The lattice thermal conductivity of CsX (X = Cl, Br, and I) and its pressure dependence are investigated using first-principles third-order anharmonic force constants. Contrary to the expectation that compounds with heavier atoms usually exhibit lower lattice thermal conductivity ( k L ), the k L of CsI is higher than those of CsCl and CsBr. This anomalous behavior is examined by analyzing the group velocity, phonon lifetime, three-phonon scattering phase space and Grüneisen parameters. The higher k L of CsI can be attributed to its longer phonon lifetimes due to weaker absorption processes in the range of 1 ∼ 2.1 THz. It is found that the lattice thermal conductivity of CsI is more sensitive to hydrostatic pressure, and the k L of CsI becomes lower than those of CsCl and CsBr at -2 GPa due to the shorter phonon lifetimes and the smaller group velocities. Moreover, the changes in the bulk modulus and Bader charge of CsX are also discussed to provide further insight into its anomalous thermal behavior.
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