Abstract
X-ray single-crystal structure analysis was performed for the novel compound V1.84Fe1.16B2 ≡ (V1-xFex)2FeB2 at x = 0.08 (P4/mbm; a = 0.555931(9) nm, c = 0.306781(5) nm; U3Si2-type). Consequently, structural identity is obvious between (V0.92Fe0.08)2FeB2 and the precipitates V∼2Fe∼1B2 earlier identified in the UGISTAB215XH permanent magnet. Magnetic and 57Fe Mössbauer studies of (V0.92Fe0.08)2FeB2 reveal a magnetically ordered ground state with Tc∼110 K. Mössbauer spectra point towards a ferrimagnetic spin arrangement.Enthalpy of formations (DFT calculations) for (Fe,V), VB, V3B2, and the hypothetical solution V3-xFexB2 (x<1.5) clearly document that the tie-line between (Fe,V) and VB is more stable than the continuous solid solution of the U3Si2-type phase. This explains the experimental observation that (i) binary V3B2 only dissolves a minor amount of Fe (replacing V), and that (ii) binary V3B2 and isotypic V1.84Fe1.16B2 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 V1.84Fe1.16B2 is 442 GPa. The higher anisotropy in the ternary boride V2FeB2 is concluded from the significantly higher difference between C11 and C33 in V2FeB2 (192.1 GPa) with respect to V3B2 (117.0 GPa).
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