Electrons in non-collinear antiferromagnets exhibit abundant transfer properties of interest to next-generation innovative devices. As two of the most important properties of electrons, both charge and spin must be simultaneously transferred. This will certainly influence many surface reaction processes like the hydrogen evolution reaction (HER). We grow a Mn₃Pt bulk single-crystal that having a room-temperature long-range magnetic order at the Mn sites, which showed Pt-like activity, and excellent stability as a catalyst for HER. Experiments and density-functional-theory calculations reveal that the electronic structure can be modified owing to the spin polarization of the Mn atoms. This further affects the adsorption energy of the reaction intermediate by tailoring the arrangement and filling of d-electrons. With this strategy, a similar Gibbs free energy for hydrogen adsorption was obtained between Mn-Mn hollow sites and Pt sites. In other words, more actives sites beyond Pt are created. This study paves the way for the design of high-efficiency electrocatalysts through the interplay between the spin states and the adsorption-desorption behaviors.

We report on our investigation on the magnetism of the iridate double perovskite Sr$_2$CoIrO$_6$, a nominally Ir$^{5+}$ Van Vleck $J_{eff}=0$ system. Using x-ray absorption (XAS) and x-ray magnetic circular dichroism (XMCD) spectroscopy at the Ir-$L_{2,3}$ edges, we found a nearly zero orbital contribution to the magnetic moment and thus an apparent breakdown of the $J_{eff}=0$ ground state. By carrying out also XAS and XMCD experiments at the Co-$L_{2,3}$ edges and by performing detailed full atomic multiplet calculations to simulate all spectra, we discovered that the compound consists of about 90% Ir$^{5+}$ ($J_{eff}=0$) and Co$^{3+}$ ($S=2$) and 10% Ir$^{6+}$ ($S=3/2$) and Co$^{2+}$ ($S=3/2$). The magnetic signal of this minority Ir$^{6+}$ component is almost equally strong as that of the main Ir$^{5+}$ component. We infer that there is a competition between the Ir$^{5+}$-Co$^{3+}$ and the Ir$^{6+}$-Co$^{2+}$ configurations in this stoichiometric compound.

The discovery of high thermoelectric performance in n-type polycrystalline Mg3(Sb,Bi)2-based Zintl compounds has ignited intensive research interest. However, some fundamental questions concerning the anisotropic transport properties and the origin of intrinsically low thermal conductivity are still elusive, requiring the investigation of single crystals. In this work, high-quality p-type Mg3Sb2 and Mg3Bi2 single crystals have been grown by using a self-flux method. The electrical resistivity ρ of Mg3Bi2 single crystal displays an anisotropy with ρ in-plane twice larger than out-of-plane. The low-temperature heat capacity and lattice thermal conductivity of Mg3Sb2 and Mg3Bi2 single crystals have been investigated by using the Debye–Callaway model, from which the existence of low-lying vibration mode could be concluded. Large Grüneisen parameters and strong anharmonicity are found responsible for the intrinsically low thermal conductivity. Moreover, grain boundary scattering does not contribute significantly to suppress the lattice thermal conductivity of polycrystalline Mg3Sb2. Our results provide insights into the intrinsic transport properties of Mg3X2 and could pave a way to realize enhanced thermoelectric performance in single-crystalline Mg3X2-based Zintl compounds.

