Unlocking Ultralow Thermal Conductivity in α‐CuTeI via Specific Symmetry Breaking in Cu Sublattice.

Lattice softening is an intricate mechanism utilized to modulate lattice thermal conductivity (κ<italic>lat</italic>). However, experimental observations are often complicated by numerous factors including thermal regimes, elemental matrices, and crystalline topographies, making the fundamental mechanisms complex. In this study, the temperature gradients are meticulously harnessed during phase transitions, both in heating and cooling trajectories, to ascertain that atomic configuration acts as the paramount factor modulating phonon propagation. Within CuTeI, the predilection between the tetragonal (β) and orthorhombic (α) phases is deftly manipulated via specific thermal pathways to a juncture of 273 K. A salient 44% variance in κ<italic>lat</italic> is observed consequent to a singular alteration in the structural disposition of the bridging Cu atoms. Such atomic configurations delineate pronounced differential effects on the transmission dynamics of transverse and longitudinal phonons. The theoretical analysis indicates that the transverse acoustic velocity plays a more pivotal role in dictating κ<italic>lat</italic> than its longitudinal counterpart due to its greater contribution to the Grüneisen parameter. The synergistic interplay of lattice softening and anharmonicity enhancement culminates in an exceptionally diminished κ<italic>lat</italic> in α‐CuTeI, registering a record low κ<italic>lat</italic> of 0.21 W/(m × K) among inorganic materials dominated by phonon–phonon scattering at 273 K. The revelations proffer avant‐garde perspectives for the nuanced modulation of phonon velocities and κ<italic>lat</italic>. [ABSTRACT FROM AUTHOR]