Local modes of Fe and Co atoms in NiAl intermetallics

Using nuclear inelastic scattering of synchrotron radiation and inelastic neutron scattering we have investigated the vibrational properties of Fe and Co impurities in NiAl intermetallics, and compared them with the density functional theory calculations. The Fe phonon spectra show two sharp peaks appearing in the frequency gap between the Ni and Al phonon bands, and originating from the local modes of Fe residing in Ni, or Al sublattices. Co, which replaces Ni, vibrates within a narrow frequency interval occurring close to the upper edge, but still within the Ni phonon band. The ab initio calculations show that these effects are expected when impurity-host force constants differ from host-host force constants causing a dynamic mismatch between guest and host vibrations.

Figure 1

Ab initio calculated (a) phonon dispersion relations of NiAl. Experimental points are taken from Ref. 13. (b) Phonon density of states of NiAl.

Figure 2

Solid squares: measured phonon density of states of ${}^{57}\mathrm{Fe}$ in NiAl alloy with $1\mathrm{at.}\%$ Fe. Error bars are as large as the square points. Open squares: ab initio calculated partial phonon density of states of Fe on the Ni sublattice. Open circles: ab initio calculated partial phonon density of states of Fe on the Al sublattice. The inset shows the nuclear inelastic scattering data from ${}^{57}\mathrm{Fe}$ in a NiAl alloy with $1\mathrm{at.}\%$ Fe.

Figure 3

Ab initio calculated (a) total phonon density of states of ${\mathrm{Ni}}_{43.75}{\mathrm{Fe}}_{6.25}{\mathrm{Al}}_{50}$; (b) Fe partial phonon density of states of ${\mathrm{Ni}}_{43.75}{\mathrm{Fe}}_{6.25}{\mathrm{Al}}_{50}$; (c) total density of states of ${\mathrm{Ni}}_{50}{\mathrm{Al}}_{43.75}{\mathrm{Fe}}_{6.25}$; (d) Fe partial density of states of ${\mathrm{Ni}}_{50}{\mathrm{Al}}_{43.75}{\mathrm{Fe}}_{6.25}$ supercells.

Figure 4

Ab initio calculated (a) total phonon density of states, and (b) Co partial phonon density of states of ${\mathrm{Ni}}_{43.75}{\mathrm{Co}}_{6.25}{\mathrm{Al}}_{50}$ supercell.

Figure 5

The neutron weighted intensity differences $(I\left[{\mathrm{Ni}}_{1-x}\left({\mathrm{Co}}_x\right)\mathrm{Al}\right]-I\left[\mathrm{Ni}\mathrm{Al}\right])$ measured by neutron scattering for ${\mathrm{Ni}}_{43.75}{\mathrm{Co}}_{6.25}{\mathrm{Al}}_{50}$ (solid circles) and ${\mathrm{Ni}}_{40}{\mathrm{Co}}_{10}{\mathrm{Al}}_{50}$ (solid squares). Open circles: the same difference obtained from ab initio calculations for ${\mathrm{Ni}}_{43.75}{\mathrm{Co}}_{6.25}{\mathrm{Al}}_{50}$ model. The inset shows the neutron weighted density of states for three samples. The solid triangles denote the NiAl specimen.