Boron-phil and boron-phob structure units in novel borides Ni₃Zn₂B and Ni₂ZnB: experiment and first principles calculations

Abstract

The crystal structures of two novel borides in the Ni-Zn-B system, τ-NiZnB and τ-NiZnB, 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 = 0.0219 for NiZnB, 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 = 0.0163 for NiZnB). 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-NiZnB, while B-B pairs exist in τ-NiZnB. The crystal structure of NiZnB is closely related to that of τ-NiZnB, i.e. NiZnB can be formed by removing the nearly planar nickel layer in NiZnB and shifting the origin of the unit cell to the center of the B-B pair. The electrical resistivity and specific heat of τ-NiZnB 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-NiZnB with some indications of lattice softening in τ-NiZnB, which could be related to the increasing metal content and the absence of B-B bonding in τ-NiZnB. For the newly found phases, τ-NiZnB and τ-NiZnB as well as for τ-NiZnB and τ-NiZnB 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 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.

The electronic pseudo band gap states and electronic transport of the full-Heusler compound Fe2VAl

“…The difference Dq at = q scf at À q sup at is defined as the charge transfer for each atom as listed in Table 1. 48 Choosing free atoms as a reference, the charge transfer Dq free at = q scf at À q free at is much larger (see Table 1) and seems much less reasonable. The differences of Bader charges Dq at reflect the change of the charge density after switching on the Schrödinger equation, i.e.…”

Density functional theory study of structural and thermodynamical stabilities of ferromagnetic MnX (X = P, As, Sb, Bi) compounds

“…For deriving a physically meaningful charge transfer and ionicity we calculated the atomic Bader charges for the selfconsistently derived charge density and the Bader charges for the superposed atomic charge densities in the corresponding atomic Bader volumes V Bader as listed in table 4. Their difference was then taken as charge transfer for each atom [42].…”

Electronic and Structural Properties of MPtxB6–2x (M = Y, Yb): Structural Disorder in an Octahedral Boron Framework

“…The maxima of ELF are also present within the faces of the [B 4 Pt 2 ] octahedra (Figure S9e,f) ranging from 0.82 (at/slightly above the B 3 faces) to 0.60 (at BPt 2 faces) indicating B–B and B–Pt covalent bonding (Figure d). It should be noted that the electron localization maxima between transition metal and boron atoms (evidencing M-B covalent bonding) were earlier observed in the compounds Mg 8 Rh 4 B, Mg 1– x RhB, MNi 21 B 20 (M = In, Sn), and Ni 3 ZnB 2 …”

“…It should be noted that the electron localization maxima between transition metal and boron atoms (evidencing M-B covalent bonding) were earlier observed in the compounds Mg 8 Rh 4 B, 102 Mg 1– x RhB, 103 MNi 21 B 20 (M = In, Sn), 91 and Ni 3 ZnB 2 . 104 …”