Li5Sn, the Most Lithium-Rich Binary Stannide: A Combined Experimental and Computational Study.

From reaction of excess lithium with tin, we isolate well-crystallized Li5Sn and solve the crystal structure from single-crystal X-ray diffraction data. The orthorhombic structure (space group Cmcm) features the same coordination polyhedra around tin and lithium as previously predicted by electronic structure calculations for this composition, however differently arranged. An extensive ab initio analysis, including thermodynamic integration using Langevin dynamics in combination with a machine-learning potential (moment tensor potential), is conducted to understand the thermodynamic stability of this Cmcm Li5Sn structure observed in our experiments. Among the 108 Li5Sn structures systematically derived using the structure enumeration algorithm, including the experimental Cmcm structure and those obtained in previous ab initio studies, another new structure with the space group Immm is found to be energetically most stable at 0 K. This computationally discovered Immm structure is also found to be thermodynamically more stable than the Cmcm structure at finite temperatures, indicating that the Cmcm Li5Sn structure observed in our experiments is favored likely due to kinetic reasons rather than thermodynamics.

The Reduced Nitridogermanates(III) Ca6 [Ge2 N6 ] and Sr6 [Ge2 N6 ] with Ge-Ge Bonds.

The first nitridogermanates(III) Ca6 [Ge2 N6 ] and Sr6 [Ge2 N6 ] were synthesized from sodium flux and structurally characterized by powder and single crystal X-ray diffraction, respectively. They crystallize isostructurally to each other and homeotypic to Ca6 [Cr2 N6 ]H in space group R 3 ‾ . They feature unprecedented, mutually isolated, ethane-like [GeIII 2 N6 ]12- anions in a staggered conformation. The compounds are semiconductors according to resistivity measurements and electronic structure calculations, yielding band gaps of 1.1 eV for Ca6 [Ge2 N6 ] and 0.2 eV for Sr6 [Ge2 N6 ].

Approaching Dissolved Species in Ammonoacidic GaN Crystal Growth: A Combined Solution NMR and Computational Study.

Solutions of gallium trihalides GaX3 (X=F, Cl, Br, I) and their ammoniates in liquid ammonia were studied at ambient temperature under autogenous pressure by multinuclear (71 Ga, 35 Cl, 81 Br) NMR spectroscopy. To unravel the role of pH, the analyses were done both in absence and in presence of ammonium halides, which are employed as mineralizers during ammonoacidic gallium nitride crystal growth. While gallium trifluoride and its ammoniate were found to be too sparingly soluble to give rise to a NMR signal, the spectra of solutions of the heavier halides reveal the presence of a single gallium-containing species in all cases. DFT calculations and molecular dynamics simulations suggest the identification of this species as consisting of a [Ga(NH3 )6 ]3+ cation and up to six surrounding halide anions, resulting in an overall trend towards negative complex charge. Quantitative 71 Ga NMR studies on saturated solutions of GaCl3 containing various amounts of additional NH4 Cl revealed a near linear increase of GaCl3 solubility with mineralizer concentration of about 0.023 mol GaCl3 per mol NH4 Cl at room temperature. These findings reflect the importance of Coulombic shielding for the inhibition of oligomerization and precipitation processes and help to rationalize both the low solubility of gallium halides in neutral ammonia solution and, in turn, the proliferating effect of the mineralizer during ammonoacidic gallium nitride formation.

Thermal History Dependent Al Distribution in Aluminum Substituted Strontium Hexaferrite.

Single crystals of aluminum substituted strontium hexaferrite SrFe12-xAlxO19 were grown from sodium oxide based flux. The substitution level aimed for was x = 1.2. Annealing experiments performed on single crystals show that the Al distribution on the five iron sites of the hexaferrite structure depends on the annealing time at 900 °C. Single crystal X-ray diffractometry shows that annealing a crystal after the initial synthesis has an impact on the Al content on the octahedrally and tetrahedrally coordinated sites. Furthermore, it was found that heating in a corundum crucible increases the overall Al content. Magnetic measurements show that annealing in a platinum or corundum crucible decreases coercivity and remanence while the saturation magnetization is hardly influenced.

