Structural and magnetic characterization of Mn3IrGe and Mn3Ir(Si1−xGex): experiments and theory

The structural and magnetic properties of a new ternary Ir-Mn-Ge phase, Mn₃IrGe, as well as the solid solution Mn₃Ir(Si_1−xGe_x), 0?x?1, have been investigated by means of X-ray and neutron powder diffraction, magnetization measurements and first principles calculations. The crystal structure is cubic, of the AlAu₄-type (an ordered form of the β-Mn structure), Z=4, space group P2₁3, and the unit-cell dimension varies linearly with the silicon content. For all compositions, antiferromagnetic ordering is found below a critical temperature of about 225 K. The magnetic structure is noncollinear, as a result of frustrated magnetic interactions on a triangular network of Mn atoms, on which the moments rotate 120° around the triangle axes. The magnitude of the magnetic moment at 10 K is 3.39(4) μ_B for Mn₃IrGe. The theoretical calculations reproduce with very good accuracy the magnitudes as well as the directions of the experimentally observed magnetic moments.     

Crystal,electronic structure and hydrogenation properties of the Mg5.57Ni16Ge7.43 cluster phase with a new type of polyhedron

The ternary germanide Mg_5.57Ni₁₆Ge_7.43 (cubic, space group Fmm, cF116) belongs to the structural family based on the Th₆Mn₂₃-type. The Ge1 and Ge2 atoms fully occupy the 4a (mm symmetry) and 24d (m.mm) sites, respectively. The Ni1 and Ni2 atoms both fully occupy two 32f sites (.3m symmetry). The Mg/Ge statistical mixture occupies the 24e site with 4m.m symmetry. The structure of the title compound contains a three-core-shell cluster. At (0,0,0), there is a Ge1 atom which is surrounded by eight Ni atoms at the vertices of a cube and consequently six Mg atoms at the vertices of an octahedron. These surrounded eight Ni and six Mg atoms form a [Ge1Ni₈(Mg/Ge)₆] rhombic dodecahedron with a coordination number of 14. The [GeNi₈(Mg/Ge)₆] rhombic dodecahedron is encapsulated within the [Ni₂₄] rhombicuboctahedron, which is again encapsulated within an [Ni₃₂(Mg/Ge)₂₄] pentacontatetrahedron; thus, the three-core-shell cluster [GeNi₈(Mg/Ge)₆@Ni₂₄@Ni₃₂(Mg/Ge)₂₄] results. The pentacontatetrahedron is a new representative of Pavlyuk's polyhedra group based on pentagonal, tetragonal and trigonal faces. The dominance of the metallic type of bonding between atoms in the Mg_5.57Ni₁₆Ge_7.43 structure is confirmed by the results of the electronic structure calculations. The hydrogen sorption capacity of this intermetallic at 570 K reaches 0.70 wt% H₂.     

Barrelane-like germanium clusters in Eu3Ge5: Crystal structure, chemical bonding and physical properties

Formation and crystal structure of the binary germanide Eu₃Ge₅ were investigated in detail. The compound forms peritectically at 1008 °C and does not undergo any phase transition down to room temperature. The crystal structure was determined first from X-ray powder diffraction data and was later confirmed by single-crystal X-ray diffraction: structure type Pu₃Pd₅, space group Cmcm (no. 63), , , . The main building blocks are Ge₅⁶⁻ cluster anions surrounded by Eu²⁺ cations. The nearly tetragonal-pyramidal shape is suggested by the interatomic distances. Contrary to that, the bonding analysis with the electron localization function (ELF) reveals only two- and three-bonded germanium atoms forming a strongly distorted [1.1.1]-barrelane-like cluster. Despite the formal electron deficiency, compared to the barrelane C₅H₈, the electron counting in the cluster anion and its conformation cannot be interpreted applying the Wade's rules. In accordance with the calculated electronic density of states, Eu₃Ge₅ shows a metal-like temperature dependence of the electrical resistivity with a sharp change of ρ(T) slope at the Néel point. Above the Néel point the inverse magnetic susceptibility reveals Curie-Weiss behavior with an effective moment of 8.11 μ_B (Eu²⁺, 4f⁷ configuration) in agreement with the analysis of the chemical bonding. The 4f⁷ electronic configuration of europium is confirmed by Eu-L_III X-ray absorption spectroscopy.     

Darstellung und Kristallstruktur neuer AM2X2-Verbindungen in den Systemen Erdalkalimetall-Platinmetall-Germanium

Preparation and Crystal Structure of New AM₂X₂ Compounds in the Systems Earthalkali Metal/Platinum Metal/Germanium . Four new ternary compounds in the systems earthalkali metal/platinum metal/germanium have been prepared and characterised by single crystal X-ray investigation. BaRu₂Ge₂ crystallizes orthorhombically, space group Fddd, a=634.4(1) pm, b=1 056.5(3) pm, c=1 273,1(3) pm. SrRu₂Ge₂ (a=430.6(1) pm, c=1 030.3(2) pm), BaRh₂Ge₂ (a=418.9(5) pm, c=1 175.7(10) pm) and SrRh₂Ge₂ (a=418.3(3) pm, c=1 071.8(6) pm) crystallize in the ThCr₂Si₂-type structure (tetragonal, space group I4/mmm).     

