Synthesis and characterization of the rare-earth dicyanamides Ln[N(CN)2]3 with Ln=La, Ce, Pr, Nd, Sm, and Eu

The rare-earth dicyanamides Ln[N(CN)₂]₃ (Ln=La, Ce, Pr, Nd, Sm, Eu) were obtained via ion exchange in aqueous medium and subsequent drying: The crystal structures were solved and refined based on X-ray powder diffraction data and they were found to be isotypic: Ln[N(CN)₂]₃; Cmcm (no. 63), Z=4, Ln=La: , , ; Ce: , , ; Pr: , , ; Nd: , , ; Sm: , , ; Eu: , , ). The compounds represent the first dicyanamides with trivalent cations. The Ln³⁺ ions are coordinated by three bridging N atoms and six terminal N atoms of the dicyanamide ions forming a three capped trigonal prism. The structure type is related to that of PuBr₃. The novel compounds Ln[N(CN)₂]₃ have been characterized by IR and Raman spectroscopy (Ln=La) and the thermal behavior has been monitored by differential scanning calorimetry (Ln=Ce, Nd, Eu).     

Crystal growth, characterization and physical properties of PrNiSb3, NdNiSb3 and SmNiSb3

The crystal structures of three new intermetallic ternary compounds in the LnNiSb₃ (Ln=Pr, Nd and Sm) family have been characterized by single crystal X-ray diffraction. PrNiSb₃, NdNiSb₃ and SmNiSb₃ all crystallize in an orthorhombic space group, Pbcm (No. 57), Z=12, with , , , and ; , , , and ; and , , , and , for Ln=Pr, Nd and Sm, respectively. These compounds consist of rare-earth atoms located above and below layers of nearly square, buckled Sb nets, along with layers of highly distorted edge- and face-sharing NiSb₆ octahedra. Resistivity data indicate metallic behavior for all three compounds. Magnetization measurements show antiferromagnetic behavior with (PrNiSb₃), 4.6 K (NdNiSb₃), and 2.9 K (SmNiSb₃). Effective moments of 3.62 μ_B, 3.90 μ_B and 0.80 μ_B are found for PrNiSb₃, NdNiSb₃ and SmNiSb₃, respectively, and are consistent with Pr³⁺ (f ²), Nd³⁺ (f ³), and Sm³⁺ (f ⁴).     

Structures of the reduced niobium oxides Nb12O29 and Nb22O54

The crystal structure of Nb₂₂O₅₄ is reported for the first time, and the structure of orthorhombic Nb₁₂O₂₉ is reexamined, resolving previous ambiguities. Single crystal X-ray and electron diffraction were employed. These compounds were found to crystallize in the space groups P2/m (, , , β=102.029(3)^°) and Cmcm (, , ), respectively and share a common structural unit, a 4×3 block of corner sharing NbO₆ octahedra. Despite different constraints imposed by symmetry these blocks are very similar in both compounds. Within a block, it is found that the niobium atoms are not located in the centers of the oxygen octahedra, but rather are displaced inward toward the center of the block forming an apparent antiferroelectric state. Bond valence sums and bond lengths do not show the presence of charge ordering, suggesting that all 4d electrons are delocalized in these compounds at the temperature studied, T=200 K.     

Syntheses, crystal and electronic structure, and some optical and transport properties of LnCuOTe (Ln=La, Ce, Nd)

Three new compounds, LaCuOTe, CeCuOTe, and NdCuOTe, have been synthesized from the respective rare-earth elements, CuO, and a KI flux at 1023 K. The compounds, which have the ZrSiCuAs structure type, are isostructural to LaCuOS, and crystallize in space group P4/nmm of the tetragonal system with two formula units in cells of dimensions at 153 K of , , for LaCuOTe; , , for CeCuOTe; and , , for NdCuOTe. The structure of LnCuOTe (Ln=La, Ce, Nd) is composed of alternating PbO-like [Ln₂O₂] and anti-PbO-like [Cu₂Te₂] layers stacked perpendicular to [0 0 1]. The experimental optical band gaps of LaCuOTe and NdCuOTe are 2.31 and 2.26 eV, respectively. At 298 K the electrical conductivity of LaCuOTe is 1.65 S/cm and the Hall mobility is +80.6 cm² V⁻¹ s⁻¹. The positive values of the Seebeck and Hall coefficients indicate p-type electrical conduction. First-principles theoretical calculations were performed on LaCuOQ (Q=S, Se, Te). In LaCuOTe, Cu 3d and Te 5p orbitals dominate the states near the valence band maximum; the states near the conduction band minimum are composed of Cu 4s, Te 5p, and La 5d orbitals. The larger dispersion of Cu 3d orbitals and the presence of Te 5p orbitals near the valence band maximum are responsible for the larger hole mobility of LaCuOTe compared to LaCuOS and LaCuOSe.     

