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

摘要

The crystal structures of two novel borides in the Ni-Zn-B system, tau(5)-Ni3Zn2B and tau(6)-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, beta = 111.164(2)degrees, 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, beta = 93.009(4)degrees, 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 tau(5)-Ni3Zn2B, while B-B pairs exist in t(6)-Ni2ZnB. The crystal structure of Ni2ZnB is closely related to that of tau(4)-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 tau(5)-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 tau(4)-Ni3ZnB2 with some indications of lattice softening in tau(5)-Ni3Zn2B, which could be related to the increasing metal content and the absence of B-B bonding in tau(5)-Ni3Zn2B. For the newly found phases, tau(5)-Ni3Zn2B and t(6)-Ni2ZnB as well as for tau(3)-Ni21Zn2B20 and tau(4)-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