New layered compounds BaFMgPn (Pn= P, As, Sb and Bi), transition-metal-free representatives of the LaOAgS structure

Four new transition metal-free pnictide representatives of the LaOAgS structure type were predicted by DFT calculations and found in the BaFMgPn (Pn = P, As, Sb and Bi) family. The compounds adopt the tetragonal space group P4/nmm with the unit cell parameters a/c 4.3097(1) angstrom/9.5032(1) angstrom, 4.3855(1) angstrom/9.5918(1) angstrom, 4.5733(1) angstrom/9.8184(1) angstrom, and 4.6359(1) angstrom/9.8599(1) angstrom, respectively. According to the DFT calculations, these new compounds are semiconductors with band gaps steadily decreasing from Pn = P (ca. 2 eV) to Pn = Bi (ca. 1 eV). The corresponding strontium fluoride and rare-earth oxide analogs are unlikely to exist and have not been observed yet. The trends of the stability within 1111 and structurally and/or chemically related compounds based on a combined consideration of geometry and DFT calculations are discussed

X-ray diffraction study of phonon and magnon properties of Eu2Cu6P5 ferromagnet

Ternary europium copper phosphide Eu₂Cu₆P₅ was synthesized from the elements by the standard ampule method as a phase-pure product and studied by means of X-ray powder diffraction. Temperature dependencies of the tetragonal unit cell parameters and volume revealed pronounced anisotropy. The observed anomalies on the a(T) and c(T) curves at about TC ≈ 36 K were related to magnetic phase transition in the phosphide under study. The linear spontaneous magnetostrictions in the basal plane and in the direction of the c axis have different signs that indicates a pronounced anisotropy of the thermal expansion at the temperatures of magnetic ordering. Temperature changes of the unit cell volume Vu(T) were analyzed within the Debye-Einstein approximation. The parameters of the model applied (Debye and Einstein characteristic temperatures, shares of the correspondence contributions) were determined. It was shown that the ratio of the shares of the Debye and Einstein contributions to the thermal characteristics are 0.85/0.15; consequently, in the region of moderately low temperatures, the thermal properties of Eu2Cu6P5 are determined mainly by the Debye vibrations of the Cu/P framework. Experimental data on thermal expansion and calculated heat capacity were used to calculate the temperature-dependent Grüneisen parameter, γ(T), for the further analysis of the lattice dynamics of Eu2Cu6P5. The revealed monotonic growth of γ(T), which is a measure of the anharmonicity of lattice vibrations, indicates an increasing influence of anharmonicity on the thermal properties of Eu₂Cu₆P₅. The anisotropy of the vibrations of europium atoms, as well as local regions of structural heterogeneity of the sample that leads to the appearance of local stresses in the crystal structure can be the reasons for the increasing influence of anharmonicity

One disorder out of two orders: Synthesis and crystal structures of cation-ordered PbNaF2NO3, anion-ordered Pb2OFNO3, and continuous disordered (Pb,Na)2(O,F)2-δNO3 solid solution with Sillén-derived structures

Текст статьи не публикуется в открытом доступе в соответствии с политикой журнала.Two new Sillen-like layered lead fluoride nitrates, PbNaF2NO3 and Pb2OFNO3, have been prepared at 300 C. PbNaF2NO3 is a structural analog of alkaline earth – bismuth oxyhalides, BaBiO2X and SrBiO2X (X = Cl – I), but not the corresponding nitrates, SrBiO2NO3 or BaBiO2NO3. Pb2OFNO3 is analogous to the corresponding halides, Pb2OFX (X = Cl, Br, I). Both structures belong to orthorhombic symmetry and demonstrate Na/Pb and O/F ordering, respectively. A continuous solid solution is formed between PbNaF2NO3 and Pb2OFNO3 which demonstrates neither cation nor anion ordering; the structure of intermediate composition Pb1.5Na0.5F1.5O0.5NO3 was refined in tetragonal symmetry and is indeed very close to that of PbBiO2NO3 and CaBiO2NO3. In PbNaF2NO3, the O:F ratio may be varied to a slight extent, PbNaF2-2yOyyNO3, which also breaks the Na – Pb cation ordering. Analogous fluoride halides could not be prepared. Structural analogies to lead, bismuth, and antimony oxyhalides are discusse

