Ferromagnetic ordering in ThSi2 type CeAu0.28Ge1.72

The compound CeAu_0.28Ge_1.72 crystallizes in the ThSi₂ structure type in the tetragonal space group I4₁/amd with lattice parameters a=b=4.2415(6) Å c=14.640(3) Å. CeAu_0.28Ge_1.72 is a polar intermetallic compound having a three-dimensional Ge/Au polyanion sub-network filled with Ce atoms. The magnetic susceptibility data show Curie-Weiss law behavior above 50 K. The compound orders ferromagnetically at ∼8 K with estimated magnetic moment of 2.48 μ_B/Ce. The ferromagnetic ordering is confirmed by the heat capacity data which show a rise at ∼8 K. The electronic specific heat coefficient (γ) value obtained from the paramagnetic temperature range 15-25 K is∼124(5) mJ/ mol K². The entropy change due to the ferromagnetic transition is ∼4.2 J/mol K which is appreciably reduced compared to the value of R ln(2) expected for a crystal-field-split doublet ground state and/or Kondo exchange interactions.     

Synthesis, structure and physical properties of the new uranium ternary phase U3Co2Ge7

A new ternary compound, U₃Co₂Ge₇, has been synthesized from the corresponding elements by a high temperature reaction using molten tin flux. It crystallizes in the orthorhombic La₃Co₂Sn₇-type (Pearson's symbol oC24, space group Cmmm, No. 65) with lattice parameters determined from single-crystal X-ray diffraction as follows: a=4.145(2) Å; b=24.920(7); c=4.136(2) Å, V=427.2(3) Å³. Structure refinements confirm an ordered structure having two crystallographically inequivalent uranium atoms, occupying sites with dissimilar coordination. U₃Co₂Ge₇ orders ferromagnetically below 40 K and undergoes a consecutive magnetic transition at 20 K. These results have been obtained from temperature- and field-dependent magnetization, resistivity and heat-capacity measurements. The estimated Sommerfeld coefficient γ=87 mJ/mol-U K² suggests U₃Co₂Ge₇ to be a moderately heavy-fermion material.     

Structural and magnetic properties of the quaternary oxides Ba6Ln2Fe4O15 (Ln=Pr and Nd)

The crystal structures and magnetic properties of the quaternary lanthanide oxides Ba₆Ln₂Fe₄O₁₅ (Ln=Pr and Nd) are reported. They crystallize in a hexagonal structure with space group P6₃mc and have the “Fe₄O₁₅ cluster” consisting of one FeO₆ octahedron and three FeO₄ tetrahedra. Measurements of the magnetic susceptibility, specific heat, and powder neutron diffraction reveal that this cluster behaves as a spin tetramer with a ferrimagnetic ground state of S_T=5 even at room temperature. The cluster moments show a long-range antiferromagnetic ordering at 23.2 K (Ln=Pr) and 17.8 K (Nd), and the magnetic moments of the Ln³⁺ ions also order cooperatively. By applying the magnetic field (∼2 T), this antiferromagnetic ordering of the clusters changes to a ferromagnetic one. This result indicates that there exists a competition in the magnetic interaction between the clusters.     

Temperature behavior of the protonic conductor K4LiH3(SO4)4

The results of the X-ray structural study for the K₄LiH₃(SO₄)₄ single crystal are presented at a wide temperature range. The thermal expansion of the crystal using the X-ray dilatometry and the capacitance dilatometry from 8 to 500 K was carried out. The crystal structures data collection, solution and refinement at 125, 295, 443 and 480 K were performed. The K₄LiH₃(SO₄)₄ crystal has tetragonal symmetry with the P4₁ space group (Z=4) at room temperature as well as at the considered temperature range. The existence of a low-temperature, para-ferroelastic phase transition at about 120 K is excluded. The layered structure of the crystal reflects a cleavage plane parallel to (001) and an anisotropy of the protonic conductivity. The superionic high-temperature phase transition at T_S=425 K is isostructural. Nevertheless, taking into account an increase of the SO₄ tetrahedra libration above T_S, a mechanism of the Grotthus type could be applied for the proton transport explanation.     

