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For Fe2VAl the temperature-dependent Seebeck coefficient S(T) and electrical resistivity ρ(T) were calculated within the framework of density functional theory (DFT). The DFT calculations were extended in terms of a DFT/LDA+U approach with U − J values attributed to Fe-d-like states. For simulating the general features of the measured data, a large range of U − J values was scanned with U − J = 2.145 eV as the recommended value. For this value a very small negative indirect gap of E(X) − E(Γ) = −0.0093 eV is found, which is significantly reduced as compared to the DFT-GGA value of −0.164 eV. Charge transfer was derived by Bader's approach, resulting in a significant transfer of 0.75 electronic charges to each Fe atom from Al (1.03) and V (0.48). The pseudogap states around the Fermi energy were analyzed in detail in terms of density of states, band structures and charge density contours. These states almost exclusively govern S(T) and ρ(T). They have large dispersions and are centered at Γ and X. They consist of tails of localized V and Fe states dangling to the Al site. The dispersion of the band along the k space direction X–Γ was modelled in terms of a tight-binding ansatz, resulting in k-dependent matrix elements. From our DFT study, based on the findings for S(T) and ρ(T), it appears that Fe2VAl has a very small negative indirect gap in the electronic structure. By fitting the temperature-dependent Seebeck coefficient within a parabolic band model, a tiny positive band gap of around 0.003 eV is revealed which qualitatively agrees with the DFT results. [ABSTRACT FROM AUTHOR]

The crystal structures of two novel borides in the Ni–Zn–B system, τ5-Ni3Zn2B and τ6-Ni2ZnB, were determined by single crystal X-ray diffraction (XRSC) in combination with selected area electron diffraction in a transmission electron microscope (SAED-TEM) and electron probe microanalysis (EPMA). Both compounds crystallize in unique structure types (space group C2/m, a = 1.68942(8) nm, b = 0.26332(1) nm, c = 0.61904(3) nm, β = 111.164(2)°, RF = 0.0219 for Ni3Zn2B, and space group C2/m, a = 0.95296(7) nm, b = 0.28371(2) nm, c = 0.59989(1) nm, β = 93.009(4)°, RF = 0.0163 for Ni2ZnB). Both compounds have similar building blocks: two triangular prisms centered by boron atoms are arranged along the c-axis separated by Zn layers, which form empty octahedra connecting the boron centered polyhedra. Consistent with the (Ni+Zn)/B ratio, isolated boron atoms are found in τ5-Ni3Zn2B, while B–B pairs exist in τ6-Ni2ZnB. The crystal structure of Ni2ZnB is closely related to that of τ4-Ni3ZnB2, i.e. Ni2ZnB can be formed by removing the nearly planar nickel layer in Ni3ZnB2 and shifting the origin of the unit cell to the center of the B–B pair. The electrical resistivity and specific heat of τ5-Ni3Zn2B reveal the metallic behavior of this compound with an anomaly at low temperature, possibly arising from a Kondo-type interaction. Further analysis on the lattice contribution of the specific heat reveals similarity with τ4-Ni3ZnB2 with some indications of lattice softening in τ5-Ni3Zn2B, which could be related to the increasing metal content and the absence of B–B bonding in τ5-Ni3Zn2B. For the newly found phases, τ5-Ni3Zn2B and τ6-Ni2ZnB as well as for τ3-Ni21Zn2B20 and τ4-Ni3ZnB2 density functional theory (DFT) calculations were performed by means of the Vienna Ab initio Simulation Package (VASP). Total energies and forces were minimized in order to determine the fully relaxed structural parameters, which agree very well with experiment. Energies of formations in the range of −25.2 to −26.9 kJ mol−1 were calculated and bulk moduli in the range of 179.7 to 248.9 GPa were derived showing hardening by increasing the B concentration. Charge transfer is discussed in terms of Bader charges resulting in electronic transfer from Zn to the system and electronic charge gain by B. Ni charge contributions vary significantly with crystallographic position depending on B located in the neighbourhood. The electronic structure is presented in terms of densities of states, band structures and contour plots revealing Ni–B and Ni–Zn bonding features. [ABSTRACT FROM AUTHOR]

Full Heusler alloys of the series Fe2-xNixVAl,0=x=0.2, were prepared and characterized, and their physical properties, relevant to the thermoelectric performance of such materials, were studied in a wide temperature range. The starting material Fe2VAl is characterized by a pseudogap of the electronic density of states near the Fermi energy, with a gap width of the order of 1 eV. Density functional theory calculations were performed by application of two approaches. In the framework of the local-spin-density approximation and coherent potential approximation, the electronic densities of states of substitutional alloys were calculated, revealing that with increasing Ni content the Fermi energy moves toward the conduction band, and consequently, the nature of electronic transport changes from p type to n type. It appears that Ni, due to its extra electrons, provides a narrow impurity band near the Fermi level. These states can be made responsible for the experimentally observed evolution of transport properties. Furthermore, the Vienna ab initio Simulation package (vasp) was utilized for deriving electronic, structural, and vibrational properties of ordered Fe2VAl and Fe1.75Ni0.25VAl. In particular, it is found that due to Ni substitution there is a general shift to lower phonon frequencies by about 2 THz as compared to the undoped case. Associated to these modifications, the electrical resistivity, ρ(T), changes from a semiconducting-like behavior to a nonsimple metallic behavior, while the Seebeck coefficient reaches values of the order of -80 µV/K around room temperature for the sample x=0.2. The increase of the Ni content, in addition, goes along with a substantial reduction of the lattice part of the thermal conductivity. This change is analyzed in detail in terms of a disorder parameter G, characterizing the derangement of the crystalline lattice due to the substitution of Fe by Ni. Ab initio calculations of the phonon dynamics carried out for Fe2VAl and for Fe1.75Ni0.25VAl support these analyses. [ABSTRACT FROM AUTHOR]

