Structure and properties of a novel boride (V0.92Fe0.08)2FeB2 with partially ordered U3Si2-type

Abstract X-ray single-crystal structure analysis was performed for the novel compound V 1.84 Fe 1.16 B 2 ≡ (V 1-x Fe x ) 2 FeB 2 at x = 0.08 ( P 4/ mbm ; a = 0.555931(9) nm, c = 0.306781(5) nm; U 3 Si 2 -type). Consequently, structural identity is obvious between (V 0.92 Fe 0.08 ) 2 FeB 2 and the precipitates V ∼2 Fe ∼1 B 2 earlier identified in the UGISTAB215XH permanent magnet. Magnetic and 57 Fe Mossbauer studies of (V 0.92 Fe 0.08 ) 2 FeB 2 reveal a magnetically ordered ground state with T c ∼110 K. Mossbauer spectra point towards a ferrimagnetic spin arrangement. Enthalpy of formations (DFT calculations) for (Fe,V), VB, V 3 B 2 , and the hypothetical solution V 3- x Fe x B 2 ( x 3 Si 2 -type phase. This explains the experimental observation that (i) binary V 3 B 2 only dissolves a minor amount of Fe (replacing V), and that (ii) binary V 3 B 2 and isotypic V 1.84 Fe 1.16 B 2 appear as independent phases in the ternary phase diagram. Calculation of the electron localization function elf yielded a very high value (ϒ ∼0.75) between boron atoms documenting strong covalent bonding. The Young's modulus E (from nano-indentation) for V 1.84 Fe 1.16 B 2 is 442 GPa. The higher anisotropy in the ternary boride V 2 FeB 2 is concluded from the significantly higher difference between C 11 and C 33 in V 2 FeB 2 (192.1 GPa) with respect to V 3 B 2 (117.0 GPa).