Mixed crystals in the system Cu2MnGexSn1−xS4: Phase analytical investigations and inspection of tetrahedra volumes

Cu₂MnGeS₄ crystallizes orthorhombic in a wurtzite superstructure type while Cu₂MnSnS₄ crystallizes in a tetragonal sphalerite superstructure type. Lattice constants and thermal analyses of the solid solution series Cu₂MnGe_xSn_1−xS₄ are presented. A two-phase region is found from Cu₂MnGe_0.3Sn_0.7S₄ to Cu₂MnGe_0.5Sn_0.5S₄. The cell volume of the mixed crystals increases with increasing Sn content. The melting points increase smoothly with increasing Ge content to x=0.5 and then steeply for higher Ge contents. The single crystal X-ray structure analysis of Cu₂MnGe_0.55Sn_0.45S₄ is presented. The refinement converges to R=0.0270 and wR₂=0.0586, Z is 2. The volumes of the tetrahedra [MS₄] (M=Cu, Mn, Ge, Sn) are calculated. From these volumes the differences in size of the tetrahedra are derived and compared with the corresponding differences in the end members of the solid solution series. It turns out that the resulting structure type in these materials depends on the volume differences of the constituting tetrahedra [MS₄].     

Hg2M2F6S et Hg2M2F6O: Deux nouvelles familles de pyrochlores contenant du mercure et des métaux M de transition divalents

New pyrochlore compounds Hg₂M₂F₆S (M = Zn, Cu, Ni, Co, Mn, Mg) and Hg₂M₂F₆O (M = Zn, Cu, Ni, Co, Mg) have been prepared and characterized. They all have a cubic structure (Fd3m) except Hg₂Cu₂F₆O, which shows a tetragonal distortion. Refinement of crystal structures of five compounds from powder diffractometer intensities leads to a positional parameter x(48f) lying from 0.315 to 0.319. The evolution of interatomic distances and independance of two networks (Hg₂X′)²⁺ and (M₂F₆)^2? are examined.     

Two Novel Oxamidato-bridged Tetranuclear Complexes M[Cu(PMoxd)]3(ClO4)2 (M=Mn, Ni, Cu(PMoxd)=N,N′-bis(pyridyl-methyl)-oxamidatocopper(II)): Syntheses, Spectra, Magnetic and Electrochemical Properties

Two novel oxamidato-bridged Mn[Cu(PMoxd)]₃(ClO₄)₂ (1) Ni[Cu(PMoxd)]₃(ClO₄)₂ (2) tetranuclear complexes were prepared and characterized by i.r., e.p.r., electronic spectra, cyclic voltammograms, and magnetic properties. The magnetic analysis was carried out by means of the theoretical expression of the magnetic susceptibility deduced from the spin Hamiltonian H=−2JS_M(S_Cu1+ S_Cu2 + S_Cu3) (M=Mn, Ni), leading to J=−20.4 cm⁻¹; −121.1 cm⁻¹ for complexes (1) and (2) respectively. Magnetic measurements indicate that the overall magnetic behavior of the tetranuclear species are antiferromagnetic.     

Phase relationships in the systems MSCr2S3In2S3 (M = Co,Cd, Hg)

The phase diagrams of the quaternary systems MSCr₂S₃In₂S₃, with M = Co, Cd, and Hg, were studied with the help of X-ray powder photographs of quenched samples, high-temperature X-ray diffraction patterns, DTA and TG measurements, and far-infrared spectra. Because indium sulfides do react with silica tubes, alumina crucibles must be used for annealing the samples. Complete series of mixed crystals are formed among the spinel-type compounds MCr₂S₄, MIn₂S₄ (M = Cd, Hg), and In₂S₃. HgIn₂S₄ is decomposed at temperatures above 300°C. In the sections CoCr₂S₄CoIn₂S₄ and CoCr₂S₄In₂S₃ relatively large miscibility gaps exist due to the change from normal to inverse spinel structure. But the interchangeability of both systems increases with increasing temperature, and at temperatures above 1000°C, complete series of solid solutions are formed, which can be quenched to ambient temperature. Superstructure ordering like that of ordered α-In₂S₃ has been found in the In-rich region of the MIn₂S₄In₂S₃ solid solutions. The unit cell dimensions of all stoichiometric and phase boundary compounds, e.g., Cd_1.15In_1.9S₄, including the chromium spinels MCr₂S₄ (M = Mn, Zn) and ZnCr₂Se₄, are given and discussed in terms of possible deviations from stoichiometry.     