Synthesis, structure, and magnetism of a new heavy-fermion antiferromagnet, CePdGa6

A new compound, CePdGa₆, and its isostructural analog, LaPdGa₆ have been synthesized by flux growth and characterized by single-crystal X-ray diffraction. The compounds adopt a tetragonal structure with P4/mmm space group, Z=1. The lattice parameters for CePdGa₆ are and and and for LaPdGa₆. Magnetic and thermal measurement have revealed that CePdGa₆ is a heavy-fermion with the specific heat coefficient and Ce f moments order antiferromagnetically along c-axis at . Reconfiguration of spin occurs at to induce a ferromagnetic component only in the a-b plane. This strong anisotropy in the magnetism might be related to its unique layered structure.     

Synthesis and crystal structure of a novel ternary oxoborate, PbBiBO4

A novel ternary borate oxide, lead bismuth boron tetraoxide, PbBiBO₄, has been prepared by solid-state reaction at temperature below 800 °C. The single-crystal X-ray structural analysis showed that PbBiBO₄ crystallizes in the monoclinic space group P2₁/n with , , , β=91.48(1), Z=4. It represents a new structure type in which distorted BiO₆⁹⁻ octahedra are connected to each other in corner- and edge-sharing manner to form two-dimensional layers that are bridged by B atoms of BO₃ triangles giving rise to a three-dimensional framework, with channels parallel to the [0 1 0] direction accommodating the pyramidally coordinated Pb²⁺ cations.     

Crystal structure of cobalt molybdate hydrate CoMoO4·nH2O

We have determined the crystal structure of the title compound, which has a triclinic cell with cell parameters of , , , α=76.617°, β=84.188°, γ=74.510° and space group . The crystal structure suggests the chemical formula CoMoO₄·3/4H₂O. The structure consists of MoO₄ tetrahedra and CoO₆ octahedra, confirming the earlier X-ray absorption near-edge spectroscopic (XANES) investigation on the hydrate. The comparison of the crystal structures of the hydrate and the α-,β-, and hp-phases shows that the hydrate exhibits metal cation coordinations similar to those of the β-phase, but had arrangements of CoO₆ and MoO_n polyhedra similar to those of the hp-phase.     

Synthesis, structure and properties of the new rare-earth Zintl phase Yb11GaSb9

A new rare-earth rich Zintl phase Yb₁₁GaSb₉ was synthesized by direct fusion of the corresponding elements, and large single crystals of the compound were obtained from high temperature flux synthesis. Its crystal structure was determined by single-crystal X-ray diffraction to be orthorhombic in the non-centrosymmetric space group Iba2 (No. 45), Z=4 (R₁=3.24%, wR₂=6.40%) with , , measured at 90(3) K. The structure belongs to the Ca₁₁InSb₉-type and can be viewed as built of isolated Sb₄-tetrahedra centered by Ga, Sb-dimers and isolated Sb anions, which are separated by Yb²⁺ cations. Electron count according to the Zintl formalism suggests that the phase is electron-precise and charge-balanced, which is supported by the virtually temperature-independent magnetization for Yb₁₁GaSb₉. Electrical resistivity data from 2 to 400 K confirm that Yb₁₁GaSb₉ is a small band-gap semiconductor with room temperature resistivity , and low-temperature resistivity at 2 K . As such, Yb₁₁GaSb₉ and related compounds might be promising materials for thermoelectric applications, and currently, efforts to synthesize new members of this family and test their thermoelectric performance are under way.     

