Structural, thermodynamic, and electronic properties of Laves-phase NbMn2 from first principles, x-ray diffraction, and calorimetric experiments

Abstract

By combining theoretical density functional theory (DFT) and experimental studies, structural and magnetic phase stabilities and electronic structural, elastic, and vibrational properties of the Laves-phase compound ${\mathrm{NbMn}}_{2}$ have been investigated for the C14, C15, and C36 crystal structures. At low temperatures C14 is the ground-state structure, with ferromagnetic and antiferromagnetic orderings being degenerate in energy. The degenerate spin configurations result in a rather large electronic density of states at Fermi energy for all magnetic cases, even for the spin-polarized DFT calculations. Based on the DFT-derived phonon dispersions and densities of states, temperature-dependent free energies were derived for the ferromagnetic and antiferromagnetic C14 phase, demonstrating that the spin-configuration degeneracy possibly exists up to finite temperatures. The heat of formation ${\mathrm{\ensuremath{\Delta}}}_{298}{H}^{0}=\ensuremath{-}45.05\ifmmode\pm\else\textpm\fi{}3.64\phantom{\rule{0.16em}{0ex}}\mathrm{kJ}\phantom{\rule{0.16em}{0ex}}{(\mathrm{mol}\phantom{\rule{0.16em}{0ex}}\mathrm{f}.\mathrm{u}.\phantom{\rule{0.16em}{0ex}}{\mathrm{NbMn}}_{2})}^{\ensuremath{-}1}$ was extracted from drop isoperibolic calorimetry in a Ni bath. The DFT-derived enthalpy of formation of ${\mathrm{NbMn}}_{2}$ is in good agreement with the calorimetric measurements. Second-order elastic constants for ${\mathrm{NbMn}}_{2}$ as well as for related compounds were calculated.