Effects of magnesium substitution on the magnetic properties of Nd0.7Sr0.3MnO3

Effects of magnesium substitution on the magnetic properties of Nd_0.7Sr_0.3MnO₃ have been investigated by neutron powder diffraction and magnetization measurements on polycrystalline samples of composition Nd_0.7Sr_0.3MnO₃, Nd_0.6Mg_0.1Sr_0.3MnO₃, Nd_0.6Mg_0.1Sr_0.3Mn_0.9Mg_0.1O₃, and Nd_0.6Mg_0.1Sr_0.3Mn_0.8Mg_0.2O₃. The pristine compound Nd_0.7Sr_0.3MnO₃ is ferromagnetic with a transition temperature occurring at about 210 K. Increasing the Mg-substitution causes weakened ferromagnetic interaction and a great reduction in the magnetic moment of Mn. The Rietveld analyses of the neutron powder diffraction (NPD) data at 1.5 K for the samples with Mg concentration, y=0.0 and 0.1, show ferromagnetic Mn moments of 3.44(4) and 3.14(4) μ_B, respectively, which order along the [001] direction. Below 20 K the Mn moments of these samples become canted giving an antiferromagnetic component along the [010] direction of about 0.4 μ_B at 1.5 K. The analyses also show ferromagnetic polarization along [001] of the Nd moments below 50 K, with a magnitude of almost 1 μ_B at 1.5 K for both samples. In the samples with Mg substitution of 0.2 and 0.3 only short range magnetic order occurs and the magnitude of the ferromagnetic Mn moments is about 1.6 μ_B at 1.5 K for both samples. Furthermore, the low-temperature NPD patterns show an additional very broad and diffuse feature resulting from short range antiferromagnetic ordering of the Nd moments.     

Crystal structure and magnetic properties of Sr4Mn2NiO9

The crystal structure of Sr₄Mn₂NiO₉ has been refined on single crystal. This phase belongs to the series A_1+x(A^′_xB_1–x)O₃ (x=1/3) related to the 2H-hexagonal perovskite. The structure contains transition metals in chains of oxide polyhedra (trigonal prisms and octahedra); neighboring chains are separated from each other by the Sr atoms. The sequence of the face sharing polyhedra along the chains is two octahedra + one trigonal prism. Mn occupies the octahedra and Ni is disordered in the trigonal prism with ≈80% in the pseudo square faces of the prism and ≈20% at the centre. This result has been confirmed by XANES experiments at Mn K and Ni K edges, respectively. Sr₄Mn₂NiO₉ is antiferromagnetic with a Néel temperature at T=3 K. The Curie constant measured at high temperature is in good agreement with ≈80% of the Ni²⁺ ions in the spin state configuration S=0.     

Crystal and magnetic structures of Sr4MMn2O9 (M=Cu or Zn)

The crystal and magnetic structures of Sr₄MMn₂O₉ (M=Cu, Zn) have been refined from neutron powder diffraction data. These trigonal compounds (space group P321, a=9.5918(1), c=7.8114(1) Å (Cu); a=9.5894(1), c=7.5039(1) Å (Zn)) are n=3 members of the series A_3n+3M_nB_n+3O_6n+9, with each unit cell containing three offset [001] polyhedral chains, each of which ideally contains a 1:1 ratio of B₂O₉ units and MO₆ trigonal prisms. In fact anti-site disorder between Mn and M is observed, and for M=Cu the cations are disordered off the center of the prism towards a rectangular face. Both compositions show 3D anti-ferromagnetic order at 1.6 K, with an ordered magnetic moment of 1.91(6) (M=Cu) or 1.8(1) (M=Zn) μ_B per Mn. No ordered magnetic moment was detected on the trigonal prismatic site in either compound, consistent with the observed temperature dependence of the magnetic susceptibility.     

Phase equilibria in the SrS-Ga2S3 system

In the SrS-Ga₂S₃ system, there exist two individual compounds: SrGa₂S₄ (a = 2.084 nm, b = 2.050 nm, c = 1.220 nm; congruent melting at 1530 K) and Sr₂Ga₂S₅ (a = 1.253 nm, b = 1.203 nm, c = 1.117 nm; peritectic melting at 1330 K); both are orthorhombic. We discovered a compound of composition Sr₄Ga₂S₇; this compound crystallizes in cubic system with the unit cell parameter a = 0.6008 nm, space group Pa3, and decomposes by a solid-phase reaction at 870 K. Eutectic compositions are 42 and 73 mol % Ga₂S₃; eutectic melting temperatures are 1210 and 1170 K, respectively. The SrS solubility in γ-Ga₂S₃ at 1070 K reaches 4 mol %.     