The Quasi-Binary Acetonitriletriide Sr3 [C2 N]2.

The first quasi-binary acetonitriletriide Sr3 [C2 N]2 has been synthesised and characterised. The nearly colourless crystals were obtained from the reaction of Sr metal, graphite, and elemental N2 , generated by decomposition of Sr(N3 )2 , in a sealed Ni ampoule with the aid of an alkali metal flux. The structure of this compound was analysed via single-crystal X-ray diffraction and the identity of the [C2 N]3- anion was confirmed by Raman spectroscopy and further investigated by quantum-chemical methods. Computed interatomic distances within the [C2 N]3- anion strikingly match the obtained experimental data.

Ferromagnetic ε-Fe2MnN: High-Pressure Synthesis, Hardness and Magnetic Properties.

The iron manganese nitride Fe2MnN was obtained by high-pressure-high-temperature synthesis from ζ-Fe2N and elemental Mn at 15(2) GPa and 1573(200) K. The phase crystallizes isostructural to binary ε-Fe3N. In comparison to the corresponding binary iron nitride, the microhardness of ε-Fe2MnN is reduced to 6.2(2) GPa. Above about 800 K the ternary compound decomposes exothermally under loss of nitrogen. ε-Fe2MnN is ferromagnetic with a Curie temperature of roughly 402 K.

Magnetic and Structural Properties of Barium Hexaferrite BaFe12O19 from Various Growth Techniques.

Barium hexaferrite powder samples with grains in the µm-range were obtained from solid-state sintering, and crystals with sizes up to 5 mm grown from PbO, Na2CO3, and BaB2O4 fluxes, respectively. Carbonate and borate fluxes provide the largest and structurally best crystals at significantly lower growth temperatures of 1533 K compared to flux-free synthesis (1623 K). The maximum synthesis temperature can be further reduced by the application of PbO-containing fluxes (down to 1223 K upon use of 80 at % PbO), however, Pb-substituted crystals Ba1-xPbxFe12O19 with Pb contents in the range of 0.23(2) ≤ x ≤ 0.80(2) form, depending on growth temperature and flux PbO content. The degree of Pb-substitution has only a minor influence on unit cell and magnetic parameters, although the values for Curie temperature, saturation magnetization, as well as the coercivity of these samples are significantly reduced in comparison with those from samples obtained from the other fluxes. Due to the lowest level of impurities, the samples from carbonate flux show superior quality compared to materials obtained using other methods.

Transition and Alkali Metal Complex Ternary Amides for Ammonia Synthesis and Decomposition.

A new complex ternary amide, Rb2 [Mn(NH2 )4 ], which simultaneously contains both transition and alkali metal catalytic sites, is developed. This is in line with the recently reported TM-LiH composite catalysts, which have been shown to effectively break the scaling relations and achieve ammonia synthesis under mild conditions. Rb2 [Mn(NH2 )4 ] can be facilely synthesized by mechanochemical reaction at room temperature. It exhibits two temperature-dependent polymorphs, that is, a low-temperature orthorhombic and a high-temperature monoclinic structure. Rb2 [Mn(NH2 )4 ] decomposes to N2 , H2 , NH3 , Mn3 N2 , and RbNH2 under inert atmosphere; whereas it releases NH3 at a temperature as low as 80 °C under H2 atmosphere. Those unique behaviors enable Rb2 [Mn(NH2 )4 ], and its analogue K2 [Mn(NH2 )4 ], to be excellent catalytic materials for ammonia decomposition and synthesis. Experimental results show both ammonia decomposition onset temperatures and conversion rates over Rb2 [Mn(NH2 )4 ] and K2 [Mn(NH2 )4 ] are similar to those of noble metal Ru-based catalysts. More importantly, these ternary amides exhibit superior capabilities in catalyzing NH3 synthesis, which are more than 3 orders of magnitude higher than that of Mn nitride and twice of that of Ru/MgO. The in situ SR-PXD measurement shows that manganese nitride, synergistic with Rb/KH or Rb/K(NH2 )x H1-x , are likely the active sites. The chemistry of Rb2 /K2 [Mn(NH2 )x ] and Rb/K(NH2 )x H1-x with H2 /N2 and NH3 correlates closely with the catalytic performance.

