Heteroanionic materials are inorganic compounds consisting of more than one type of anion. Layered heteroanionic materials with homoleptic polyhedra are particularly interesting. The interfacial interactions between these adjacent layers can lead to large lattice anharmonicity and consequently can promote ultralow intrinsic lattice thermal conductivity. As an analog of well-known ultralow thermal conductive BiCuOSe, BiAgOSe has been predicted to be a better ultralow thermal conductive material by DFT calculations. Utilizing thermodynamic calculations and Pourbaix diagrams, we discovered a high yield hydrothermal synthesis to make the BiAgOSe phase. The thermal conductivity of BiAgOSe, as predicted, is lower compared to BiCuOSe. To study the origin of the ultralow thermal conductivity of BiAgOSe, we combined lattice dynamic calculations and synchrotron X-ray diffraction measurements and found that the low lattice thermal conductivity results from both low phonon velocities and strong lattice anharmonicity which are associated with the anisotropic layered structure, the large distortion of AgSe<sub>4</sub> tetrahedra and soft Ag−Se bonds.

Exploring hydrothermal synthesis of ultralow thermal conductive heteroanionic materials

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