PREPARATION AND CATALYTIC PERFORMANCE OF La0.5RE0.1Sr0.4MnO3 (RE: Y, Dy, Sm OR Ce) ULTRA-FINE PARTICLES in METHANE COMBUSTION

Ultra-fine particles of La_0.5RE_0.1Sr_0.4MnO₃ (RE: Y, Dy, Sm or Ce) with perovskite structure were prepared by the co-precipitation method, resulting in high surface area and good thermal stability catalysts (S_BET reached 45 and 16.5 m²/g, upon calcination at 700 and 1000^oC, respectively). The thermal stability of these ultra-fine particles was effectively improved with partial substitution of RE³⁺ (Y³⁺, Dy³⁺, or Sm³⁺) for La³⁺ in La_0.6Sr_0.4MnO₃. The catalysts exhibited high activity for the total combustion of methane. For the catalysts calcined at 1000^oC, La_0.5RE_0.1Sr_0.4MnO₃ (RE: Y or Dy) were much more active and showed much higher specific surface area than La_0.6Sr_0.4MnO₃.     

Structural transformation and oxidation of Sr2MnO3.5+x determined by in-situ neutron powder diffraction

The oxidation of the n = 1 Ruddlesden-Popper phase, Sr₂MnO_3.5+x, where 0 ≤ x ≤ 0.5 has been investigated using a combination of in-situ diffraction techniques. In agreement with previous reports the room temperature structure of Sr₂MnO_3.5+x was determined to be monoclinic crystallising in space group P2₁/c. On heating in air the material undergoes rapid oxidation at a relatively modest temperature, ∼275 °C. The oxidation process is coincident with a significant change in the structure, with the material now adopting a tetragonal I4/mmm structure. In the oxygen deficient phase where x > 0 the Mn coordination is square pyramidal, with a sixth partially occupied oxygen position giving rise to octahedral coordination. Oxidation of Sr₂MnO_3.5+x results in the filling of the partially occupied O4 positions and a resulting increase in symmetry, with the Mn coordination now adopting solely a distorted octahedral environment.     

Phase diagrams of sections in the EuS-Cu2S-Nd2S3 system

Phase equilibria in the EuS-Cu₂S-Nd₂S₃ system were studied in an isothermal (970 K) section and NdCuS₂-EuS and Cu₂S-EuNdCuS₃ polythermal sections. The complex sulfide EuNdCuS₃ has an orthorhombic crystal lattice (space group Pnma; a = 1.10438(2) nm, b = 0.40660(1) nm, c = 1.14149(4) nm), is isostructural to BaLaCuS₃, and melts incongruently at 1470 K: EuNdCuS₃ (0.50 EuS; 0.50 NdCuS₂) ai 0.18 EuS ss (0.88 EuS; 0.12 NdCuS₂) + 0.82 L (0.415 EuS; 0.585 NdCuS₂); ΔH = 17.8 kJ/mol. Within the range 0.5 mol % EuS, EuNdCuS₃-based solid solutions were not found. At 970 K, the tie lines pass from the compound EuNdCuS₃ to Cu₂S, EuS, NdCuS₂, and EuNd₂S₄ phases and lie between the NdCuS₂ phase and solid solutions (ss) of γ-Nd₂S₃ with EuNd₂S₄. Eutectics are formed between the compounds NdCuS₂ and EuNdCuS₃ at 32.0 mol % EuS T = 1318 K and between the compounds Cu₂S and EuNdCuS₃ at 20.5 mol % EuNdCuS₃ and T = 1142 K. Five main subordinate triangles were identified in the system.     

Magnesium substitution in Nd0.7Sr0.3MnO3

Magnesium substitution in Nd_0.7Sr_0.3MnO₃ has been studied by neutron powder diffraction. Polycrystalline samples of nominal compositions Nd_0.7Sr_0.3Mn_1−yMg_yO₃ with y=0.0, 0.1, 0.2 and 0.3 were synthesized by the standard solid-state reaction method. Rietveld refinements of the neutron powder diffraction data showed that all samples had distorted perovskite structure of orthorhombic symmetry. Mg initially preferred to substitute for Nd and only at Mg concentration greater than 0.1, a substantial substitution for Mn occurred. Our study also showed that Mg-substitution did not change the crystal structure of Nd_0.7Sr_0.3MnO₃.     