Ferromagnetic-phase transition in the spinel-type CuCr2Te4

Ferromagnetic-phase transition in spinel-type CuCr₂Te₄ has been clearly observed. CuCr₂Te₄ is a telluride-spinel with the lattice constant , which has been synthesized successfully. The heat capacity exhibits a sharp peak due to the ferromagnetic-phase transition with the Curie temperature . This value of T_C corresponds exactly to that of the negative peak of dM/dT in low field of 1.0 Oe. The magnetic susceptibility shows the Curie-Weiss behavior between 380 and 650 K with the effective magnetic moment /Cr-ion and the Weiss constant . The low temperature magnetization indicates the spin-wave excitations, where the existence of first term of Bloch T^3/2 law and the next T^5/2 term are verified experimentally. This spin-wave excitation is detected up to approximately 250 K which is a fairly high temperature.     

Inherent instabilities of some W5Si3-type binary phases with large electropositive metal atoms: Six examples in the Ba-Pb, Ca-Sn-Mg,Cu,Zn, and La-Ga-Al,Zn systems

A particular pathology of certain W₅Si₃-type A₅B₃ structures (I4/mcm) appears to arise because of unduly close approaches of the A1-type atoms on the cell faces at , 0, () that occur with the larger and more electropositive A and/or in the presence of smaller B atoms. A structure refinement of binary Ba_4.81Pb₃ indicates such a marginal stability in that the Ba atoms in the facial Ba_0.81 chains exhibit an extreme displacement ellipsoid along . Although Ca₅Sn₃ and La₅Ga₃ binaries are unknown in this structure type, five stable ternary derivatives of these have been synthesized via substitution reactions and characterized by single crystal X-ray diffraction means: Ca₄Sn_3.223(4)Mg_0.777, Ca₄Sn₃Cu_1.30(4), Ca_4.66(6)Sn₃Zn_0.704(4), La_4.81(1)Ga_1.38(2)Al_1.62, and La_4.762(5)Ga_1.5(1)Zn_1.5. Only the Ca-Sn-Zn phase exhibits lower symmetry, P4/mbm. The problematic A1 sites exhibit diverse changes in these, whereas the surrounding B2 tetrahedra are largely unaltered. The Ca-Sn results are, respectively: direct Mg/Sn substitution at the Ca1 site; mixed fractional distribution of the smaller Cu at two sites around the A1 position with an unresolved disorder; a pair of apparently independent modes, fractional Ca in the normal position and fractional Zn rectangles thereabout. The two La-Sn phases contain normal Ga,Al (Ga,Zn) tetrahedral chains with pairs of fractional disordered La atoms along , 0, z. Each can be rationalized in terms of a reasonable incommensurate structure. Electronic effects may also be operable.     

Fabrication and luminescent properties of rare earths-doped Gd2(WO4)3 thin film phosphors by Pechini sol-gel process

Rare earth ions (Eu³⁺ and Dy³⁺)-doped Gd₂(WO₄)₃ phosphor films were prepared by a Pechini sol-gel process. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), atomic force microscopy (AFM) and photoluminescence (PL) spectra as well as lifetimes were used to characterize the resulting powders and films. The results of XRD indicate that the films begin to crystallize at 600°C and the crystallinity increases with the elevation of annealing temperatures. The film is uniform and crack-free, mainly consists of closely packed fine particles with an average grain size of 80 nm. Owing to an energy transfer from WO₄²⁻ groups, the rare earth ions show their characteristic emissions in crystalline Gd₂(WO₄)₃ phosphor films, i.e., (J=0, 1, 2, 3; J′=0, 1, 2, 3, 4, not in all cases) transitions for Eu³⁺ and (J=13/2, 15/2) transitions for Dy³⁺, with the hypersensitive transitions (Eu³⁺) and (Dy³⁺) being the most prominent groups, respectively. Both the lifetimes and PL intensity of the Eu³⁺ () and Dy³⁺ () increase with increasing the annealing temperature from 500°C to 800°C, and the optimum doping concentrations for Eu³⁺ and Dy³⁺ are determined to be 30 and 6 at% of Gd³⁺ in Gd₂(WO₄)₃ film host lattices, respectively.     