Investigating the phase relations of the system {Th,U}-Pt-B at 900℃ the formation of two compounds has been observed: cubic ThPt3B with Pm3m structure as a representative of the perovskites, and tetragonal UPt3B with P4mm structure being isotypic to the noncentrosymmetric structure of CePt3B. The crystal structures of the two compounds are defined by combined x-ray diffraction and transmission electron microscopy. Characterization of physical properties for ThPt3B reveals a superconducting transition at 0.75 K and an upper critical field at T=0 exceeding 0.4 T. For nonsuperconducting UPt3B a metallic resistivity behavior was found in the entire temperature range; at very low temperatures spin fluctuations become evident and the resistivity ρ(T) follows non-Fermi liquid characteristics, ρ = ρ0+ATn with n=1.6. Density functional theory (DFT) calculations were performed for both compounds for both types of structures. They predict that the experimentally claimed cubic structure of ThPt3B is thermodynamically not stable in comparison to a tetragonal phase, with a very large enthalpy difference of 25 kJ/mol, which cannot be explained by the formation energy of B vacancies. However, the presence of random boron vacancies possibly stabilizes the cubic structure via a local strain compensation mechanism during the growth of the crystal. For UPt3B the DFT results agree well with the experimental findings. [ABSTRACT FROM AUTHOR]

Density functional theory (DFT) calculations for deriving enthalpies of formation [Formula: see text] H for ferromagnetic MnX (X [Formula: see text] P, As, Sb, Bi) compounds were made for the two competing structures, hexagonal [Formula: see text] and orthorhombic [Formula: see text]. Standard calculations were performed by using pseudopotentials with the generalized-gradient-approximation (PBE) as exchange-correlation functional. Enhanced exchange-correlation interactions were included by making use of a so-called DFT[Formula: see text]U approach which requires [Formula: see text] as a parameter. Application of PBE potentials for all compounds and elementary phases (all-PBE) resulted in negative values of [Formula: see text] H for MnP and MnAs in both structures whereby the result for MnP [Formula: see text] agrees very well with experiment. For MnSb and MnBi the all-PBE calculation gives a positive nonbonding [Formula: see text] H disagreeing with experiment. To overcome this discrepancy for MnSb and MnBi a DFT[Formula: see text]U ansatz was employed for all compounds and elemental Mn. The values for [Formula: see text] ranging between 0.7 for MnBi and 1.4 eV for MnAs were determined by fitting the DFT results to measured data of [Formula: see text] H. As a reference for pure Mn the [Formula: see text]-Mn phase was taken with [Formula: see text] eV by which choice the experimental volume is fitted. Atomic volumes and ionicities were derived applying Bader's concept resulting in ionicities of Mn less than [Formula: see text].

The crystal structures of two novel borides in the Ni-Zn-B system, τ 5 -Ni 3 Zn 2 B and τ 6 -Ni 2 ZnB, were determined by single crystal X-ray diffraction (XRSC) in combination with selected area electron diffraction in a transmission electron microscope (SAED-TEM) and electron probe microanalysis (EPMA). Both compounds crystallize in unique structure types (space group C2/m, a = 1.68942(8) nm, b = 0.26332(1) nm, c = 0.61904(3) nm, β = 111.164(2)°, R F = 0.0219 for Ni 3 Zn 2 B, and space group C2/m, a = 0.95296(7) nm, b = 0.28371(2) nm, c = 0.59989(1) nm, β = 93.009(4)°, R F = 0.0163 for Ni 2 ZnB). Both compounds have similar building blocks: two triangular prisms centered by boron atoms are arranged along the c-axis separated by Zn layers, which form empty octahedra connecting the boron centered polyhedra. Consistent with the (Ni+Zn)/B ratio, isolated boron atoms are found in τ 5 -Ni 3 Zn 2 B, while B-B pairs exist in τ 6 -Ni 2 ZnB. The crystal structure of Ni 2 ZnB is closely related to that of τ 4 -Ni 3 ZnB 2 , i.e. Ni 2 ZnB can be formed by removing the nearly planar nickel layer in Ni 3 ZnB 2 and shifting the origin of the unit cell to the center of the B-B pair. The electrical resistivity and specific heat of τ 5 -Ni 3 Zn 2 B reveal the metallic behavior of this compound with an anomaly at low temperature, possibly arising from a Kondo-type interaction. Further analysis on the lattice contribution of the specific heat reveals similarity with τ 4 -Ni 3 ZnB 2 with some indications of lattice softening in τ 5 -Ni 3 Zn 2 B, which could be related to the increasing metal content and the absence of B-B bonding in τ 5 -Ni 3 Zn 2 B. For the newly found phases, τ 5 -Ni 3 Zn 2 B and τ 6 -Ni 2 ZnB as well as for τ 3 -Ni 21 Zn 2 B 20 and τ 4 -Ni 3 ZnB 2 density functional theory (DFT) calculations were performed by means of the Vienna Ab initio Simulation Package (VASP). Total energies and forces were minimized in order to determine the fully relaxed structural parameters, which agree very well with experiment. Energies of formations in the range of -25.2 to -26.9 kJ mol -1 were calculated and bulk moduli in the range of 179.7 to 248.9 GPa were derived showing hardening by increasing the B concentration. Charge transfer is discussed in terms of Bader charges resulting in electronic transfer from Zn to the system and electronic charge gain by B. Ni charge contributions vary significantly with crystallographic position depending on B located in the neighbourhood. The electronic structure is presented in terms of densities of states, band structures and contour plots revealing Ni-B and Ni-Zn bonding features.