Structure and thermoelectric properties of the silver lead bismuth selenides Ag5Pb9Bi19Se40 and AgPb3Bi7Se14

In the system Ag/Pb/Bi/Se, two new thermoelectric phases derived from lillianite (Pb3Bi2S6) have been characterized. The crystal structures correspond to the L-8,L-8- and L-5,L-5-types, respectively, both in the space group Cmcm. The room-temperature unit-cell parameters of L-8,L-8-Ag5Pb9Bi19Se40 are a = 4.2151(8) angstrom, b = 13.951(3) angstrom and c = 35.284(7) angstrom and those of L-5,L-5-AgPb3Bi7Se14 are a = 4.2337(8) angstrom, b = 13.864(2) angstrom and c = 24.653(3) angstrom. The temperature-dependent evolution of the lattice parameters of Ag5Pb9Bi19Se40 becomes steeper at temperatures above 300 degrees C and hints at the mobility of Ag+ ions. Samples containing both phases exhibit thermoelectric figures of merit up to ZT = 0.23 at 250 degrees C. HRTEM investigations on such samples showed well-ordered areas of the lillianite-like phases separated by large slabs that exhibit a high concentration of defects and may compensate for lattice misfits between the lillianite-type phases.