Searching novel catalysts for efficient and cost-effective hydrogen evolution is one of the most important subjects in the area of catalysis. The traditional method for enhancing the activity of a catalyst is to increase the concentration of active sites and/or enhancing the metallicity. Our study proposes and demonstrates a different principle that goes beyond local site optimization by utilizing topological electronic states to enhance the catalytic activity. The band inversion in topological materials leads to numerous favourable physical properties such as robust surface states, extremely high conductivity, and high carrier mobility, which significantly influence the interfacial charge transfer and transport behaviour in the catalytic process. In addition, the topological surface states (TSS) can act as both electron acceptors or donators for small adsorbed molecules, consequently tailoring the adsorption energy and Gibbs free energy by choosing a topological phase with a specific electronic structure. We tested various topological materials such as Weyl semimetals, 1T’-MoTe2, NbP, TaP, NbAs, and TaAs; topological insulators (TI) Bi2Se3, Bi2Te2Se, Bi2Te3, and Sb2Te3; triple-point metal MoP; and nodal line semimetal PtSn4. The MoTe2 and TaAs family of Weyl semimetals were proved to be excellent catalysts in dye-sensitised visible-light photocatalytic hydrogen evolution experiments. Comparison of NbP with some typical catalysts demonstrated that the former works better than the semiconducting TiO2 or metal catalysts such as Ni under similar experimental conditions. We also established for the first time that bismuth selenide and telluride TIs are active for hydrogen evolution catalysis, where the two-dimensional topological surface states play a crucial role. Moreover, topological MoP and PtSn4 exhibit significantly small overpotentials in the electrocatalytic hydrogen evolution reaction. In a short span of time, we have demonstrated that the topological features of a material can be exploited as an effective tool to achieve high catalytic activity. Our long-term goal is to tune the catalytic reaction pathway by topological materials employing external parameters such as magnetic field.

The thermoelectric properties of the n-type semiconductor TiNiSn were optimized by partial substitution with metallic MnNiSb in the half Heusler structure. Herein, we study the transport properties and intrinsic phase separation in the Ti1-xMnxNiSn1-xSbx system. The alloys were prepared by arc-melting and annealed at temperatures obtained from differential thermal analysis and differential scanning calorimetry results. The phases were characterized using powder X-ray diffraction patterns, energy-dispersive X-ray spectroscopy, and differential scanning calorimetry. After annealing, the majority phase was TiNiSn with some Ni-rich sites, and the minority phases were primarily Ti6Sn5, Sn and MnSn2. The Ni-rich sites were caused by Frenkel defects; this led to metal-like behavior in the semiconductor specimens at low temperature. For x ≤ 0.05 the samples showed an activated conduction, whereas for x > 0.05 they showed metallic character. The figure of merit for x = 0.05 was increased by 61% (zT = 0.45) in comparison with the pure TiNiSn.

Received 17 March 2017DOI:https://doi.org/10.1103/PhysRevLett.118.259702© 2017 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasKondo effectVacanciesPhysical SystemsPnictidesTechniquesTransport techniquesCondensed Matter, Materials & Applied Physics

We address the origin of the magnetic-field-independent -|A|T^{1/2} term observed in the low-temperature resistivity of several As-based metallic systems of the PbFCl structure type. For the layered compound ZrAs_{1.58}Se_{0.39}, we show that vacancies in the square nets of As give rise to the low-temperature transport anomaly over a wide temperature regime of almost two decades in temperature. This low-temperature behavior is in line with the nonmagnetic version of the two-channel Kondo effect, whose origin we ascribe to a dynamic Jahn-Teller effect operating at the vacancy-carrying As layer with a C_{4} symmetry. The pair-breaking nature of the dynamical defects in the square nets of As explains the low superconducting transition temperature T_{c}≈0.14  K of ZrAs_{1.58}Se_{0.39} compared to the free-of-vacancies homologue ZrP_{1.54}S_{0.46} (T_{c}≈3.7  K). Our findings should be relevant to a wide class of metals with disordered pnictogen layers.

Li0.44Eu3[B3N6] was synthesized from the metathesis reaction of Li3[BN2] and EuCl3 at 850 °C. Li0.44Eu3[B3N6] crystallizes in the trigonal space group R3¯c (No. 167) with a=12.0225(2) Å, c=6.8556(2) Å and Z=6. In the crystal structure, isolated, planar cyclic [B3N6]9− units are charge-balanced by the mixed-valence Eu3+/Eu2+ and Li+ cations. Li+ occupies partially (44%) the Wyckoff site 6b and is sandwiched between the [B3N6]9− anions. Mössbauer spectroscopy results show the resonance lines of Eu2+ and Eu3+, respectively, indicating the heterogeneous mixed valency of the Eu atoms. X-Band ESR investigations between 5 and 300 K reveal an intense signal over the whole temperature range originating from Eu2+. Magnetic susceptibility measurements indicate a Curie–Weiss behavior with an experimental effective magnetic moment of μeff=8.28 μB per formula unit.

