refinement the compound was found to be a G-type antiferromagnet (space group Pn^′m^′a) at room temperature, with the magnetic moments of cobalt atoms directed along chains of tetrahedra in the BM structure. Electron diffraction and electron microscopy studies revealed disorder in the crystallites, which can be interpreted as the presence of slabs with BM-type structure of Pnma and I2mb symmetry.     

The study of Bi3B5O12: synthesis, crystal structure and thermal expansion of oxoborate Bi3B5O12

The paper presents a new data on the crystal structure, thermal expansion and IR spectra of Bi₃B₅O₁₂. The Bi₃B₅O₁₂ single crystals were grown from the melt of the same stoichiometry by Czochralski technique. The crystal structure of Bi₃B₅O₁₂ was refined in anisotropic approximation using single-crystal X-ray diffraction data. It is orthorhombic, Pnma, a=6.530(4), b=7.726(5), c=18.578(5) Å, V=937.2(5) Å³, Z=4, R=3.45%. Bi³⁺ atoms have irregular coordination polyhedra, Bi(1)O₆ (d(B-O)=2.09-2.75 Å) and Bi(2)O₇ (d(B-O)=2.108-2.804 Å). Taking into account the shortest bonds only, these polyhedra are considered here as trigonal Bi(1)O₃ (2.09-2.20 Å) and tetragonal Bi(2)O₄ (2.108-2.331 Å) irregular pyramids with Bi atoms in the tops of both pyramids. The BiO₄ polyhedra form zigzag chains along b-axis. These chains alternate with isolated anions [B₂^IVB₃^IIIO₁₁]⁷⁻ through the common oxygen atoms to form thick layers extended in ab plane. A perfect cleavage of the compound corresponds to these layers and an imperfect one is parallel to the Bi-O chains. The Bi₃B₅O₁₂ thermal expansion is sharply anisotropic (α₁₁≈α₂₂=12, α₃₃=3×10⁻⁶ °C⁻¹) likely due to a straightening of the flexible zigzag chains along b-axis and decreasing of their zigzag along c-axis. Thus the properties like cleavage and thermal expansion correlate to these chains.     

Synthesis and crystal structure of MgB12Si2—The first ternary compound in the system B/Mg/Si

We report on the synthesis of MgB₁₂Si₂ the first ternary compound in the system B/Mg/Si. Yellow transparent single crystals were yielded from the elements at 1600 °C in h-BN crucibles welded in Ta ampoules. MgB₁₂Si₂ crystallizes orthorhombic in the space group Pnma with , , and Z=4. The crystal structure is characterized by layers of B₁₂ icosahedra, connected by isolated Si atoms to a three-dimensional framework. Mg atoms are placed in voids of the framework. Each icosahedron forms 8 B-Si bonds and 4 exohedral B-B bonds. The Si atoms are tetrahedrally coordinated by B atoms of the B₁₂ icosahedra.     

Synthesis and crystal structure of MgB12

Single crystals of MgB₁₂ were synthesized from the elements in a Mg/Cu melt at 1600 °C. MgB₁₂ crystallizes orthorhombic in space group Pnma with , and . The crystal structure (Z=30, 5796 reflections, 510 variables, R₁(F)=0.049, wR₂(I)=0.134) consists of a three dimensional net of B₁₂ icosahedra and B₂₁ units in a ratio 2:1. The B₂₁ units are observed for the first time in a solid compound. Mg is on positions with partial occupation. The summation reveals the composition MgB_12.35 or Mg_0.97B₁₂ , respectively. This is in good agreement with the value of MgB_11.25 as expected by electronic reasons to stabilize the boron polyhedra and .     

