Coordination engineering of thermal transport in AB compounds: Insights from polymorphic CsBr

The atomic-level coordination environment plays a decisive role in governing thermal transport; however, a unified framework linking coordination character to thermal transport remains lacking in binary AB compounds. Here, based on a dimensionless parameter b=fᵢ+rA/rB, combining bond ionicity and the cation--anion radius ratio, we classify 18 prototypical AB compounds into tetrahedral, octahedral, and cubic coordination configurations. Adopting polymorphic CsBr as a platform, we further elucidate the intrinsic effect of coordination environment on thermal transport and reveal that, because of stronger phonon anharmonicity, the cubic phase consistently exhibits lower thermal transport than its octahedral counterpart. Specifically, we ascribe this enhanced anharmonicity in the cubic phase to its dominant quartic anharmonicity, which stiffens phonons and thus expands the scattering phase space with rising temperature, whereas the octahedral phase, where cubic anharmonicity is slightly stronger than quartic anharmonicity, exhibits slight phonon softening and a weaker temperature dependence of the scattering rates. Anomalously, in sharp contrast to the conventional low-frequency peak observed in the cubic phase, the principal spectral peak of the particle-like thermal conductivity in the octahedral phase originates from the highest-frequency transverse acoustic modes, which exhibit a nearly harmonic character. These findings establish a direct link from coordination geometry to thermal transport, paving the way for lattice thermal conductivity modulation through coordination engineering.