Various preparation routes led to a large number of complex ternary transition-metal hydrides and hydridometalates containing d- and f-elements, together with alkali, alkaline-earth, and rare-earth metals. Here, the parameter pressure is a crucial tool for the selective synthesis: increasing the reaction pressure leads to higher oxidation states of the transition element. The crystal structures of hydridometalates show a broad variety of different complex formations, whereby higher valence states of transition metals are favored, with alkali metal as the counterion. Moreover, the mobility of hydrogen atoms causes numerous order–disorder transitions.

The magnetic properties and electronic structure of thulium diboride were investigated as it is a magnetically unknown phase between ferromagnetic (FM) REB2 (RE=Tb,Dy,Ho,Er) and antiferromagnetic YbB2. TmB2 adopts the AlB2-type crystal structure (P6/mmm) with a=3.26016(5) A and c=3.75351(8) A and exhibits long-range FM order with the specific heat C-p exhibiting a peak at T-C=7.2 K at the Curie temperature. A Curie-Weiss fit of the magnetic susceptibility yielded mu(eff)=7.49 mu(B)/Tm (Tm3+) and weak FM interactions. A Schottky anomaly in C-p was used to investigate crystal electric field (CEF) effects. From a measurement of C-p(T) of the isostructural LuB2 the magnetic entropy S-mag of TmB2 is determined. The ordered state of the Tm ions involves three CEF levels. Electronic-structure calculations, performed within the local spin density approximation, indicate a Ruderman-Kittel-Kasuya-Yoshida-type mechanism for the magnetic order. With respect to the total energy, ferromagnetic order is found to be more stable than the A-type antiferromagnetically ordered state, in agreement with the experiment.

The binary systems alkaline-earth metal-nitrogen and their intermediate compounds have been subject of numerous discussions for many years. At the beginning of the third millennium, the reactive gas high-pressure synthesis [1] led to single phase products of nitride-diazenides for the first time. Using this method it was possible to synthesize nitrogen rich compounds [2-4], which are free of impurities such as hydrogen, carbon and oxygen. The most important variable with regard to phase formation and composition is the reactive gas pressure. The formation of the different phases can be described in terms of reversible redox-intercalation processes, which are exemplified for the Sr-N system: Starting from Sr2N as the host structure, diazenide-dumb-bells are inserted accompanied by simultaneous oxidation of an equivalent amount of strontium to Sr. The stepwise intercalation proceeds with increasing the reaction pressure (up to 6000 bar) until the final state with the chemical composition SrN2 is reached [2, 3]. Additional studies in the system Ba-N clearly show that starting from Ba2N an insertion of nitrogen in the host structure succeeds in the same manner by increasing N2-reaction pressure, however, the pressure regions for obtaining pure intercalated phases are significantly lower and smaller. Thus, the pure diazenide BaN2 [4] is already obtained at 200 bar N2 reaction pressure, and reaction pressures in an even lower pressure range reveal the existence of Ba4N3 and BaN. The nitrogen content was ascertained by chemical analysis using the carrier gas hot extraction (CGHE) method (Fig. 1). For quantification the CGHE method was modified to allow controlled heating of the binary nitride diazenides in such a way that the determination of the thermal decomposition process was possible [5]. The crystal structures of the three compounds Ba4N3, BaN [6] and BaN2 [4] differ from that of the analogous strontium nitrogen compounds by different orientations of the [N2] dumb-bells. For a more detailed investigation of the intermediate crystal structures of the Ba-series, we intend to carry out synchrotron radiation studies. Obviously, the parameter gas pressure is a crucial tool for the selective synthesis of specific nitrogen compounds, in which the oxidation state is gained by adjusting the redoxpotential of the reaction gas by varying its pressure, thus increasing reaction pressures facilitate access to nitrogen rich compounds. Recently, the investigations of the intercalation processes in the subnitride Sr2N was extended to the reaction with molecular hydrogen [7]. The experimental results confirm that the pathway of gas pressure synthesis enables access to single phase products. The strontium-nitride-hydride (Sr2N)H is obtained as brown-yellow single phase powder by the reaction of strontium subnitride (Sr2N) with hydrogen (400 bar, 620 K) and subsequent treatment under vacuum (10 bar, 870 K). The structure determination was carried out by a combination of X-ray and neutron diffraction experiments on a deuterated sample, and the elemental composition of the ternary compound was confirmed by means of chemical analyses. Strontium (Sr) in the crystal structure of (Sr2N)D (Fig. 2) is arranged in a slightly distorted ccpmotif. Nitrogen (N) and deuterium (D) occupy the octahedral voids of the strontium-matrix in an ordered manner resulting in an alternating Redox-Intercalation of Hydrogen and Nitrogen in Alkaline-Earth Subnitrides Gudrun Auffermann, René Chemnitzer, Yurii Prots, and Rüdiger Kniep