La3Ru8B6 and Y3Os8B6, new members of a homologous series R(A)nM3n−1B2n

Two new compounds, La₃Ru₈B₆ and Y₃Os₈B₆, were synthesized by arc melting the elements. Their structural characterization was carried out at room temperature on as-cast samples by using X-ray diffractometry. According to X-ray single-crystal diffraction results these borides crystallize in Fmmm space group (no. 69), Z=4, a=5.5607(1) Å, b=9.8035(3) Å, c=17.5524(4) Å, ρ=8.956 Mg/m³, μ=25.23 mm⁻¹ for La₃Ru₈B₆ and a=5.4792(2) Å, b=9.5139(4) Å, c=17.6972(8) Å, ρ=13.343 Mg/m³, μ=128.23 mm⁻¹ for Y₃Os₈B₆. The crystal structure of La₃Ru₈B₆ was confirmed from Rietveld refinement of X-ray powder diffraction data. Both La₃Ru₈B₆ and Y₃Os₈B₆ compounds are isotypic with the Ca₃Rh₈B₆ compound and their structures are built up from CeCo₃B₂-type and CeAl₂Ga₂-type structural fragments taken in ratio 2:1. They are the members of structural series R(A)_nM_3n−1B_2n with n=3 (R is the rare earth metal, A the alkaline earth metal, and M the transition metal). Structural and atomic parameters were also obtained for La_0.94Ru₃B₂ compound from Rietveld refinement (CeCo₃B₂-type structure, P6/mmm space group (no. 191), a=5.5835(9) Å, c=3.0278(6) Å).     

Single crystal growth and structure of La4Cu3MoO12

We report the synthesis and structure determination of single crystals of La₄Cu₃MoO₁₂ grown from a CuO/KCl flux. This material, whose structure had previously been reported based solely on polycrystalline diffraction data, shows frustrated magnetic behavior and an anti-ferromagnetic ordering of spin-1/2 triangles at low temperatures. The structural and atomic parameters determined from the single crystal data are in very good agreement with those reported previously. However, HREM data showed evidence for disorder in the stacking of the Cu₃MoO₄ planes, and thus a twinned structural refinement in space group P2₁/m was replaced by an equivalent disordered structural model in space group Pm. This development of a synthetic route to single crystals of La₄Cu₃MoO₁₂ will allow a more detailed investigation of its complex electronic and magnetic properties.     

Single crystal studies on boron-rich τ-borides Ni23−xMxB6 (M=Zn, Ga, In, Sn, Ir)—The surprising occurrence of B4-tetraheda as a normal case?

Single crystals of the cubic τ-borides Ni_23−xM_xB₆ (M=Zn, Ga, In, Sn, Ir) were synthesised from the elements at temperatures between 1200 and 1500 °C. The structure refinements show that the existence of boron-rich phases is quite common. Starting from the idealised composition Ni₂₀M′₃B₆ a part of the metal atoms on site 8c is substituted by B₄ tetrahedra. For M′=Ga a complete exchange seems to be possible leading to the composition Ni₂₀GaB₁₄. For M′=Zn and Sn the formation of solid solutions is less extended. For M′=In no exchange is observed but an unusual pattern of Ni/In distribution is observed. With M=Ir mixed occupations occur for all sites and the boron content varies, too. All compositions were confirmed by EDX measurements.     

Synthesis, crystal structure and optical properties of a novel sodium lead pentaborate, NaPbB5O9

A novel sodium lead pentaborate, NaPbB₅O₉, has been successfully synthesized by standard solid-state reaction. The single-crystal X-ray structural analysis showed that NaPbB₅O₉ crystallizes in the monoclinic space group P2₁/c with a=6.5324(10) Å, b=13.0234(2) Å, c=8.5838(10) Å, β=104.971(10)°, and Z=4. The crystal structure is composed of double ring [B₅O₉]³⁻ units, [PbO₇] and [NaO₇] polyhedra. [B₅O₉]³⁻ groups connect with each other forming two-dimensional infinite _∞[B₅O₉]³⁻ layers, while [PbO₇] and [NaO₇] polyhedra are located between the layers. [PbO₇] polyhedra linked together via corner-sharing O atom forming novel infinite _∞[PbO₆] chains along the c axis. The thermal behavior, IR spectrum and the optical diffuse reflectance spectrum of NaPbB₅O₉ were reported.     