The structural investigation of Ba4Bi3F17

The anion-excess ordered fluorite-related phase Ba₄Bi₃F₁₇ has been synthesized by a solid state reaction of BaF₂ and BiF₃ at 873 K. The crystal structure of Ba₄Bi₃F₁₇ has been studied using electron diffraction and X-ray powder diffraction (a=11.2300(2) Å, c=20.7766(5) Å, S.G. , R_I=0.020, R_P=0.036). Interstitial fluorine atoms in the Ba₄Bi₃F₁₇ structure are considered to form isolated cuboctahedral 8 : 12 : 1 clusters. The structural relationship between Ba₄Bi₃F₁₇ and similar rare-earth-based phases is discussed.     

Enhanced ferromagnetic transition and magnetoresistance in granular Ag-added La0.7Ca0.3MnO3

Granular Ag-added La_0.7Ca_0.3MnO₃ (LCMO) samples were prepared by a sol-gel chemical route. Significant enhancements in Curie temperature (T_C), metal−insulator transition (T_p) and magnetoresistance (MR) effects near room temperature are observed in as-obtained samples. 10 wt% addition of Ag in LCMO causes T_C shift from 272 to 290 K, T_p boost up for more than 100 K and resistivity decrease by more than 3 orders of magnitude. X-ray diffraction patterns, thermal analysis and energy dispersive analysis of X-rays evidently show the existence of metal silver in LCMO matrices. High-resolution electron microscopy illustrates a well crystallization for LCMO grains in existence of Ag. It is argued that improved grain boundary effect and better crystallization caused by Ag addition are responsible for the enhancements.     

Ion-beam irradiation of lanthanum compounds in the systems La2O3-Al2O3 and La2O3-TiO2

Thin crystals of La₂O₃, LaAlO₃, La_2/3TiO₃, La₂TiO₅, and La₂Ti₂O₇ have been irradiated in situ using 1 MeV Kr²⁺ ions at the Intermediate Voltage Electron Microscope-Tandem User Facility (IVEM-Tandem), Argonne National Laboratory (ANL). We observed that La₂O₃ remained crystalline to a fluence greater than 3.1×10¹⁶ ions cm⁻² at a temperature of 50 K. The four binary oxide compounds in the two systems were observed through the crystalline-amorphous transition as a function of ion fluence and temperature. Results from the ion irradiations give critical temperatures for amorphisation (T_c) of 647 K for LaAlO₃, 840 K for La₂Ti₂O₇, 865 K for La_2/3TiO₃, and 1027 K for La₂TiO₅. The T_c values observed in this study, together with previous data for Al₂O₃ and TiO₂, are discussed with reference to the melting points for the La₂O₃-Al₂O₃ and La₂O₃-TiO₂ systems and the different local environments within the four crystal structures. Results suggest that there is an observable inverse correlation between T_c and melting temperature (T_m) in the two systems. More complex relationships exist between T_c and crystal structure, with the stoichiometric perovskite LaAlO₃ being the most resistant to amorphisation.     

Crystal structure, characterization and thermoelectric properties of the type-I clathrate Ba8−ySryAl14Si32 (0.6≤y≤1.3) prepared by aluminum flux

The title compound was prepared as single crystals using an aluminum flux technique. Single crystal and powder X-ray diffraction indicate that this composition crystallizes in the clathrate type-I structure, space group Pm3?n. Electron microprobe characterization indicates the composition to be Ba_8−ySr_yAl_14.2(2)Si_31.8(2) (0.77<y<1.3). Single-crystal X-ray diffraction data (90 and 12 K) were refined with the Al content fixed at the microprobe value (12 K data: R₁=0.0233, wR₂=0.0441) on a crystal of compositions Ba. The Sr atom preferentially occupies the 2a position; mixed Al/Si occupancy was found on all framework sites. These refinements are consistent with a fully occupied framework and nearly fully occupied cation guest sites as found by microprobe analysis. Temperature dependent electrical resistivity and thermal conductivity have been measured from room temperature to 1200 K on a hot-pressed pellet. Electrical resistivity reveals metallic behavior. The negative Seebeck coefficient indicates transport processes dominated by electrons as carriers. Thermal conductivity is between 22 and 25 mW/cm K. The sample shows n-type conductivity with a maximum figure of merit, zT of 0.3 at 1200 K. A single parabolic band model predicts a five-fold increase in zT at 800 K if carrier concentration is lowered.     

