Phase stabilities of Cmcm and Pnma SnSe studied by phonon quasiparticle approach

We investigated the structural stability of SnSe from 0 to 800 K by the phonon quasiparticle approach combining first-principles molecular-dynamics (MD) simulations and lattice dynamics. At high temperature, we witness the dynamic stability of the Cmcm phase and reveal the coupling of the polarization of phonon modes and the phase transition between the Cmcm and Pnma phases. Specifically, in real space, the probability distribution of atomic displacements from first-principles MD simulations successfully captures the structural instability at low temperature and the structural stability at high temperature for Cmcm SnSe. An analysis of phonon power spectra of several modes also delivers the dynamic stabilization of Cmcm at high temperature. Particularly, the soft modes of the Y1 mode at the q=Y(12,12,0) point and the Γ1 mode at the q=Γ(0,0,0) point of the Cmcm phase in the harmonic approximation become relatively rigid at elevated temperature, in agreement with experimental and previous theoretical results. The calculated anharmonic phonon dispersions and density of states are strongly temperature-dependent, and some phonon modes adopt giant frequency shifts. These aspects demonstrate the heavy anharmonicity in SnSe. At low temperature, the transition from the Cmcm to the Pnma phase induces a symmetry breaking of structure. Consequently, the degeneracy of associated electronic states (mainly p states) is lifted, thus lowering the energy of the Pnma phase.