Anomalous thermal transport in MgSe with diamond phase under pressure

Most materials exhibit a monotonically increasing lattice thermal conductivity with pressure. However, in this work, we report anomalous thermal transport properties of MgSe with a diamond phase under high pressure by first-principles calculations. Unlike other selenides, the lattice thermal conductivity of MgSe decreases with increasing pressure. We attribute this anomalous thermal transport to both harmonic and anharmonic effects. The harmonic effect is related to the anomalous downward shift of the transverse acoustic modes, which leads to decreasing phonon group velocities. This anomalous harmonic effect can also be confirmed by anomalous soft elastic properties. The anharmonic effect, on the other hand, is related to the stronger anharmonic scattering strength under pressure. Through bonding analysis, we found that the alternating bonding and antibonding states, combined with the unique lattice structure, are the main reason behind this anomaly. Our results reveal that the abnormal soft elastic properties under pressure can be a new way to judge whether lattice thermal conductivity decreases with pressure.

Figure 1

The crystal structure of (a) $Fm\overline{3}m$ and (b) $F\overline{4}3m$ space groups. The primitive cell is marked by red lines. The ${\kappa }_l$ of (c) $Fm\overline{3}m$ and (d) $F\overline{4}3m$ space group structures as a function of pressure.

Figure 2

The ${\kappa }_l$ of (a) BeSe, (b) $\alpha \text{−}\mathrm{MgSe}$, and (c) $\beta \text{−}\mathrm{MgSe}$ under different pressures as a function of temperature. The phonon dispersion of (d) BeSe, (e) $\alpha \text{−}\mathrm{MgSe}$, and (f) $\beta \text{−}\mathrm{MgSe}$ under different pressures.

Figure 3

The phonon group velocity of (a) BeSe, (b) $\alpha \text{−}\mathrm{MgSe}$, and (c) $\beta \text{−}\mathrm{MgSe}$ at different pressures. The group velocities for acoustic phonon modes of $\alpha \text{−}\mathrm{MgSe}$ are given at pressures of (d) 0 GPa, (e) 5 GPa, and (f) 10 GPa. The phonon relaxation time of (g) BeSe, (h) $\alpha \text{−}\mathrm{MgSe}$, and (i) $\beta \text{−}\mathrm{MgSe}$ at different pressures.

Figure 4

The Grüneisen parameters of (a) BeSe, (b) $\alpha \text{−}\mathrm{MgSe}$, and (c) $\beta \text{−}\mathrm{MgSe}$ at different pressures. The Grüneisen parameters for acoustic phonon modes of $\alpha \text{−}\mathrm{MgSe}$ at pressure of (d) 0 GPa, (e) 5 GPa, and (f) 10 GPa.

Figure 5

Growth ratio of (a) second IFCs and (b) the lattice constant as a function of pressure.

Figure 6

The orbital-resolved projected crystal orbital Hamilton population (COHP) of (a) BeSe, (b) $\alpha \text{−}\mathrm{MgSe}$, and (c) $\beta \text{−}\mathrm{MgSe}$ under different pressures. Deformation charge density, given in units of electrons per cubic angstrom, for the (d) $\alpha$-MgSe 3D, (e) $\beta$-MgSe 3D, (f) $\alpha$-MgSe 2D, and (g) $\beta$-MgSe 2D views.

Figure 7

The elastic properties of BeSe, $\alpha \text{−}\mathrm{MgSe}, \beta \text{−}\mathrm{MgSe}$ structures as a function of pressure: (a) $C_{11}$, (b) $C_{12}$, (c) $C_{44}$, (d) Young's modulus, (e) shear modulus, and (f) Poisson's ratio.