Ab initio/DFT theory and multichannel RRKM study on the mechanisms and kinetics for the CH3S + CO reaction

The potential energy surface for the reaction of CH₃S with CO was calculated at the G3MP2//B3LYP/6-311++G(d,p) level. The rate constants for feasible channels leading to several products were calculated by TST and multichannel-RRKM theory. The results show that addition–elimination mechanism is dominant, while hydrogen abstraction mechanism is uncompetitive. The major channel is the addition of CO to CH₃S leading to an intermediate CH₃SCO which then decomposes to CH₃ + OCS. In the temperature range of 200–3000 K, the overall rate constants are positive temperature dependence and pressure independence, and it can be described by the expression as k = 1.10 × 10⁻¹⁶T^1.57exp(−3359/T) cm³ molecule⁻¹ s⁻¹. At temperature between 208 and 295 K, the calculated rate constants are in good agreement with the experimental upper limit data. At T = 1000 and 2000 K, the major product is CH₃ + OCS at lower pressure; while at higher pressure, the stabilization of IM1 is dominant channel.     

Determination of the enthalpy of fusion of K3TaF8 and K3TaOF6

The areas of the fusion and crystallization peaks of K₃TaF₈ and K₃TaOF₆ have been measured using the DSC mode of the high-temperature calorimeter (SETARAM 1800 K). On the basis of these quantities and the temperature dependence of the used calorimetric method sensitivity, the values of the enthalpy of fusion of K₃TaF₈ at temperature of fusion 1039 K: Δ_fusH_m(K₃TaF₈; 1039 K) = (52 ± 2) kJ mol⁻¹ and of K₃TaOF₆ at temperature of fusion 1055 K: Δ_fusH_m(K₃TaOF₆; 1055 K) = (62 ± 3) kJ mol⁻¹ have been determined.     

Synthesis, structure and physical properties of GdCu4Al and GdCu4Ga

Two non-stoichiometric Gd compounds, GdCu_5−xTr_x (Tr=Al, Ga) have been synthesized from the corresponding elements by high temperature reactions in sealed tantalum containers. They crystallize in the hexagonal CaCu₅-type (Pearson's symbol hP6, space group P6/mmm, No. 191) with lattice parameters determined from single-crystal X-ray diffraction at room temperature as follows: a=5.0831(10) Å; c=4.156(2) Å for GdCu_3.98(4)Al_1.02(4), and a=5.1025(10) Å; c=4.155(2) Å for GdCu_3.9(1)Ga_1.1(1), respectively. Structure refinements from single crystal X-ray diffraction data reveal that substitution of Cu for Al or Ga takes place preferably on one of the two transition metal sites with site symmetry mmm (3g). Both compounds order antiferromagnetically below ∼40 K and ∼36 K, respectively, as determined from temperature dependent dc-magnetization, resistivity and heat-capacity measurements.     

Crystal and magnetic structures of the brownmillerite compound Ca2Fe1.039(8)Mn0.962(8)O5

The crystal and magnetic structures of the brownmillerite material, Ca₂Fe_1.039(8)Mn_0.962(8)O₅ were investigated using powder X-ray and neutron diffraction methods, the latter from 3.8 to 700 K. The compound crystallizes in Pnma space group with unit cell parameters of a=5.3055(5) Å, b=15.322(2) Å, c=5.4587(6) Å at 300 K. The neutron diffraction study revealed the occupancies of Fe³⁺ and Mn³⁺ ions in both octahedral and tetrahedral sites and showed some intersite mixing and a small, ∼4%, Fe excess. While bulk magnetization data were inconclusive, variable temperature neutron diffraction measurements showed the magnetic transition temperature to be 407(2) K below which a long range antiferromagnetic ordering of spins occurs with ordering wave vector k=(000). The spins of each ion are coupled antiferromagnetically with the nearest neighbors within the same layer and coupled antiparallel to the closest ions from the neighboring layer. This combination of intra- and inter-layer antiparallel arrangement of spins forms a G-type magnetic structure. The ordered moments on the octahedral and tetrahedral sites at 3.8 K are 3.64(16) and 4.23(16) μ_B, respectively.     

