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

The crystal structures of two novel borides in the Ni-Zn-B system, τ ₅-Ni ₃Zn ₂B and τ ₆-Ni ₂ZnB, 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 ₃Zn ₂B, 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 ₂ZnB). 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 ₃Zn ₂B, while B-B pairs exist in τ ₆-Ni ₂ZnB. The crystal structure of Ni ₂ZnB is closely related to that of τ ₄-Ni ₃ZnB ₂, i.e. Ni ₂ZnB can be formed by removing the nearly planar nickel layer in Ni ₃ZnB 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 ₃Zn ₂B 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 ₃ZnB ₂ with some indications of lattice softening in τ ₅-Ni ₃Zn ₂B, which could be related to the increasing metal content and the absence of B-B bonding in τ ₅-Ni ₃Zn ₂B. For the newly found phases, τ ₅-Ni ₃Zn ₂B and τ ₆-Ni ₂ZnB as well as for τ ₃-Ni ₂₁Zn ₂B ₂₀ and τ ₄-Ni ₃ZnB 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.