High-pressure synthesis and crystal structure analysis of NaMn2O4 with the calcium ferrite-type structure

Single crystals of a new sodium manganese oxide, NaMn₂O₄, were synthesized for the first time using a high-temperature and high-pressure technique. The NaMn₂O₄ single crystal is black, has a needle shape, and crystallizes in the orthorhombic calcium ferrite-type structure, space group Pnam with , , , , and Z=4. The structure was determined from a single-crystal X-ray study and refined to the conventional values R=0.041 and wR=0.034 for 1190 observed reflections. The framework structure is built up from edge-sharing chains of MnO₆ octahedra that condense to form one-dimensional tunnels in which the sodium atoms are located. The Mn-O bond distance and bond valence analyses revealed the manganese valence Mn³⁺/Mn⁴⁺ ordering in the two “double rutile” chains of NaMn₂O₄.     

Synthesis and characterization of a Ruddlesden-Popper compound: Sr3FeMoO7

A powder sample of Sr₃FeMoO₇ was synthesized by solid-state reaction in reduced atmosphere (5% H₂/Ar). At room temperature, Sr₃FeMoO₇ crystallizes in a typical Ruddlesden-Popper (n=2) structure in the space group I4/mmm, and . The structure refinement indicates that the Fe and Mo ions are randomly distributed in a single B-site with small fraction of B-site and oxygen vacancies. At low temperature, long-range magnetic interaction was observed. The antiferromagnetic magnetic interaction can be described with a large unit cell, and c_m=c_n, in the magnetic space group An′.     

A comparison of the structure and localized magnetism in Ce2PdGa12 with the heavy fermion CePdGa6

Single crystals of Ce₂PdGa₁₂ have been synthesized in Ga flux and characterized by X-ray diffraction. This compound crystallizes in the tetragonal P4/nbm space group, Z=2 with lattice parameters of and . It shows strongly anisotropic magnetism and orders antiferromagnetically at T_N∼11 K. A field-induced metamagnetic transition to the ferromagnetic state is observed below T_N. Structure-property relationships with the related heavy-fermion antiferromagnet CePdGa₆ are discussed.     

PbI4Cu2(PPh3)4: A heterometallic iodide with unusual cis-divacant octahedral coordination sphere for lead: Synthesis, structure, red-infrared fluorescence and theoretical studies

A new heterometallic iodide, PbI₄Cu₂(PPh₃)₄, was synthesized by reactions of PbI₂, CuI and triphenylphosphine (PPh₃) in DMF solution. The single-crystal X-ray diffraction analyses show that Pb(II) center adopts an unusual cis-divacant octahedral geometry. Crystal data: triclinic, space group , , , , α=106.623(4)°, β=103.478(6)°, γ=93.574(5)°, and Z=2. Density function theory (DFT) calculations and fragment orbital interaction analyses reveal the presence of a three-center four-electron (3c-4e) hypervalent bonding about lead; and the formation of the unusual cis-divacant [PbI₄]²⁻ octahedron is energetically favorable. The title yellow compound has an optical bandgap of 2.69 eV and shows remarkable red-infrared fluorescence emission at 732 nm with lifetime of 24 μs which is assigned as an iodine 5p-lead 6s to PPh₃-lead 6p charge transfer (XM-LM-CT).     

Synthesis, crystal structure and optical properties of BiMgVO5

The new vanadate BiMgVO₅ has been prepared and its structure has been determined by single crystal X-ray diffraction: space group P2₁/n, , , , β=107.38(5)°, wR₂=0.0447, R=0.0255. The structure consists of [Mg₂O₁₀] and [Bi₂O₁₀] dimers sharing their corners with [VO₄] tetrahedra. The ranges of bond lengths are 2.129-2.814 Å for Bi-O; 2.035-2.167 Å for Mg-O and 1.684-1.745 Å for V-O. V-O bond lengths determined from Raman band wavenumbers are between 1.679 and 1.747 Å. An emission band overlapping the entire visible region with a maximum around 650 nm is observed.     

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.     

Enthalpies of dilution of aqueous Li2B4O7 solutions at 298.15 K and application of ion-interaction model

The enthalpies of dilution have been measured for aqueous Li₂B₄O₇ solutions from 0.0212 to 2.1530 mol kg⁻¹ at 298.15 K. The relative apparent molar enthalpies, L_?, and relative partial molar enthalpies of the solvent and solute, and were calculated. The thermodynamic properties of the complex aqueous solutions were represented with a modified Pitzer ion-interaction model.