Cationic lone-pair electrons are often associated with distinctive phonon properties (e.g., strong lattice anharmonicity) that lead to low lattice thermal conductivity (${\kappa }_{\mathrm{L}}$). However, the reliance on specific cations (e.g., ${\mathrm{Sb}}^{3+}$ or ${\mathrm{Ge}}^{2+}$) severely restricts the broader applicability of this lone-pair-based strategy. In this Letter, anionic lone-pair-like electrons (LPEs) were successfully introduced in the high-symmetry diamondoid compound ${\mathrm{AgInSnSe}}_4$ through cation-deficient cross-substitution, achieving an ultralow ${\kappa }_{\mathrm{L}}$ of $0.23\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ at 773 K. The LPEs of Se atoms significantly weaken the long-range Se-Se interactions and Ag-Se bonding. This effect localizes the optical phonon modes, resulting in ultralow group velocities and strong scattering rates among optical phonons, minimizing the particlelike propagation contribution to heat transport. This finding extends the concept of LPEs-driven phonon blocking from cations to anions, revealing a mechanism fundamentally distinct from the anharmonicity-driven suppression observed in cationic lone-pair systems.

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• Chemical bonding
• Lattice dynamics
• Thermal conductivity
• Thermal properties
• Thermoelectric effects