Synthesis, Crystal Structure, and Magnetic Properties of a Novel Layered Manganese Oxide Sr2MnGaO5+δ

New Sr₂MnGaO_4.97 complex oxide was synthesized by solid state reaction in sealed silica tubes at 950–1000°C. The Sr₂MnGaO_4.97 crystal structure was refined from X-ray powder diffraction data. Sr₂MnGaO_4.97 is based on the Ima2 brownmillerite-type structure with apically elongated MnO₆ octahedra due to a Jahn–Teller effect. Electron diffraction and high-resolution electron microscopy showed that local ordering of the left-and right-hand chains of GaO₄ tetrahedra in Sr₂MnGaO_4.97 leads to a superstructure with a doubling of the b parameter of the orthorhombic unit cell. The formal oxidation state of Mn (V_Mn) can be varied by thermal treatments at elevated oxygen pressure (450°C, 20 bar of O₂). The oxygen insertion induces a structure transformation in oxidized Sr₂MnGaO_5.47 material with the formation of a tetragonal perovskite-like structure (aa_p, c2c_p) with oxygen vacancies located in the Ga layers. The oxidation is accompanied by a significant compression of the Mn–O apical distances and a suppression of the Jahn–Teller deformation. Both Sr₂MnGaO_4.97 and Sr₂MnGaO_5.47 can probably be treated as canted antiferromagnets with T_N=150 and 80 K, respectively.     

Synthesis, crystal structure and magnetic properties of the Sr2Al0.78Mn1.22O5.2 anion-deficient layered perovskite

A new layered perovskite Sr₂Al_0.78Mn_1.22O_5.2 has been synthesized by solid state reaction in a sealed evacuated silica tube. The crystal structure has been determined using electron diffraction, high-resolution electron microscopy, and high-angle annular dark field imaging and refined from X-ray powder diffraction data (space group P4/mmm, a=3.89023(5) Å, c=7.8034(1) Å, R_I=0.023, R_P=0.015). The structure is characterized by an alternation of MnO₂ and (Al_0.78Mn_0.22)O_1.2 layers. Oxygen atoms and vacancies, as well as the Al and Mn atoms in the (Al_0.78Mn_0.22)O_1.2 layers are disordered. The local atomic arrangement in these layers is suggested to consist of short fragments of brownmillerite-type tetrahedral chains of corner-sharing AlO₄ tetrahedra interrupted by MnO₆ octahedra, at which the chain fragments rotate over 90°. This results in an averaged tetragonal symmetry. This is confirmed by the valence state of Mn measured by EELS. The relationship between the Sr₂Al_0.78Mn_1.22O_5.2 tetragonal perovskite and the parent Sr₂Al_1.07Mn_0.93O₅ brownmillerite is discussed. Magnetic susceptibility measurements indicate spin glass behavior of Sr₂Al_0.78Mn_1.22O_5.2. The lack of long-range magnetic ordering contrasts with Mn-containing brownmillerites and is likely caused by the frustration of interlayer interactions due to presence of the Mn atoms in the (Al_0.78Mn_0.22)O_1.2 layers.     

MnOx-SnO2 Catalysts Synthesized by a Redox Coprecipitation Method for Selective Catalytic Reduction of NO by NH3

MnO_x-SnO₂ composite oxides prepared by a redox coprecipitation route were tested in selective catalytic reduction of NO by NH₃ at low temperatures. The results showed that the MnO_x-SnO₂ catalyst with a Mn/(Mn+Sn) molar ratio of 75% exhibited the best performance, on which NO conversion of 100% could be achieved at temperatures of 120–200 °C. The characterization results of N₂ adsorption-desorption, X-ray diffraction, and X-ray photoelectron spectroscopy indicated that the higher surface area, the formation of solid solution between manganese and tin oxides, and the high oxidation state manganese species were responsible for the high catalytic activity of the MnO_x-SnO₂ catalyst.     

MnO2-0.39IrOx复合氧化物用于酸性介质中水氧化反应的性能

通过Adams方法成功制备MnO₂-0.39IrO_x(0.39为Ir/Mn的原子比)催化剂并将其用于酸性介质中高效析氧反应(OER)。电化学测试发现,MnO₂-0.39IrO_x仅需253 mV的过电势即可驱动10 mA·cm^-2的水氧化电流密度,并可稳定运行200 h。在1.50 V(vs RHE)电势下,MnO₂-0.39IrO_x的贵金属Ir的质量活性为61.3 mA·mg^-1,是IrO₂的35.8倍,说明MnO₂掺杂大大提升了贵金属利用率。结构分析发现MnO₂-0.39IrO_x独特的片状结构大幅度提高了催化剂的电化学活性表面积,并且Ir位点与Mn位点之间存在一定的电子相互作用。催化过程分析表明,MnO₂-0.39IrO_x表面出现一定的重构现象,并且Mn组分对Ir位点的化学环境实现了持续优化,从而实现了催化剂的高效酸性OER性能。     