High-Pressure NiAs-Type Modification of FeN.

The combination of laser-heated diamond anvil cells and synchrotron Mössbauer source spectroscopy were used to investigate high-temperature high-pressure chemical reactions of iron and iron nitride Fe2 N with nitrogen. At pressures between 10 and 45 GPa, significant magnetic hyperfine splitting indicated compound formation after annealing at 1300 K. Subsequent in situ X-ray diffraction reveals a new modification of FeN with NiAs-type crystal structure, as also rationalized by first-principles total-energy and chemical-bonding studies.

Three Oxidation States of Manganese in the Barium Hexaferrite BaFe12-xMnxO19.

The coexistence of three valence states of Mn ions, namely, +2, +3, and +4, in substituted magnetoplumbite-type BaFe12-xMnxO19 was observed by soft X-ray absorption spectroscopy at the Mn-L2,3 edge. We infer that the occurrence of multiple valence states of Mn situated in the pristine purely iron(III) compound BaFe12O19 is made possible by the fact that the charge disproportionation of Mn3+ into Mn2+ and Mn4+ requires less energy than that of Fe3+ into Fe2+ and Fe4+, related to the smaller effective Coulomb interaction of Mn3+ (d4) compared to Fe3+ (d5). The different chemical environments determine the location of the differently charged ions: with Mn3+ occupying positions with (distorted) octahedral local symmetry, Mn4+ ions prefer octahedrally coordinated sites in order to optimize their covalent bonding. Larger and more ionic bonded Mn2+ ions with a spherical charge distribution accumulate at tetrahedrally coordinated sites. Simulations of the experimental Mn-L2,3 XAS spectra of two different samples with x = 1.5 and x = 1.7 led to Mn2+:Mn3+:Mn4+ atomic ratios of 0.16:0.51:0.33 and 0.19:0.57:0.24.

New synthesis route for ternary transition metal amides as well as ultrafast amide-hydride hydrogen storage materials.

K2[Mn(NH2)4] and K2[Zn(NH2)4] were successfully synthesized via a mechanochemical method. The mixture of K2[Mn(NH2)4] and LiH showed excellent rehydrogenation properties. In fact, after dehydrogenation K2[Mn(NH2)4]-8LiH fully rehydrogenates within 60 seconds at ca. 230 °C and 5 MPa of H2. This is one of the fastest rehydrogenation rates in amide-hydride systems known to date. This work also shows a strategy for the synthesis of transition metal nitrides by decomposition of the mixtures of M[M'(NH2)n] (where M is an alkali or alkaline earth metal and M' is a transition metal) and metal hydrides.

Trigonal-Bipyramidal Coordination in First Ammoniates of ZnF2: ZnF2(NH3)3 and ZnF2(NH3)2.

Single crystals of ZnF2(NH3)3 and ZnF2(NH3)2 were obtained under ammonothermal conditions (250 °C, 196 MPa and 500 °C, 136 MPa). Upon thermal decomposition of both ZnF2(NH3)3 and ZnF2(NH3)2, a microcrystalline powder of ZnF2(NH3) was obtained. ZnF2(NH3)3 and ZnF2(NH3)2 represent probable intermediates in a conceivable ammonothermal synthesis of the semiconductor Zn3N2 and manifest a rare trigonal-bipyramidal coordination of F(-) and NH3 ligands around Zn(2+) according to single-crystal X-ray diffraction. Thermal analysis of all three compounds showed not only ZnF2(NH3) but also ZnF2(NH3)2 to be decomposition intermediates of ZnF2(NH3)3 prior to the formation of ZnF2. All three compounds demonstrate hydrogen bonds, as indicated by the intensities and half-widths of the bands in the vibrational spectra and by short N-H···F distances in the crystal structures of ZnF2(NH3)3 and ZnF2(NH3)2. With ZnF2(NH3)3, ZnF2(NH3)2, and ZnF2(NH3), we present the first ammoniates of ZnF2.

