Filling-Fraction Fluctuation Leading to Glasslike Ultralow Thermal Conductivity in Caged Skutterudites

It is generally believed that filling atoms randomly and uniformly distribute in caged crystals, such as skutterudite compounds. Here, we report first-principles and experimental discovery of a multiscale filling-fraction fluctuation in the $R{\mathrm{Fe}}_4{\mathrm{Sb}}_{12}$ system. ${\mathrm{La}}_{0.8}{\mathrm{Ti}}_{0.1}{\mathrm{Ga}}_{0.1}{\mathrm{Fe}}_4{\mathrm{Sb}}_{12}$ spontaneously separates into La-rich and La-poor skutterudite phases, leading to multiscale strain field fluctuations. As a result, glasslike ultralow lattice thermal conductivity approaching the theoretical minimum is achieved, mainly due to strain field scattering of high-energy phonons. These findings reveal that an uneven distribution of filling atoms is efficient to further reduce the lattice thermal conductivity of caged crystals.