Lattice thermal conductivity below Cahill limit: Negative thermal expansion in Pr doped BiCuSeO

Fundamental understanding of lattice softening and phonon transport in crystalline solids is crucial for thermoelectric applications, since the design of thermally insulating solids has gained significant interest over the years. In this work, we demonstrate strain induced lattice softening through negative thermal expansion (NTE) in Pr doped BiCuSeO, which breaks the theoretical Cahill limit, κlmin ∼ 0.55 W m−1 K−1, achieving a low κl of 0.24 W m−1 K−1 at 740 K. The Differential Scanning Calorimetry (DSC) and Raman analyses experimentally confirm NTE-driven lattice softening, which influences glass-like thermal transport beyond the theoretical limit. Meanwhile, the negative Gruneisen parameter (−1.13) calculated from reduced average sound velocity, νs∼1870 ms−1, with shrinkage of cell volume illustrates the NTE behavior. Furthermore, the observed weak carrier–phonon coupling μW/κl indicates the absence of conventional carrier scattering through NTE, which leads to a 54% increment in zT compared to undoped BiCuSeO (0.5 at 740 K) for Bi0.97Pr0.03CuSeO.