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).     

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′.     

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 growth, transport, and magnetic properties of Ln3Co4Sn13 (Ln=La, Ce) with a perovskite-like structure

Ln₃Co₄Sn₁₃ (Ln=La, Ce) have been synthesized by flux growth and characterized by single crystal X-ray diffraction. These compounds adopt the Yb₃Rh₄Sn₁₃-type structure and crystallize in the cubic space group (No. 223) with Z=2. Lattice parameters at 298 K are , , and , for the La and Ce analogues, respectively. The crystal structure consists of an Sn-centered icosahedron at the origin of the unit cell, which shares faces with eight Co trigonal prisms and 12 Ln-centered cuboctahedra. Magnetization data at 0.1 T show paramagnetic behavior down to 1.8 K for Ce₃Co₄Sn₁₃, with per Ce³⁺, while conventional type II superconductivity appears below 2.85 K in the La compound. Electrical resistivity and specific heat data for the La compound show a corresponding sharp superconducting transition at T_c∼2.85 K. The entropy and resistivity data for Ce₃Co₄Sn₁₃ show the existence of the Kondo effect with a complicated semiconducting-like behavior in the resistivity data. In addition, a large enhanced specific heat coefficient at low T with a low magnetic transition temperature suggests a heavy-fermionic character for the Ce compound. Herein, the structure and physical properties of Ln₃Co₄Sn₁₃ (Ln=La, Ce) are discussed.     

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.     

The crystal chemistry of L-Ta2O5 and related structures

Single crystals of a new form of L-Ta₂O₅ with a 19×b superstructure have been synthesised by flux growth. The phase is most likely stabilised by the incorporation of a small amount of lithium (0.14 wt% Li) from the flux. The phase has C-centred monoclinic symmetry with , (), , γ=90.00(1)°. The structure was refined in space group C112/m to R₁=0.044 for 814 unique reflections with F>4σ(F). The structure can be described as comprising chains of edge-shared TaO₇ pentagonal bipyramids that are regularly folded at (010) planes to give sinusoidal chains along [010]. These chains are interconnected along [100] and [001] by corner sharing, creating inter-chain regions that are occupied by isolated TaO₆ octahedra and pairs of corner-shared octahedra. A comparison with published data for high-quality refinements of related structures has led to the development of a general model that can explain the structural chemistry variations in the known L-Ta₂O₅-related structures. A shorthand notation is presented for representing the structures, based on the sequence along [010] of the interchain octahedra.     

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.     

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.     

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.     

A Raman spectroscopic study of the phase transition of BaZr(PO4)2: Evidence for a trigonal structure of the high-temperature polymorph

We have studied the structural evolution of monoclinic BaZr(PO₄)₂ during heating up to 835 K by Raman spectroscopy. In agreement with previous studies we found a first-order phase transition at about 730 K during heating while upon cooling the reverse transition occurs at 705 K. However, some disagreement about the crystal structure of the high-temperature polymorph occurs in the literature. While the space group has not yet been determined, the X-ray diffraction pattern of the high-temperature phase has been indexed on either an orthorhombic or a hexagonal unit cell. We found that the number of Raman active internal PO₄ vibrational modes decrease from nine to six during the transition. A group theoretical survey through all orthorhombic, trigonal, and hexagonal factor groups revealed that the observed number of vibrations would only be consistent with the Ba and Zr atoms located at a site, the P and two O atoms at a C_3v(3m), and six O atoms at a C_s(m) site in the D_3d factor group. Based on our Raman data, the space group of the high-temperature polymorph is thus either , , or .     

High-pressure synthesis, crystal structures, and characterization of CdVO3−δ and solid solutions CdVO3-NaVO3

CdVO_3−δ and solid solutions of Cd_1−xNa_xVO₃ with the GdFeO₃-type perovskite structure were prepared using a high-pressure (6 GPa) and high-temperature technique. No significant oxygen and cation deficiency was found in CdVO₃. Cd_1−xNa_xVO₃ are formed in the compositional range of 0?x?0.2. CdVO₃ and Cd_1−xNa_xVO₃ demonstrate metallic conductivity and Pauli paramagnetism between 2 and 300 K. A large electronic contribution to the specific heat (γ=13.4 and ) for CdVO₃ and Cd_0.8Na_0.2VO₃, respectively) was observed at low temperatures due to the strongly correlated electrons. Crystal structures of CdVO₃ and Cd_0.8Na_0.2VO₃ were refined by X-ray powder diffraction: space group Pnma; Z=4; , , and for CdVO₃ and , , and for Cd_0.8Na_0.2VO₃.     