Microstructural and transport properties in substituted Bi2Sr2CaCu2O8+δ-modulated compounds

X-ray powder diffraction and resistivity measurements were performed on Bi₂Sr₂CaCu₂O_8+δ ceramics substituted by Y and Zn for Ca and Cu sites, respectively. X-ray diffraction patterns show an incommensurate modulated structure along the b-axis. The structural refinements were carried out using the four-dimensional space group Bbmb(0β1)0 0 0. From the X-ray peak profiles analysis, an anisotropic line-shape broadening was observed. The use of the “Williamson and Hall” method allows distinguishing the origin of broadening as mainly due to microstrains. A large transition from a metallic to semiconductor behaviour is observed on the resistivity curves at x≈0.4 for Bi₂Sr₂Ca_1−xY_xCu₂O_8+δ and at x^′≈0.36 for Bi₂Sr₂Ca_1−xY_xCu_1.94Zn_0.06O_8+δ, which can be also correlated to the defects. Oppositely to the metallic behaviour, which satisfies the Mathiessen's rule, the semiconducting one can be modelled by a variable range hopping process.     

Reverse micellar synthesis and properties of nanocrystalline GMR materials (LaMnO3, La0.67Sr0.33MnO3 and La0.67Ca0.33MnO3): Ramifications of size considerations

Nanoparticles of complex manganites (viz. LaMnO₃, La_0.67Sr_0.33MnO₃ and La_0.67Ca{0.33}MnO₃) have been synthesized using the reverse micellar route. These manganites are prepared at 800‡C and the monophasic nature of all the oxides has been established by powder X-ray diffraction studies. TEM studies show an average grain size of 68, 80 and 50 nm for LaMnO₃, La_0.67Sr_0.33MnO₃ and La_0.67Ca{0.33}MnO₃respectively. Ferromagnetic ordering is observed at around 250 K for LaMnO₃, 350 K for La_0.67Sr_0.33MnO₃ and 200 K for La_0.67Ca{0.33}MnO₃. These Curie temperatures correspond well with those reported for bulk materials with similar composition.     

Charge and structural ordering in the brownmillerite phases: La1−xSrxMnO2.5 (0.2<x<0.4)

The topotactic reduction of La_1−xSr_xMnO₃ (0.2<x<0.4) perovskite phases to the corresponding La_1−xSr_xMnO_2.5 brownmillerite phases with NaH is described. Neutron and electron diffraction data show the x=0.25 and 0.2 phases adopt structures with an unusual ordered L-R-L-R alternation of twisted chains of Mn(II) tetrahedra within each anion-deficient layer. This is accompanied by Mn(II)/(III) charge ordering within the remaining MnO₆ octahedral layers. In contrast, the x=0.4 phase adopts a structure in which the twisted chains of tetrahedra are disordered.     

Three-layer Aurivillius phases containing magnetic transition metal cations: Bi2−xSr2+x(Nb,Ta)2+xM1−xO12, M=Ru, Ir, Mn, x≈0.5

We report the synthesis of Aurivillius-type phases incorporating magnetic M⁴⁺ cations (M=Mn, Ru, Ir), based on the substitution of M⁴⁺ for Ti⁴⁺ in Bi₂Sr₂(Nb,Ta)₂TiO₁₂. The key to incorporating these magnetic transition metal cations appears to be the partial substitution of Sr²⁺ for Bi³⁺ in the α-PbO-type layer of the Aurivillius phase, leading to a concomitant decrease in the M⁴⁺ content; i.e., the composition of the prepared compounds was Bi_2−xSr_2+x(Nb,Ta)_2+xM_1−xO₁₂, x≈0.5. These compounds only exist over a narrow range of x, between an apparent minimum (x≈0.4) Sr²⁺ content in the α-PbO-type [Bi₂O₂] layer required for Aurivillius phases to form with magnetic M⁴⁺ cations, and an apparent maximum (x≈0.6) Sr²⁺ substitution in this [Bi₂O₂] layer. Rietveld-refinement of synchrotron X-ray powder diffraction data making use of anomalous dispersion at the Nb and Ru K edges show that the overwhelming majority of the incorporated M cations occupy the central of the three MO₆ octahedral layers in the perovskite-type block. Magnetic susceptibility measurements are presented and discussed in the context of the potential for multiferroic (magnetoelectric) properties in these materials.