Understanding the underlying mechanisms that suppress thermal conduction in solids is of paramount importance for the targeted design of materials for thermal management and thermoelectric energy conversion applications. Bismuth copper oxychalcogenides, BiOCuQ (Q = Se, Te), are highly crystalline thermoelectric materials with an unusually low lattice thermal conductivity of ∼0.5 Wm(-1) K(-1), a value normally found in amorphous materials. Here we unveil the origin of the unusual thermal transport properties of these phases. First principles calculations of the vibrational properties combined with analysis of in-situ neutron diffraction data, demonstrate that weak bonding of copper atoms within the structure leads to an unexpected vibrational mode at low frequencies, which is likely to be a major contributor to the low thermal conductivity of these materials. In addition, we show that anharmonicity and the large Grüneisen parameter in these oxychalcogenides are mainly related to the low frequency copper vibrations, rather than to the Bi(3+) lone pairs.

a b s t r a c tBi 2 O 2 Te was synthesised from a stoichiometric mixture of Bi, Bi 2 O 3 and Te by a solid state reaction. Analysis of powder X-ray diffraction data indicates that this material crystallises in the anti-ThCr 2 Si 2 structure type (space group I4/mmm), with lattice parameters a¼ 3.98025(4) and c¼ 12.70391(16) Å. The electrical and thermal transport properties of Bi 2 O 2 Te were investigated as a function of temperature over the temperature range 300 rT (K)r 665. These measurements indicate that Bi 2 O 2 Te is an n-type semiconductor, with a band gap of 0.23 eV. The thermal conductivity of Bi 2 O 2 Te is remarkably low for a crystalline material, with a value of only 0.91 W m À 1 K À 1 at room temperature.