Syntheses, crystal structures and Raman spectra of Ba(BF4)(PF6), Ba(BF4)(AsF6) and Ba2(BF4)2(AsF6)(H3F4); the first examples of metal salts containing simultaneously tetrahedral BF4 and octahedral AF6 anions

In the system BaF₂/BF₃/PF₅/anhydrous hydrogen fluoride (aHF) a compound Ba(BF₄)(PF₆) was isolated and characterized by Raman spectroscopy and X-ray diffraction on the single crystal. Ba(BF₄)(PF₆) crystallizes in a hexagonal space group with a=10.2251(4) Å, c=6.1535(4) Å, V=557.17(5) Å³ at 200 K, and Z=3. Both crystallographically independent Ba atoms possess coordination polyhedra in the shape of tri-capped trigonal prisms, which include F atoms from BF₄⁻ and PF₆⁻ anions. In the analogous system with AsF₅ instead of PF₅ the compound Ba(BF₄)(AsF₆) was isolated and characterized. It crystallizes in an orthorhombic Pnma space group with a=10.415(2) Å, b=6.325(3) Å, c=11.8297(17) Å, V=779.3(4) Å³ at 200 K, and Z=4. The coordination around Ba atom is in the shape of slightly distorted tri-capped trigonal prism which includes five F atoms from AsF₆⁻ and four F atoms from BF₄⁻ anions. When the system BaF₂/BF₃/AsF₅/aHF is made basic with an extra addition of BaF₂, the compound Ba₂(BF₄)₂(AsF₆)(H₃F₄) was obtained. It crystallizes in a hexagonal P6₃/mmc space group with a=6.8709(9) Å, c=17.327(8) Å, V=708.4(4) Å³ at 200 K, and Z=2. The barium environment in the shape of tetra-capped distorted trigonal prism involves 10 F atoms from four BF₄⁻, three AsF₆⁻ and three H₃F₄⁻ anions. All F atoms, except the central atom in H₃F₄ moiety, act as μ₂-bridges yielding a complex 3-D structural network.     

Syntheses,crystal structures,and properties of Ni(II) and Co(III) complexes derived from tripod polypyridine ligands containing N7 or N6 donor set atoms

New complexes [Ni^II(pbpaen)](ClO₄)₂ (1) and [Co^III(pbpaen)](ClO₄)₃ (2) (pbpaen = N′-(pyridin-2-ylmethyl)-N,N-bis {2-[(pyridin-2-ylmethyl)amino]ethyl}ethane-1,2-diamine) have been synthesized and characterized by IR and UV–Vis spectroscopies. An X-ray structure of the nickel(II) complex shows that [Ni(pbpaen)](ClO₄)₂ (1) crystallizes in the monoclinic space group P2_1/c. The cation [Ni(pbpaen)]²⁺ is pseudo-octahedral with one of the three pyridyl nitrogen atom uncoordinated. The crystal lattice of this complex is stabilized by intra and intermolecular hydrogen bonding systems, giving one-dimensional sheets like arrays. All attempts to obtain nickel or cobalt complexes with protonated forms of the ligand resulted in isolation of only [Co^III(bpaen)](ClO₄)₃ (3) compound in which the tripod pbpaen ligand has lost one of the three pyridylmethyl groups, procuring then bpaen ligand {bpaen = N,N-bis{2-[(pyridin-2-ylmethyl)amino]ethyl}ethane-1,2-diamine}. The X-ray crystal structure reveals that the compound 3 crystallizes in the orthorhombic space group Pna2 with the Co³⁺ ion having a distorted-octahedral environment. These two ligands with strong-field N donor stabilise the +3 oxidation state of the Co center.     