One disorder out of two orders: Synthesis and crystal structures of cation-ordered PbNaF2NO3, anion-ordered Pb2OFNO3, and continuous disordered (Pb,Na)2(O,F)2-δNO3 solid solution with Sillén-derived structures

Текст статьи не публикуется в открытом доступе в соответствии с политикой журнала.Two new Sillen-like layered lead fluoride nitrates, PbNaF2NO3 and Pb2OFNO3, have been prepared at 300 C. PbNaF2NO3 is a structural analog of alkaline earth – bismuth oxyhalides, BaBiO2X and SrBiO2X (X = Cl – I), but not the corresponding nitrates, SrBiO2NO3 or BaBiO2NO3. Pb2OFNO3 is analogous to the corresponding halides, Pb2OFX (X = Cl, Br, I). Both structures belong to orthorhombic symmetry and demonstrate Na/Pb and O/F ordering, respectively. A continuous solid solution is formed between PbNaF2NO3 and Pb2OFNO3 which demonstrates neither cation nor anion ordering; the structure of intermediate composition Pb1.5Na0.5F1.5O0.5NO3 was refined in tetragonal symmetry and is indeed very close to that of PbBiO2NO3 and CaBiO2NO3. In PbNaF2NO3, the O:F ratio may be varied to a slight extent, PbNaF2-2yOyyNO3, which also breaks the Na – Pb cation ordering. Analogous fluoride halides could not be prepared. Structural analogies to lead, bismuth, and antimony oxyhalides are discusse

Water-assisted generation of catalytic interface: The case of interfacial Pt-FeOx(OH)y sites active in preferential carbon monoxide oxidation

The surface of supported heterogeneous catalysts often contains adsorbed water and hydroxyl groups even when water is not directly added to the reaction stream. Nonetheless, the reactivity of adsorbed water and hydroxyl groups is rarely considered. We demonstrate that water and hydroxyl groups can not only directly participate in the catalytic oxidation processes but are also able to generate and stabilize the catalytically active metal-oxide interface. We show that the reduction of Pt-Fe-supported catalysts with hydrogen in the presence of adsorbed water or steam allows for achieving one of the highest preferential carbon monoxide oxidation activities at ambient temperature. These conditions create active iron-associated hydroxyl groups next to platinum nanoparticles with enhanced reactivity towards carbon mon-oxide oxidation. Density functional theory calculations suggest that hydroxylation of oxidic iron species stabilizes the FeOx(OH)y/Pt interface, via strong metal-support interaction, which is confirmed by chemisorption measurements. Kinetic experiments, including those with 18O-labeled water, in combination with operando infrared spectroscopy, show that water and hydroxyl groups directly participate in preferential carbon monoxide oxidation. A quantitative correlation between the catalytic activity of Pt-FeOx(OH)y/γ-Al2O3 catalysts and the Fe2+ concentration, obtained using operando X-ray absorption spectroscopy, shows that the number of active Fe2+ sites and the carbon monoxide oxidation rate per active site can be significantly increased by water-assisted pretreatment with hydrogen. This work provides a new example of positive role of strong metal-support interaction for the design of more active catalysts

Water-assisted generation of catalytic interface: The case of interfacial Pt-FeOₓ(OH)ᵧ sites active in preferential carbon monoxide oxidation

The surface of supported heterogeneous catalysts often contains adsorbed water and hydroxyl groups even when water is not directly added to the reaction stream. Nonetheless, the reactivity of adsorbed water and hydroxyl groups is rarely considered. We demonstrate that water and hydroxyl groups can not only directly participate in the catalytic oxidation processes but are also able to generate and stabilize the catalytically active metal-oxide interface. We show that the reduction of Pt-Fe-supported catalysts with hydrogen in the presence of adsorbed water or steam allows for achieving one of the highest preferential carbon monoxide oxidation activities at ambient temperature. These conditions create active iron-associated hydroxyl groups next to platinum nanoparticles with enhanced reactivity towards carbon monoxide oxidation. Density functional theory calculations suggest that hydroxylation of oxidic iron species stabilizes the FeOx(OH)y/Pt interface, via strong metal-support interaction, which is confirmed by chemisorption measurements. Kinetic experiments, including those with 18O-labeled water, in combination with operando infrared spectroscopy, show that water and hydroxyl groups directly participate in preferential carbon monoxide oxidation. A quantitative correlation between the catalytic activity of Pt-FeOx(OH)y/γ-Al2O3 catalysts and the Fe2+ concentration, obtained using operando X-ray absorption spectroscopy, shows that the number of active Fe2+ sites and the carbon monoxide oxidation rate per active site can be significantly increased by water-assisted pretreatment with hydrogen. This work provides a new example of positive role of strong metal-support interaction for the design of more active catalysts.ISSN:0021-9517ISSN:1090-269

