  The anharmonic phonon properties of SnSe in the Pnma phase were investigated
with a combination of experiments and first-principles simulations. Using
inelastic neutron scattering (INS) and nuclear resonant inelastic X-ray
scattering (NRIXS), we have measured the phonon dispersions and density of
states (DOS) and their temperature dependence, which revealed a strong,
inhomogeneous shift and broadening of the spectrum on warming. First-principles
simulations were performed to rationalize these measurements, and to explain
the previously reported anisotropic thermal expansion, in particular the
negative thermal expansion within the Sn-Se bilayers. Including the anisotropic
strain dependence of the phonon free energy, in addition to the electronic
ground state energy, is essential to reproduce the negative thermal expansion.
From the phonon DOS obtained with INS and additional calorimetry measurements,
we quantify the harmonic, dilational, and anharmonic components of the phonon
entropy, heat capacity, and free energy. The origin of the anharmonic phonon
thermodynamics is linked to the electronic structure.