Structural, magnetic, and thermal characteristics of the phase transitions in Gd5GaxGe4−x magnetocaloric materials

Temperature-dependent, single crystal and powder X-ray diffraction studies as well as magnetization, and heat capacity measurements were carried out on two phases of the Gd₅Ga_xGe_4−x system: for x=0.7 and 1.0. Gd₅Ga_0.7Ge_3.3 shows three structure types as a function of temperature: (i) from 165 K to room temperature, the orthorhombic Sm₅Ge₄-type structure exists; (ii) below 150 K, it transforms to a orthorhombic Gd₅Si₄-type structure; and (iii) a monoclinic Gd₅Si₂Ge₂-type component is observed for the intermediate temperature range of 150 K≤T≤165 K. This is the first time that all these three structure types have been observed for the same composition. For Gd₅Ga_1.0Ge_3.0, the room temperature phase belongs to the orthorhombic Pu₅Rh₄-type structure with interslab contacts between main group atoms of 2.837(4) Å. Upon heating above 523 K, it transforms to a Gd₅Si₄-type structure with this distance decreasing to 2.521(7) Å before decomposing above 573 K.     

New Compounds of the ThCr2Si2-Type and the Electronic Structure of CaM2Ge2 (M: Mn-Zn)

Two new compounds were synthesized by heating mixtures of the elements at 975-1025 K and characterized by single-crystal X-ray methods. CaZn₂Si₂ (a=4.173(2) Å, c=10.576(5) Å) and EuZn₂Ge₂ (a=4.348(2) Å, c=10.589(9) Å) crystallize in the ThCr₂Si₂-type structure (space group I4/mmm; Z=2). Magnetic susceptibility measurements of EuZn₂Ge₂ show Curie-Weiss behavior with a magnetic moment of 7.85(5)μ_B/Eu and a paramagnetic Curie temperature of 10(1) K. EuZn₂Ge₂ orders antiferromagnetically at T_N=10.0(5) K and undergoes a metamagnetic transition at a low critical field of about 0.3(2) T. The saturation magnetization at 2 K and 5.5 T is 6.60(5) μ_B/Eu. ¹⁵¹Eu Mössbauer spectroscopic experiments show one signal at 78 K at an isomer shift of −11.4(1) mm/s and a line width of 2.7(1) mm/s compatible with divalent europium. At 4.2 K full magnetic hyperfine field splitting with a field of 26.4(4) T is detected. The already known compounds CaM₂Ge₂ (M: Mn-Zn) also crystallize in the ThCr₂Si₂-type structure. Their MGe₄ tetrahedra are strongly distorted with M=Ni and nearly undistorted with M=Mn or Zn. According to LMTO electronic band structure calculations, the distortion is driven by a charge transfer from M-Ge antibonding to bonding levels.     

From ordered antiferromagnet to spin glass: A new phase Mn4Ir7−xMnxGe6

A new ternary phase, Mn₄Ir_7−xMn_xGe₆ (0?x?1.3), was studied by X-ray and neutron powder diffraction and SQUID magnetometry. The crystal structure is cubic, of the U₄Re₇Si₆ type, space group , Z=2, with the lattice parameter at 295 K. Within the limited range of homogeneity small variations of the composition yield dramatic changes of the magnetic structure. For x=0 long-range antiferromagnetic order is formed below the transition temperature 228 K, with large magnetic moments on Mn, 4.11(9) μ_B at 10 K, in a magnetic unit cell , c_M=2a_C. In contrast, for x=1.3 spin glass behavior is observed below 90 K. The Mn atoms form an ideal cubic framework, on which geometric frustration of competing nearest and next nearest neighbor antiferromagnetic interactions is suggested to explain the composition sensitive magnetic properties. A TiNiSi-type phase, IrMnGe, is found in samples of 1:1:1 composition quenched from the melt.     

Structures and energetics of BenGen (n = 1–5) and Be2nGen (n = 1–4) clusters

Cluster geometries and energies of Be_nGe_n (n = 1–5) and Be_2nGe_n (n = 1–4) have been examined in theoretical electronic structure calculations. Structure optimisations were carried out using DFT B3LYP/6-31G(2df) and the energies of the optimum geometries were ordered in QCISD(T) calculations. Be and Ge bond to each other and to other atoms of their own kind, creating a great variety of low-energy clusters in a variety of structural types. Comparisons of the germanide clusters with previously explored silicide and carbide structures reveal some structural similarities, but the germanides have much more in common with the beryllium silicides than with the carbides. However, germanide clusters show a greater tendency to form cage-like structures with potential in technological applications.     

Magnetic structure of the Mn5Si3-type Er5Ge3 compound

The Er₅Ge₃ compound (Mn₅Si₃-type, hP16, P6₃/mcm) at 4 K shows magnetic ordering of the antiferromagnetic type. Its magnetic structure consists of sine modulated collinear magnetic moments of Er that are parallel to the c axis (with a propagation vector k=[0 0 ±0.3]). This corresponds to the magnetic unit cell (a a 10c), the values of the magnetic moment of the Er atoms being, as a general formula, M_z≈M₀ cos [2π(Z–1/4)(1–kZ)], with M₀=9.2(2) μ_B at 4 K.     

Magnetic phase transitions in RMnGe (R=Tb, Dy) compounds induced by high magnetic fields

Magnetization isotherms for polycrystalline TbMnGe and DyMnGe compounds were measured at the temperature 4.2 K in pulsed magnetic fields (up to 360 kOe). The received isotherms demonstrate the appearance of metamagnetic transitions at low temperatures. Both compounds crystallize in the orthorhombic TiNiSi-type structure. It was found that the magnetic phase transitions with a destruction of ferrimagnetic spiral structure occur in the TbMnGe and DyMnGe compounds in high magnetic fields. An attempt was undertaken to explain the nature of these transitions using the results of the powder neutron diffraction and the qualitative estimation of the exchange interaction values on the basis of the molecular field theory.