Comprehending lattice dynamics and thermal conductivity is crucial for energy conversion devices. This work examines thermal transport in $\mathrm{CaSn}{\mathrm{S}}_3$, a member of the chalcogen perovskite family, which is increasingly favored in energy applications. The temperature-dependent phonon frequency renormalization including loop and bubble diagram framework, in conjunction with higher-order four-phonon scattering and the wavelike contribution of phonon transport, predicts an ultralow lattice thermal conductivity (${\kappa }_l)$ of $\sim 0.75\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ at 300 K and ∼0.63 $\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ at 700 K. Weak bonding and bond inhomogeneity are significant structural characteristics that introduce anharmonicity to the system, with an estimated average Grüneisen parameter of ∼1.92. Strongly anharmonic phonon branches below 60 $\mathrm{c}{\mathrm{m}}^{-1}$ determine the weightage of particle and wavelike phonon characteristics. At room temperature, the particlelike propagation ${\kappa }_l^{\mathrm{p}}$ is $\sim 0.4 \mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$, while the estimated wavelike coherent contribution ${\kappa }_l^{\mathrm{c}}$ is $\sim 0.35 \mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$. At 300 K, the wavelike component accounts for $\sim 46\%$ of the total ${\kappa }_l$, rising to 56% at 700 K, underscoring the impact of vibrational state coupling and interbranch phonon tunneling in $\mathrm{CaSn}{\mathrm{S}}_3$, also comparable to glasses. The high density of closely spaced phonon branches makes it favorable for wavelike tunneling by providing a larger phonon linewidth than the interbranch spacing. Hence, guided by strong anharmonicity in $\mathrm{CaSn}{\mathrm{S}}_3$, the interplay of higher-order anharmonic self-energies and wavelike tunneling pathways results in a minimal temperature dependence $\sim T^{-0.20}$. This work not only quantifies lattice thermal conductivity but also highlights the potential discrepancies while accounting for three-phonon scattering only and neglecting bubble correction in phonon self-energy, emphasizing the necessity of adopting the right approach. Overall, while emphasizing lattice anharmonicity and glasslike thermal conduction, this study will help to improve the understanding of chalcogen perovskite design for multiple technological applications.

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• Thermoelectric generators
• Thermoelectrics
• 3-dimensional systems
• Dichalcogenides