Achieving Exceptionally Enhanced Thermal Conductivity and Bulk Modulus in Polar Insulators Via Modification of Chemical Bonding.

Materials capable of altering their physical properties in response to external stimuli are highly desirable for a wide range of applications. In particular, materials that exhibit substantial changes in thermal conductivity hold promise for advanced thermal management systems including thermal diodes, rectifiers, and switches. Despite significant interest, achieving substantial tunability in thermal transport has remained a challenge, with current approaches, primarily based on phase change materials, typically limited to ∼ 4× changes in thermal conductivity. Here, by employing first-principles-based atomistic simulations, we demonstrate that pressure can be used to modulate the thermal conductivity of lithium halides by up to 2 orders of magnitude, enabling a transition from a thermal insulator to an efficient heat conductor within the same material system. This exceptional tunability arises from pressure-induced changes in their chemical bonding, inducing a transition from predominantly ionic to more covalent character, and their polar nature, characterized by a significant reduction in the longitudinal-optical and transverse-optical phonon mode splitting at the Brillouin zone center. These changes lead to unique anharmonic phonon scattering behavior, which underlies the extraordinary thermal transport modulation. Moreover, our results also show exceptional enhancements in the bulk modulus by as much as 15-fold at pressures near 90 GPa for the lithium halides. Our findings highlight the strong coupling between chemical bonding and vibrational dynamics in polar insulators and open new pathways for designing stimuli-responsive materials with highly tunable macroscopic properties. These insights hold potential for a wide array of applications, including next-generation biomedical devices, sensors, and thermal circuits.