Structural Origins of Phonon Anharmonicity in Alkali Halide Rock Salt Structures

Thermal management in solid-state technologies such as batteries, microprocessors, thermal barrier coatings, superconductors, fuel cells, and more depends on understanding phonons, the quantum vibrations of atoms described by lattice dynamics. Anharmonicity, the nonlinear dependence of interatomic forces on displacement, governs key properties like thermal conductivity and thermal expansion. Mode Grüneisen parameters quantify this anharmonicity by assessing phonon frequency shifts with lattice deformations, linking microscopic vibrations to macroscopic thermal behavior. Previously, Grüneisen parameters have been correlated to atomic coordination environments, but the fundamental roles of atomic structure and crystal chemistry in phonon anharmonicity have yet to be fully discerned. Herein, an analysis of the vibrational structures and mode Grüneisen parameters of nine alkali halide rock salt structures demonstrates the importance of phonon eigenvectors in anharmonic analysis. A mode-matching algorithm is developed that allows for direct comparison of their anharmonicities, where materials with similar mode shapes are found to have similar mode anharmonicities. This suggests that eigenvector alignment relative to structural motifs plays a central role in determining the distribution and magnitude of phonon anharmonicity. Thus, materials design will greatly benefit from a combined consideration of atomic structure and bonding in tuning anharmonic properties of solids.