Physical properties of the ternary borides Ni21Zn2B20 and Ni3ZnB2

Abstract Mechanical, electrical and magnetic properties have been determined for the compounds Ni 21 Zn 2 B 20 and Ni 3 ZnB 2 , which reveal a different boron–boron aggregation and bonding due to their different boron/metal ratio. Resonant Ultrasound Spectroscopy (RUS) measurements show significantly higher elastic moduli for Ni 21 Zn 2 B 20 ( E  = 264 GPa) with a unique cluster of a cage of 20 B-atoms nesting a Ni-metal octahedron than for Ni 3 ZnB 2 ( E  = 223 GPa), which contains covalently bonded B–B chain fragments consisting of 4 B-atoms. Similarly Vickers hardness (for a load of 200 ponds) is higher for Ni 21 Zn 2 B 2 ( HV  = 6.87 GPa) than for Ni 3 ZnB 2 ( HV  = 4.99 GPa). The higher boron/metal ratio for Ni 21 Zn 2 B 20 is reflected in a smaller thermal expansion coefficient α  = 8 × 10 −6  K −1 with respect to Ni 3 ZnB 2 with α  = 10.4 × 10 −6  K −1 (both determined for the range 150–250 K). At elevated temperatures, the specific heat C p of both compounds, Ni 21 Zn 2 B 20 and Ni 3 ZnB 2 , is clearly dominated by the lattice contribution. Analyses of C p in terms of Debye and Einstein models indicate that the boron related high energy Einstein modes display at similar energies with spectral weights being in clear correspondence with their boron stoichiometries. Temperature dependent magnetization measurements reveal a paramagnetic behavior for both borides suggesting almost filled 3d states of Ni by contributions from B and Zn valence electrons. Electrical resistivities attest a dominant metallic behavior for both Ni 21 Zn 2 B 20 and Ni 3 ZnB 2 yielding negative Seebeck coefficients (electrons as the majority charge carriers) in the range of a few μV/K.