Ternary phases in the systems Zr-As-Se and Zr-As-Te were studied using single crystals of ZrAs1.40(1)Se0.50(1) and ZrAs1.60(2)Te0.40(1) (PbFCl-type of structure, space group P4/nmm) as well as ZrAs0.70(1)Se1.30(1) and ZrAs0.75(1)Te0.25(1) (NbPS-type of structure, space group Immm). The characterization covers chemical compositions, crystal structures, homogeneity ranges and electrical resistivities. At 1223 K, the Te-containing phases can be described with the general formula ZrAsxTe2-x, with 1.53(1)<= x <= 1.65(1) (As-rich) and 0.58(1)<= x <= 0.75(1) (Te-rich). Both phases are located directly on the tie-line between ZrAS(2) and ZrTe2, with no indication for any deviation. Similar is true for the Se-rich phase ZrAsxSe2-x with 0.70(1)<= x <= 0.75(1). However, the compositional range of the respective As-rich phase ZrAsx-ySe2-x (0.03(l)<= y <= 0.10(l); 1.42(l),<= x <= 1.70(l)) is not located on the tie-line ZrAS(2)-ZrSe2, and exhibits a triangular region of existence with intrinsic deviation of the composition towards lower non-metal contents. Except for ZrAS(0.75)Se(1.25), from the homogeneity range of the Se-rich phase, all compounds under investigation show metallic characteristics of electrical resistivity at temperatures > 20K. Related uranium and thorium arsenide selenides display a typical magnetic field-independent rise of the resistivity towards lower temperatures, which has been explained by a non-magnetic Kondo effect. However, a similar observation has been made for ZrAs1.40Se0.50, which, among the Zr-based arsenide chalcogenides, is the only system with a large concentration of intrinsic defects in the anionic substructure. (c) 2007 NIMS and Elsevier Ltd. All rights reserved.

Nitride-diazenides and hydridometalates (exemplified by the chemical systems Sr (Ba)-N-2 and Rb-Pt-H-2) are obtained by reactive gas pressure syntheses in autoclaves starting from the elements and binary precursors.. respectively. The formation of the nitride-diazenides can be described in terms of reversible redox-intercalation processes and that of the hydridometalates as reversible solid-gas redox reactions. Phase formation and stability regions of the intermediate compounds of the chemical systems under consideration are particularly controlled by the level of the applied gas pressure.

