Lattice dynamical properties of antiferromagnetic MnO, CoO, and NiO, and the lattice thermal conductivity of NiO

Lattice dynamical properties of antiferromagnetic rocksalt oxides are often interpreted using the cubic space group $Fm\overline{3}m$, although below Néel temperature their magnetic substructure possesses a lower symmetry. For example, in the case of NiO, a rhombohedral structural distortion lowers the symmetry to trigonal space group $R\overline{3}m$ below 525 K. We performed hybrid density functional theory calculations on the phonon dispersion relations of MnO, CoO, and NiO, and the lattice thermal conductivity of NiO using both $Fm\overline{3}m$ and $R\overline{3}m$ space groups. The calculated acoustic phonon frequencies of all oxides agree well with the available experimental data, while the optical modes of MnO and CoO show somewhat larger discrepancies. Our calculations show the phonon density of states to be very similar with both studied space groups. The experimental thermal conductivity of antiferromagnetic NiO is reproduced well below the Néel temperature by solving the linearized phonon Boltzmann transport equation.

Figure 1

Unit cell of an antiferromagnetic rocksalt oxide such as NiO ($Fm\overline{3}m$, gray: metal, red: oxygen). The colored planes are perpendicular to the [111] direction. Metal atoms on the green plane have the same spin while the atoms on the blue plane have the opposite spin, as indicated by the arrows.

Figure 2

Phonon dispersions of MnO, CoO, and NiO obtained using cubic primitive cells. In the case of MnO, blue crosses are experimental data points from Wagner et al. and green crosses from Haywood et al. For CoO and NiO, the green crosses correspond to experimental data from Sakurai et al. and Reichardt et al., respectively [9, 10, 11, 12, 13].

Figure 3

Phonon dispersions and atom-projected phonon density of states (DOS) of NiO using two different primitive cells. The phonon dispersions of the cubic primitive cell were plotted in directions corresponding to the trigonal primitive cell, folded, and labeled according to the trigonal case. The following reciprocal space points were used for the cubic primitive cell: $L=(-1/2,1/2,1/2), Z=(1/2,1/2,1/2)$, and $F=(-1/2,1/2,0)$. Corresponding points for the trigonal primitive cell are: $L=(0,1/2,0), Z=(1/2,1/2,1/2)$, and $F=(1/2,1/2,0)$. In the DOS plots the green and blue lines correspond to Ni and O contributions, respectively.

Figure 4

Lattice thermal conductivity of NiO calculated using both the trigonal (red line) and cubic (black line) primitive cells. Crosses denote the different experimental data points [48, 49, 50, 51, 52].

Figure 5

Phonon lifetimes of NiO at 300 K calculated using the cubic and trigonal primitive cells. (a) Cumulative phonon lifetimes for both primitive cells. (b) and (c) Phonon lifetimes plotted on top of the phonon density of states for the cubic and trigonal primitive cells, respectively. Color map on the right is for the density of points on the lifetime-frequency plane. The $y$ axis has been cut in such way that only the highest-density region in the frequency-lifetime plane is shown. Plots with full $y$ axis and phonon lifetimes up to 250 ps are shown in the Supplemental Material [33].