Synthesis, crystal and electronic structures of Pt-rich phosphides EuPt3P and EuPt6P2

Two new ternary Pt-rich phosphides, EuPt6P2 and EuPt3P, have been prepared via a two-step solid state reaction. Their crystal structures have been determined from powder XRD data. EuPt6P2 is isostructural to SrPt6P2 (cubic, Pa3, a = 8.4603(1) angstrom); its crystal structure comprises corner-sharing Pt6P trigonal prisms hosting Eu2+ cations in the cuboctahedral voids of the framework. EuPt3P is isostructural to the SrPt3P anti-perovskite (P4/nmm, a = 5.7452(1) angstrom and c = 5.4212(1) angstrom). Magnetization measurements reveal the magnetic response caused by the Eu2+(4f 7) cations. EuPt6P2 is paramagnetic exhibiting no phase transitions down to 1.8 K, whereas EuPt3P orders ferromagnetically below 19 K. Similar to SrPt6P2 and SrPt3P, the new compounds are metallic with states near the Fermi level predominantly formed by the 5d orbitals of Pt

Effect of Transition Metal Substitution on the Structure and Properties of a Clathrate-Like Compound Eu7Cu44As23

A series of substitutional solid solutions—Eu7Cu44−xTxAs23 (T = Fe, Co, Ni)—based on a recently discovered clathrate-like compound (Eu7Cu44As23) were synthesized from the elements at 800 °C. Almost up to 50% of Cu can be substituted by Ni, resulting in a linear decrease of the cubic unit cell parameter from a = 16.6707(1) Å for the ternary compound to a = 16.3719(1) Å for the sample with the nominal composition Eu7Cu24Ni20As23. In contrast, Co and Fe can only substitute less than 20% of Cu. Crystal structures of six samples of different composition were refined from powder diffraction data. Despite very small differences in scattering powers of Cu, Ni, Co, and Fe, we were able to propose a reasonable model of dopant distribution over copper sites based on the trends in interatomic distances as well as on Mössbauer spectra for the iron-substituted compound Eu7Cu36Fe8As23. Ni doping increases the Curie temperature to 25 K with respect to the parent compound, which is ferromagnetically ordered below 17.5 K, whereas Fe doping suppresses the ferromagnetic ordering in the Eu sublattice

EuNi2P4, the first magnetic unconventional clathrate prepared via a mechanochemically assisted route

For the first time, a magnetic unconventional clathrate EuNi2P4 has been prepared from the elements via a combined mechanochemical and solid-state route. Its crystal and electronic structure, magnetic and transport properties as well as lattice dynamics were elucidated. It crystallizes in the orthorhombic space group Fddd (Z = 8) with unit cell parameters a = 5.1852(1) Å, b = 9.4834(1) Å, and c = 18.9893(2) Å, V = 933.78(1) Å3 at room temperature. Its crystal structure can be described as a covalent Ni–P framework forming a twisted Kelvin cell with Eu2+ guest cations occupying the voids of this framework, which is supported by chemical bonding analysis based on the electron localization function topology. Eu2+ guests sit inside the oversized cages with a coordination number of 24 and exhibit strong rattling, which manifests in both thermodynamic properties and Raman spectra. The compound is metallic and exhibits a rather low thermal conductivity of ∼4 W K−1 m−1 at high temperature along with a positive Seebeck coefficient. The magnetism of EuNi2P4 is predetermined by Eu2+ (4f7) with dominant antiferromagnetic interactions. According to the magnetic susceptibility, heat capacity and resistivity measurements, EuNi2P4 undergoes three phase transitions at ∼2.5 K, 6.1 K and 11.3 K in zero magnetic field