On phase equilibria and crystal structures in the systems Ce-Pd-B and Yb-Pd-B. Physical properties of R2Pd13.6B5 (R=Yb, Lu)

Phase equilibria and crystal structures of ternary compounds were determined in the systems Ce-Pd-B and Yb-Pd-B at 850 °C in the concentration ranges up to 45 and 33 at% of Ce and Yb, respectively, employing X-ray single crystal and powder diffraction. Phase relations in the Ce-Pd-B system at 850 °C are governed by formation of extended homogeneity fields, τ₂-CePd₈B_2−x (0.10<x<0.48); τ₃-Ce₃Pd_25−xB_8−y (1.06<x<1.87; 2.20<y<0.05), and CePd₃B_x (0<x<0.65) the latter arising from binary CePd₃. Crystallographic parameters for the new structure type τ₂-CePd₈B_2−x (space group C2/c, a=1.78104(4) nm, b=1.03723(3) nm, c=1.16314(3), β=118.515(1)° for x=0.46) were established from X-ray single crystal diffraction. The crystal structures of τ₂-CePd₈B_2−x and τ₃-Ce₃Pd_25−xB_3−y are connected in a crystallographic group-subgroup relationship. Due to the lack of suitable single crystals, the novel structure of τ₁-Ce₆Pd_47−xB₆ (x=0.2, C2/m space group, a=1.03594(2) nm, b=1.80782(3) nm, c=1.01997(2) nm, β=108.321(1)°) was determined from Rietveld refinement of X-ray powder diffraction data applying the structural model obtained from single crystals of homologous La₆Pd_47−xB₆ (x=0.19) (X-ray single crystal diffraction, new structure type, space group C2/m, a=1.03988(2) nm, b=1.81941(5) nm, c=1.02418(2) nm, β=108.168(1)°).The Yb-Pd-B system is characterized by one ternary compound, τ₁-Yb₂Pd₁₄B₅, forming equilibria with extended solution YbPd₃B_x, YbB₆, Pd₅B₂ and Pd₃B. The crystal structures of both Yb₂Pd₁₄B₅ and isotypic Lu₂Pd₁₄B₅ were determined from X-ray Rietveld refinements and found to be closely related to the Y₂Pd₁₄B₅-type (I4₁/amd). The crystal structure of binary Yb₅Pd_2−x (Mn₅C₂-type) was confirmed from X-ray single crystal data and a slight defect on the Pd site (x=0.06) was established.The three structures τ₁-Ce₆Pd_47−xB₆, τ₂-CePd₈B_2−x and τ₃-Ce₃Pd_25−xB_8−y are related and can be considered as the packings of fragments observed in Nd₂Fe₁₄B structure with different stacking of common structural blocks.Physical properties for Yb₂Pd_13.6B₅ (temperature dependent specific heat, electrical resistivity and magnetization) yielded a predominantly Yb-4f¹³ electronic configuration, presumably related with a magnetic instability below 2 K. Kondo interaction and crystalline electric field effects control the paramagnetic temperature domain.     

Synthesis and crystal structure of the Sr2MnGa(O,F)6 oxyfluorides

The fluorine-containing derivatives of Sr₂MnGaO_5.5 were prepared by treatment with XeF₂ at temperatures ranging from 300°C to 600°C. The compounds crystallize in a tetragonal unit cell with a_t≈a_p, c_t≈2a_p (a_p—the parameter of the perovskite subcell). An increase in fluorine content is accompanied by a reduction of the Mn oxidation state due to a partial replacement of oxygen by fluorine. The crystal structure of Sr₂MnGaO_4.78F_1.22 was determined by electron diffraction and X-ray powder diffraction (a=3.85559(2) Å, c=7.78289(6) Å, S.G. P4/mmm, R_I=0.012, R_P=0.019). The structure consists of alternating (MnO₂), (SrO) and (GaO_0.78F_1.22) layers. The Ga atoms are situated in slightly elongated octahedra, the MnO₆ octahedra are characterized by two short apical Mn-O distances of 1.876(8) Å and four long equatorial ones of 1.9278(1) Å. This is interpreted as an “apically compressed” type of Jahn-Teller distortion, in contrast to the “apically elongated” one in the Sr₂MnGaO_5+δ brownmillerites with different oxygen content. Possible structural reasons for the reversed Jahn-Teller effect are discussed.