Ternary Amides Containing Transition Metals for Hydrogen Storage: A Case Study with Alkali Metal Amidozincates.

The alkali metal amidozincates Li4 [Zn(NH2)4](NH2)2 and K2[Zn(NH2)4] were, to the best of our knowledge, studied for the first time as hydrogen storage media. Compared with the LiNH2-2 LiH system, both Li4 [Zn(NH2)4](NH2)2-12 LiH and K2[Zn(NH2)4]-8 LiH systems showed improved rehydrogenation performance, especially K2[Zn(NH2)4]-8 LiH, which can be fully hydrogenated within 30 s at approximately 230 °C. The absorption properties are stable upon cycling. This work shows that ternary amides containing transition metals have great potential as hydrogen storage materials.

On copper(I) fluorides, the cuprophilic interaction, the preparation of copper nitride at room temperature, and the formation mechanism at elevated temperatures.

Our attempts to synthesize the hitherto unknown binary copper(I) fluoride have led to first successes and a serendipitious result: By conproportionation of elemental copper and copper(II) fluoride in anhydrous liquid ammonia, two copper(I) fluorides were obtained as simple NH3 complexes. One of them presents an example of ligand-unsupported "cuprophilic" interactions in an infinite [Cu2 (NH3 )4 ](2+) chain with alternating Cu-Cu distances. We discovered that both copper(I) fluorides can easily be converted into Cu3 N at room temperature, just by applying a vacuum. Additionally, we investigated the formation mechanism of the classical synthesis route of Cu3 N that starts with CuF2 and flowing NH3 in the temperature range between ambient and 290 °C by means of thermal analysis and in situ neutron diffraction. The reaction proceeds at elevated temperatures through the formation of a blue and amorphous ammoniate Cu(NH3 )2 F2 , the reformation of CuF2 , and finally the redox reaction to form Cu3 N.

Synthesis and Characterization of Superconducting Ca1-xNaxFFeAs.

A representative of the fluoride-containing iron pnictide high-temperature superconductors, namely CaFFeAs, was doped with sodium up to the composition Ca0.86Na0.14FFeAs for the first time. Single crystals with an edge length in the range of 0.1 - 2.0 mm were obtained via solid-state and flux syntheses, respectively. The composition of the crystals was verified by means of single crystal X-ray diffractometry and energy dispersive X-ray spectroscopy (EDX). Measurements of the electrical resistivity, as well as the magnetization on a crystal of Ca0.89Na0.11FFeAs both show a transition to the superconducting state on cooling to 34.5 K. Investigations of the upper critical fields reveal an anisotropy ratio of about five. The lattice parameters and molar volumes increase with rising sodium content. This effect is clearly observable for the c-axis and the volume, whereas the increase of the a-axis is rather minor.

Garnet-type Mn(3)Cr(2)(GeO(4))(3).

Single crystals of garnet-type trimanganese(II) dichrom-i-um(III) tris-[orthogermanate(IV)], Mn(II) (3)Cr(III) (2)(GeO(4))(3), were obtained by utilizing a chemical transport reaction. Corres-ponding to the mineral garnet with the general formula A(II) (3)B(III) (2)(SiO(4))(3), each of the four elements occupies only one crystallographically distinct position. Mn(2+) occupies the respective A position (Wyckoff site 24c, site symmetry 2.22), being surrounded by eight O atoms that form a distorted cube [d(Mn-O) = 2.291 (2) and 2.422 (2) Å, 4× each], while Cr(3+) on the B position (Wyckoff site 16a, site symmetry .-3.) is situated in a slightly distorted octa-hedron of six O(2-) anions [d(Cr-O) = 1.972 (2) Å, 6×]. In addition, the O atoms on general site 96h form isolated [GeO(4)](4-) tetra-hedra with Ge(4+) on site 24d [site symmetry -4..; d(Ge-O) = 1.744 (2) Å, 4×].

