Phonon anharmonicity and negative thermal expansion in SnSe

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© 2016 American Physical Society.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.






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Bansal, D, J Hong, CW Li, AF May, W Porter, MY Hu, DL Abernathy, O Delaire, et al. (2016). Phonon anharmonicity and negative thermal expansion in SnSe. Physical Review B - Condensed Matter and Materials Physics, 94(5). 10.1103/PhysRevB.94.054307 Retrieved from

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Olivier Delaire

Associate Professor of the Thomas Lord Department of Mechanical Engineering and Materials Science

The Delaire group investigates atomistic transport processes of energy and charge, and thermodynamics in energy materials. We use a combined experimental and computational approach to understand and control microscopic energy transport for the design of next-generation materials, in particular for sustainable energy applications. Current materials of interest include superionic conductors, photovoltaics, thermoelectrics, ferroelectrics/multiferroics, and metal-insulator transitions. Our group's studies provide fundamental insights into  atomic dynamics and elementary excitations in condensed-matter systems (phonons, electrons, spins), their couplings and their effects on macroscopic properties. We probe the microscopic underpinnings of transport and thermodynamics properties by integrating neutron and x-ray scattering, optical spectroscopy, and thermal characterization, together with quantum-mechanical computer simulations.

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