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We show how tuning the proximity to the soft optical mode phase transition via chemical composition affects the lattice thermal conductivity κ of Pb1-xGexTe alloys. Using first-principles virtual-crystal simulations, we find that the anharmonic contribution to κ is minimized at the phase transition due to the maximized acoustic-optical anharmonic interaction. Mass disorder significantly lowers and flattens the dip in the anharmonic κ over a wide composition range, thus shifting the κ minimum away from the phase transition. The total κ and its anharmonic contribution vary continuously between the rocksalt and rhombohedral phases as expected for the second-order phase transition. The actual phase and its strength of resonant bonding play a less prominent role in reducing the κ of Pb1-xGexTe alloys than the proximity to the phase transition and the atomic mass. Our results show that alloys with soft optical mode transitions are promising materials for achieving low thermal conductivity and possibly high thermoelectric efficiency.

The main focus of this work is to investigate how electronic excitation changes the forces between atoms in materials, causing large amplitude collective atomic motion, referred to as coherent phonon motion. The recent development of ultrafast optical spectroscopy has greatly advanced our understanding of these phenomena. However, many aspects of ultrafast phenomena are still poorly understood. For instance, experiments on a variety of different materials have shown that symmetry-breaking coherent modes have a much smaller amplitude than fully symmetric coherent modes. Recent experimental work in the group V semimetals have shown that the symmetry-breaking $E_g$ mode is driven by a force which decays in $\sim 10$ fs, a timescale much shorter than the period of the mode, explaining its small amplitude. However, the exact mechanism for this decay has remained unclear. We show that the $E_g$ force in optically excited group V semimetals decays due to coupling of the excited electrons to thermal vibrations. We propose a method for increasing the impact of photoexcitation on symmetry-breaking modes by a suitable choice of photon energy, which would apply to a variety of materials. We are also interested in the dynamics of the coherent phonons themselves. Recently, it has been shown that optical excitation can drive collective structural transformations to phases that do not exist on the equilibrium phase diagram of materials as diverse as transition metal oxides, transition metal chalcogenides, and the group V semimetals. Experimentally reaching many of these states requires a high degree of photoexcitation, with a significant fraction ($1$-$5$\%) of the valence electrons excited to the conduction bands. We use first-principles calculations to simulate coherent phonon dynamics in Sb, and show that at high enough excited carrier concentration, Sb undergoes a reversal of its Peierls distortion. Our results are in good agreement with pump-probe experiments carried out by our experimental collaborators, providing good evidence that such a reversal can be induced by optical excitation.