Hierarchical Bonding Induced Promising Thermoelectric Performance in Monoclinic Cs2SnSe3

Thermoelectric materials capable of directly converting heat into electricity are of great interest for sustainable energy utilization and waste-heat recovery. Here, we systematically investigate the structural, vibrational, thermal transport, and electronic transport properties of Cs2SnSe3 by combining first-principles calculations, self-consistent phonon theory, machine-learning interatomic potentials, and Boltzmann transport analysis. Our results show that Cs2SnSe3 crystallizes in a monoclinic C2/m structure with hierarchical bonding characteristics, in which ionic Cs-Se interactions coexist with covalent Sn-Se bonds. This bonding heterogeneity, together with the presence of heavy constituent atoms and weakly bonded structural units, gives rise to strong lattice anharmonicity. We find that Cs2SnSe3 is dynamically and mechanically stable, while its renormalized phonon spectrum exhibits anomalous temperature dependence, namely the hardening of low-frequency modes and the softening of high-frequency modes with increasing temperature. The low Debye temperature and large atomic mean-square displacements further confirm the soft and strongly anharmonic lattice. As a consequence, Cs2SnSe3 exhibits intrinsically ultralow and anisotropic lattice thermal conductivity, with values below 0.25 W m-1 K-1 at 300 K along all crystallographic directions. Furthermore, four-phonon scattering plays an important role in suppressing thermal transport, especially in the low-frequency region below 2 THz. Electronic structure calculations reveal that Cs2SnSe3 is a semiconductor with an Heyd-Scuseria-Ernzerhof (HSE) band gap of 3.09 eV. Carrier transport calculations further show that the thermoelectric performance strongly depends on carrier type, carrier concentration, and temperature, with p-type doping being much more favorable than n-type doping. Theoretical calculations predict that with p-type doping, the ZT value can reach a maximum of approximately 1.1 at 500 K.