Crystal chemistry and physical properties of the nonmagnetic Kondo compound HfAs1.7Se0.2.

Single crystals of HfAs(1.7)Se(0.2) are grown by chemical transport reaction and their chemical composition characterized in detail by various analytical methods. Chemical analyses and crystal structure investigations by single-crystal X-ray diffraction as well as powder diffraction with synchrotron radiation reveal a tetragonal PbFCl structure type with strong disorder caused by a significant arsenic deficiency (As(0.9)) on the 2a site and mixed occupancy of the 2c site (As(0.8)Se(0.2)). HfAs(1.7)Se(0.2) is a diamagnetic metal which transforms into a superconducting state at T(c)=0.52 K. Similar to other PbFCl-type arsenide selenides, the title compound displays a magnetic-field-independent -AT(1/2) term in the low-temperature electrical resistivity. This unusual term presumably originates from the electron scattering of structural two-level systems. According to the experimental results, HfAs1.7Se0.2 appears to be a rare example of a nonmagnetic Kondo material.

Stacking design of inverse perovskites: the systems (Sr3-xBaxN)E, E = Bi, Sb.

The first representatives of 4H (BaMnO3-type structure, P63/mmc, Z = 4) and 9R (BaMnO3-type structure, Rm, Z = 9) inverse Perovskite phases are presented. The phases are obtained within the solid solutions (Sr3-xBaxN)E with E = Bi, Sb. The crystal structures and homogeneity ranges were studied by combined X-ray and neutron diffraction as well as chemical analyses. The cubic Perovskite phase with Bi (Sb) is stable in the range of 0.00 < or = x < or = 0.90(5) (0.00 < or = x < or = 1.30(5)), the 4H variant is stable for 1.55(5) < or = x < or = 2.10(5) (1.85(5) < or = x < or = 2.45(5)), the 9R structure is stable for 2.50(2) < or = x < or = 2.55(2) (2.56(2) < or = x < or = 2.60(2)), and the 2H phase is stable for 2.75(5) < or = x < or = 3.00 (2.80(5) < or = x < or = 3.00). Ba occupies preferable sites in the hexagonal stacking of close packed layers of alkaline earth metal ions and E3-; Sr is mainly located in cubic stacked layers. The phase order upon going from cubic (Sr3N)E to 2H-type (Ba3N)E concomitant to the pronounced Sr/Ba partial order can, in general, be rationalized considering the Coulomb repulsion of nitride ions, as well as the size and charge density of the alkaline earth metal ions.

Li3[ScN2]: the first nitridoscandate(III)-tetrahedral Sc coordination and unusual MX2 framework.

Li(3)[ScN(2)] was prepared from Li(3)N with Sc or ScN in a nitrogen atmosphere at 1020 K as a light yellow powder with an optical band gap of about 2.9 eV. The crystal structure was refined based on X-ray and neutron powder diffraction data (Ia$\bar 3$, Z=16, X-ray diffraction: R(profile)=0.078, R(Bragg)=0.070; Neutron diffraction: R(profile)=0.077, R(Bragg)=0.074; Rietfeld: a=1003.940(8) pm, Guinier: a=1004.50(3) pm). Li(3)[ScN(2)] is an isotype of Li(3)[AlN(2)] and Li(3)[GaN(2)] and crystallizes in an ordered superstructure of the Li(2)O structure type, leading to a three-dimensional framework of all-vertex-sharing tetrahedra 3[infinity[ScN[4/2][3-]]. Li is displaced from the center of a tetrahedron of N atoms in the direction of one trigonal face. Li(3)[ScN(2)] decomposes above 1050 K to form ScN and Li(3)N. Calculations of the periodic nodal surface (PNS) and of the electron localization function (ELF) support the picture of a covalent Sc-N network separated from isolated Li cations, whereby scandium d orbitals are involved in the chemical bonding.