Acoustic phonon softening and reduced thermal conductivity in Mg2Si1−xSnx solid solutions

The measured thermal conductivity κph of Mg2Si1−xSnx solid solutions exhibits a dramatic decrease compared with Mg2Si and Mg2Sn. By solving the full Boltzmann equation, the significantly reduced κph was quantitatively reproduced by our calculations. Besides the expected increase in phonon-phonon scattering, we also observed acoustic phonon softening in Mg2Si1−xSnx, despite their smaller average atomic mass relative to Mg2Sn. In agreement with measurements, the lowest κph appears in Mg2Si0.375Sn0.625, which has the shortest mean free path and lowest group velocity. According to our calculated cumulative curve, nanoinclusions of tens of nanometers are suggested for the further reduction of κph in Mg2Si1−xSnx solid solution.