Pressure-induced low-lying phonon modes softening and enhanced thermal resistance inβ−Mg2Al4Si5O18

Lattice thermal conductivities of $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{M}{\mathrm{g}}₂\mathrm{A}{\mathrm{l}}₄\mathrm{S}{\mathrm{i}}₅{\mathrm{O}}₁₈$ were predicted at various hydrostatic pressures based on some theoretical models. An abnormal decrement on lattice thermal conductivity is observed for compressed crystal structure. A rigorous analysis of structural stability, bonding characteristics, vibration modes, group velocities, and mode Gr\"uneisen parameters helps us to recognize the origin of this anomalous behavior. We attribute the negative dependent trend to the softening of low frequency phonons and strengthening of anharmonicity at elevated pressure, both of which arise from the specific corner-linked tetrahedral framework in the crystal structure. To validate theoretical calculations, we synthesized pure and dense $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{M}{\mathrm{g}}₂\mathrm{A}{\mathrm{l}}₄\mathrm{S}{\mathrm{i}}₅{\mathrm{O}}₁₈$ ceramic by using a two-step processing method and determined its intrinsic lattice thermal conductivity by successfully eliminating the phonon scattering from defects and high-temperature thermal radiation. The experimental intrinsic values agreed quite well with the theoretical predictions. This paper reports an anomalous pressure-induced reduction of lattice thermal conductivity and also provides a key insight into the interesting phonon modification mechanism through tailoring the crystal structure of complex compounds.