Magnetic and electronic structure of TlCo2S2

An extensive investigation of the ferromagnetic compound TlCo₂S₂ has resulted in new information on the electronic and magnetic structure. Electronic structure calculations showed that magnetic ordering is energetically favorable with a clear driving force for ferromagnetic coupling within the cobalt layers. TlCo₂S₂ is metallic and the conductivity is due to holes in the valence band. XPS single crystal measurements did not show evidence of mixed oxidation states of cobalt. Neutron powder diffraction resulted in a ferromagnetic structure with the magnetic moment in the ab-plane. The derived magnetic moment of the cobalt atom is at 10 K and is in very good agreement with the value, at 10 K, inferred from the magnetic hysteresis curve.     

Energetics of La1−xAxCrO3−δ perovskites (A=Ca or Sr)

A series of perovskites with the general formula La_1−xA_xCrO_3−δ (A=Ca or Sr) have been synthesized in the solid solution range 0.0<x?0.3 and 0.0?δ?0.5x with a variety of heat treatments. High-temperature drop solution calorimetry in molten 2PbO·B₂O₃ at 1080 K was performed to determine their enthalpies of formation from oxides at room temperature. The enthalpy of oxidation involved in the reaction is roughly independent of oxygen nonstoichiometry (δ) in each series with a given dopant composition, but varies with composition (x). The values change from −620±260 to −280±80 kJ/mol O₂ when x=0.1-0.3 for Ca-doped samples, and from −440±150 to −290±50 kJ/mol O₂ for Sr-doped ones. This dependence of enthalpy of oxidation on composition suggests oxygen vacancies are increasingly short-range ordered in reduced samples. The higher oxidation state of chromium is stabilized by the substitution of alkaline earth ions, but with increasing doping, the enthalpy of formation of the fully oxidized sample in both Ca and Sr-doped systems becomes more endothermic. This destabilization effect is attributed to the large endothermic enthalpy of oxygen vacancy formation (395±30 kJ/mol of ) for the reaction (A=Ca or Sr) that over-rides the exothermic enthalpies of oxidation. At a given composition, Sr-doped LaCrO₃ is more stable than its Ca-doped counterpart, which is consistent with basicity arguments.     

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.     

Synthesis, crystal structure, spectrum properties, and electronic structure of a novel non-centrosymmetric borate, BiCd3(AlO)3(BO3)4

A novel non-centrosymmetric borate, BiCd₃(AlO)₃(BO₃)₄, has been prepared by solid state reaction methods below 750 °C. Single-crystal XRD analysis showed that it crystallizes in the hexagonal group P6₃ with a=10.3919(15) Å, c=5.7215(11) Å, Z=2. In its structure, AlO₆ octahedra share edges to form 1D chains that are bridged by BO₃ groups through sharing O atoms to form the 3D framework. The 3D framework affords two kinds of channels that are occupied by Bi³⁺/Cd²⁺ atoms only or by Bi³⁺/Cd²⁺ atoms together with BO₃ groups. The IR spectrum further confirmed the presence of BO₃ groups. Second-harmonic-generation measurements displayed a response of about 0.5×KDP (KH₂PO₄). UV-vis diffuse reflectance spectrum showed a band gap of about 3.19 eV. Solid-state fluorescence spectrum exhibited the maximum emission peak at around 390.6 nm. Band structure calculations indicated that it is an indirect semiconductor.     

Mg8Rh4B — A new type of boron stabilized Ti2Ni structure

The new magnesium rhodium boron compound Mg₈Rh₄B has been synthesized by reaction of the metal powders with crystalline or amorphous boron or the RhB precursor. The crystal structure of Mg₈Rh₄B was solved using single-crystal X-ray diffraction data (space group , , Z=8, 174 reflections, R_F=0.016). The crystal structure can be described as a filled Ti₂Ni type where the interstitial sites 8b (), located at the center of two nested Mg₄Rh₄ tetrahedra, are occupied by boron atoms. Taking into account the absence of the Ti₂Ni-type phase in the binary Mg-Rh system, the boron atoms can be considered as stabilizing this structural motif. From the bonding analysis with the electron localization function the crystal structure is described as covalently bonded [Rh₄B]³⁻ anions, embedded in a cationic magnesium matrix.     

Synthesis, structure, and physical properties of Ce2PdGa10

A new ternary compound, Ce₂PdGa₁₀, has been synthesized using Ga flux and characterized by single-crystal X-ray diffraction. Ce₂PdGa₁₀ adopts a tetragonal structure in the I₄/mmm space group and is isostructural to Ce₂NiGa₁₀. Lattice parameters are , , , and Z=2. The compound is metallic (dρ/dT>0), with the resistance decreasing roughly linearly with temperature from 300 to 175 K. The magnetic susceptibility of Ce₂PdGa₁₀ is consistent with local-moment paramagnetism and no long-range magnetic ordering occurs down to 2 K. A large positive magnetoresistance over 200% is observed at 2 K for fields of 9 T. In this paper, we present the structure and physical properties of Ce₂PdGa₁₀ and compared them to CePdGa₆.     

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.