Cation Disorder and Bond Anharmonicity Optimize the Thermoelectric Properties in Kinetically Stabilized Rocksalt AgBiS2 Nanocrystals

High temperature rocksalt phases of AgBiS2 and AgBiS2-xSex (x = 0.05-0.1) have been kinetically stabilized at room temperature in nanocrytals (similar to 11 nm) by simple solution-based synthesis. Experimental evidence for this derives from variable temperature powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and electron diffraction analysis. The band gap of the AgBiS2 nanocrystals (similar to 1.0 eV) is blue-shifted by quantum confinement relative to that of the cubic bulk phase of AgBiS2. Moreover, systematic lower energy shift of the band gap in AgBiS2.xSex nanocrystals compared to pristine nanocrystalline AgBiS2 was observed with increasing Se concentration. Existence of fascinating order-disorder type transition in these nanocrystals was evidenced by temperature dependent electrical conductivity, thermopower, and heat capacity measurements. Disordered cation sublattice and nanoscale grain boundaries coupled with strong Bi-S bond anharmonicity allow effective phonon scattering, which leads to minimal lattice thermal conductivity of the nano crystalline AgBiS2.