Non-centrosymmetric Ba3Ti3O6(BO3)2

The compound previously reported as Ba₂Ti₂B₂O₉ has been reformulated as Ba₃Ti₃B₂O₁₂, or Ba₃Ti₃O₆(BO₃)₂, a new barium titanium oxoborate. Small single crystals have been recovered from a melt with a composition of BaTiO₃:BaTiB₂O₆ (molar ratio) cooled between 1100°C and 850°C. The crystal structure has been determined by X-ray diffraction: hexagonal system, non-centrosymmetric space group, a=8.7377(11) Å, c=3.9147(8) Å, Z=1, wR(F²)=0.039 for 504 unique reflections. Ba₃Ti₃O₆(BO₃)₂ is isostructural with K₃Ta₃O₆(BO₃)₂. Preliminary measurements of nonlinear optical properties on microcrystalline samples show that the second harmonic generation efficiency of Ba₃Ti₃O₆(BO₃)₂ is equal to 95% of that of LiNbO₃.     

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.     

The chemical and dielectric properties of epoxy/(Ba0.8Sr0.2)(Ti0.9Zr0.1)O3 composites for embedded capacitor application

The characteristics of epoxy/(Ba_0.8Sr_0.2)(Ti_0.9Zr_0.1)O₃ (BSTZ) composites are investigated for the further application in embedded capacitor device. The effects of BSTZ ceramic powder filler ratio on the chemical, physical and dielectric properties of epoxy/BSTZ composites are studied. Differential scanning calorimeter (DSC) thermal analysis is conducted to determine the optimum values of hardener agent, curing temperature, reaction heat, and glass transition temperature (T_g). The hardener reaction process starts at about 115 °C and completes at about 200 °C, for that it is appropriate to process of epoxy/BSTZ composites in the range of temperature. The highest glass transition temperature (T_g) of 155 °C is obtained at one equivalent weight ratio (hardener/epoxy). Only the BSTZ phase can be detected in the XRD patterns of epoxy/BSTZ composites. The more BSTZ ceramic powder is mixed with epoxy, the higher crystalline intensity of tetragonal BSTZ phase are revealed in the XRD patterns. The dielectric constant measured at 1 MHz increases from 5.8 to 23.6 as the content of BSTZ ceramic powder in the epoxy/BSTZ composites increases from 10 to 70 wt%. The loss tangents of the epoxy/BSTZ composites slightly increase with the increase of measurement frequency.     

Unusual oxidation states give reversible room temperature magnetocaloric effect on perovskite-related oxides SrFe0.5Co0.5O3

The magnetic properties and the magnetocaloric effect are presented for the perovskite-related oxide SrFe_0.5Co_0.5O₃ prepared using electrochemical oxidation. SrFe_0.5Co_0.5O₃ exhibits a second order paramagnetic-ferromagnetic transition close to room temperature (T_C=330 K). The maximal magnetic entropy change ΔS_M^Max , the maximal adiabatic temperature change ΔT_ad and the refrigerant capacity are found to be equal to respectively 4.0 J/kgK, 1.8 K and 258 J/kg while raising the B-field change from 0 to 5 T.     

Superconductivity and crystal structure of the solid solutions of Ba8−δSi46−xGex (0?x?23) with Type I clathrate structure

The solid solutions of barium containing Type I clathrate, Ba_8−δSi_46−xGe_x (0?x?23) were prepared under high-pressure and high-temperature conditions of 3 GPa at 800°C. All the solid solutions showed superconductivity, and the transition temperature (T_c) decreased from 8.0 to 2.0 K as the germanium content increased from x=0 to 23 in Ba_8−δSi_46−xGe_x. The single crystals with five different compositions were obtained and the structures, compositions, and site occupancies were determined from X-ray single-crystal analysis. A slight barium deficiency was observed at Ba1 (2a) sites for all the clathrates. The Ge atoms replaced the Si atoms at the Si3 (24k) site in the composition range of x<8, and then at the Si2 (16i) site. The crystals had a slight deficiency in the covalent (Si, Ge) network and the deficiency increased with the increase of the Ge content.     