Strontium-nitride-hydride is obtained as brown-yellow single phase powder by reaction of strontium-subnitride (Sr2N) with hydrogen (200 bar, 620 K) and subsequent treatment under vacuum (10(-6) bar, 870 K). The structure determination was carried out by a combination of X-ray and neutron diffraction experiments on a deuterated sample. The elemental composition of the ternary compound was confirmed by means of chemical analyses. (Sr2N)D crystallizes in the space group R (3) over barm with a = 381.91(2) pm and c = 1887.61(2) pm. Strontium (Sr2+) in the crystal structure of (Sr2N)D is arranged with an only slightly distorted ccp-motif. Nitrogen (N3-) and deuterium (D-) occupy the octahedral voids of the Sr2+ matrix in an ordered manner resulting in an alternating sequence of layers (anti-alpha-NaFeO2-type structure).

Cs3OsD9 was synthesized by the reaction of cesium deuteride with osmium powder at 870 K and a deuterium pressure above 1500 bar.Elastic neutron diffraction experiments on a powder sample at room temperature led to an atomic arrangement nearly describable in a cubic unit cell (Pm (3) over barm; a = 6.128(2) A, Z = 1). According to the formula CS3D[OsD8] the structure corresponds to that of the perovskite type. The coordination polyhedron of the deuterium atoms surrounding each osmium atom can be described crystallographically as a statistical occupation of two 24-fold positions with deuterium. A detailed investigation on the gaussian profiles of the neutron diffraction pattern leads to the result that the compound crystallizes in a tetragonal unit cell with a = 8.602(1) angstrom c = 12.250(1) angstrom and Z = 4. Below a phase transition at 110K an orthorhombic unit cell was found with a = 8.6983(4) angstrom, b = 8.6046(4) angstrom, c = 11.9437(6) angstrom and Z = 4. A provisional structure model is presented.Magnetic susceptibility measurements in the temperature range between 1.8 and 400 K reveal a weak paramagnetism. Quantum mechanical calculations confirm the experimental result and show in detail that spin momentum and orbital momentum cancel each other nearly complete with regard to the ground state.

AbstractFor Abstract see ChemInform Abstract in Full Text.

We present low-temperature heat and charge transport as well as caloric properties of a ThAsSe single crystal. An extra -AT1/2 term in the electrical resistivity, independent of magnetic fields as high as 14 T, provides evidence for an unusual scattering of conduction electrons. Additionally, both the thermal conductivity and specific heat show a glass-type temperature dependence which signifies the presence of tunneling states. These observations apparently point to an experimantal realization of a two-channel Kondo effect derived from structural two-level systems.

Neutron scattering experiments play a key role in the investigation in crystal structures and properties of solid compounds which is demonstrated with examples from the compound series A(x)M(y)X(z) (A (=) over cap alkali metal, M (=) over cap transition metal and X (=) over cap H, S or Se). Measurements on powdered samples provide as well the hydrogen positions within the crystal structures of ternary hydrides by using the analogous deuterium compounds as the lithium positions in the compounds with A (=) over cap Li. Ferromagnetic and antiferromagnetic arrangements are investigated by neutron diffraction which lead to the determination of the magnetic moments of transition metal atoms providing information about chemical bonding. Finally, it is demonstrated that inelastic neutron scattering is an independent method used in a complementary way to infrared or Raman spectroscopy, and, due to the interactions of neutrons with the magnetic states of transition metal atoms, to susceptibility or magnetisation.

Inelastic neutron scattering spectra from polycrystalline NaH, KH, RbH and CsH, measured at low temperature in the energy transfer range 3 meV < E < 500 meV, are reported. From the medium-energy regions, coinciding with the optical phonon bands, accurate hydrogen-projected densities of phonon states are extracted and compared to ab initio lattice dynamics results. The overall agreement is very good. Further lattice dynamics calculations, based on a pairwise Born-Mayer semi-empirical potential scheme, were also performed, providing only limited and qualitative agreement with the experimental data. In conclusion, incoherent inelastic neutron spectroscopy proves to be a stringent validation tool for lattice dynamics simulations of H-containing materials.