K2B12F12: A rare A2X structure for an ionic compound at ambient conditions

The anhydrous salt K₂B₁₂F₁₂ crystallized from aqueous solution and its structure was determined by single crystal X-ray diffraction. The Ni₂In-type structure it exhibits is rare for an A₂X ionic compound at 25 °C and 1 atm., consisting of an expanded hexagonal close-packed array of B₁₂F₁₂²⁻ centroids (cent?cent distances: 7.204-8.236 Å) with half of the K⁺ ions filling all of the O_h holes and half of the K⁺ ions filling all of the D_3h trigonal holes in the close-packed layers that are midway between two “empty” T_d holes. The structure is also unusual in that the bond-valence sum for the K⁺ ions in O_h holes is less than or equal to 0.73 (the bond-valence sum for the other type of K⁺ ion is 1.16). A variation of the Ni₂In structure is exhibited by the previously published monohydrate Cs₂(H₂O)B₁₂F₁₂, for which an improved structure is also reported here. For K₂B₁₂F₁₂: monoclinic, C2/c, a = 8.2072(8), b = 14.2818(7), c = 11.3441(9) Å, β = 92.832(5)°, Z = 4, T = 120(2) K. For Cs₂(H₂O)B₁₂F₁₂: orthorhombic, P2₁2₁2₁, a = 9.7475(4), b = 10.2579(4), c = 15.0549(5) Å, Z = 4, T = 110(1) K.     

Crystal growth and structural investigation of A2BReO6 (A=Sr, Ba; B=Li, Na)

Single crystals of the double perovskite rhenates A₂BReO₆ (A=Sr, Ba; B=Li, Na) were grown out of molten hydroxide fluxes. Single crystals of orange/yellow Ba₂LiReO₆, Ba₂NaReO₆ and Sr₂LiReO₆ were solved in the cubic, Fm-3m space group with a=8.1214(11) Å, 8.2975(3) Å, and 7.9071(15) Å, respectively, while Sr₂NaReO₆ was determined to be monoclinic P2₁/n with a=5.6737(6) Å, b=5.7988(6) Å, c=8.0431(8) Å, and β=90.02(6) °. The cubic structure consists of a rock salt lattice of corner-shared ReO₆ and MO₆ (M=Li, Na) octahedra which, in the monoclinic structure, are both tilted and rotated. A discrepancy exists between the symmetry of Sr₂LiReO₆ indicated by the single-crystal refinement of flux-grown crystals (cubic, Fm-3m) and the symmetry indicated by the powder diffraction data collected on polycrystalline samples prepared by the ceramic method (tetragonal, I4/m). It is possible that the cubic crystals are a kinetic product that forms in small quantities at low temperatures, while the powder represents the more stable polymorph that forms at higher reaction temperature.     

Crystal growth of a novel oxygen-deficient layered perovskite: Ba7Li3Ru4O20

Single crystals of both Ba₇Li₃Ru₄O₂₀ and Ba₄NaRu₃O₁₂ were grown from reactive molten hydroxide fluxes. Ba₇Li₃Ru₄O₂₀ is a 7L-layer perovskite-related phase resulting from the stacking of six [AO₃] layers and one oxygen deficient [AO₂] layer, thereby creating LiO₄ tetrahedra in addition to the LiO₆ octahedra and face-sharing Ru₂O₉ bi-octahedra formed from the [AO₃] layers. The compound crystallizes in the space group with a=5.7927(1) Å and c=50.336(2) Å, Z=3. Ba₄NaRu₃O₁₂ crystallizes in the space group P6₃mc with lattice parameters of a=5.8014(2) Å and c=19.2050(9) Å, Z=2. Ba₄NaRu₃O₁₂ is identical to a previously reported neutron refinement structure. The magnetic properties of Ba₇Li₃Ru₄O₂₀ are also reported.