Semiconducting Pavonites CdMBi4Se8 (M = Sn and Pb) and Their Thermoelectric Properties

Two new compounds CdPbBi4Se8 and CdSnBi4Se8 adopt the pavonite structure type and crystallize in the monoclinic space group C2/m with a = 13.713(3) Å, b = 4.1665(8) Å, c = 15.228(3) Å, β = 115.56(3)° for CdPbBi4Se8; a = 13.679 Å, b = 4.153 Å, c = 15.127 Å, β = 115.51° for CdSnBi4Se8. Their crystal structures are composed of two different types of polyhedral slabs, one containing a mixture of one octahedron [MSe6] block and paired squared pyramids [MSe5], while the other forms distorted galena-type (or NaCl-type) lattices with three [MSe6] octahedral chains (M = Pb, Cd, Bi, Sn). Both CdPbBi4Se8 and CdSnBi4Se8 are stable up to ∼970 K. Density functional theory (DFT) calculations show that both CdPbBi4Se8 and CdSnBi4Se8 are indirect band gap semiconductors. DFT phonon dispersion calculations performed on CdSnBi4Se8 give valuable insights as to the origin of the observed low experimental lattice thermal conductivities of ∼0.58 W m–1 K–1 at 320 K. The title compounds exhibit n-type conduction, and they exhibit promising thermoelectric properties with a maximum thermoelectric figure of merit, ZT, reaching 0.63 for CdPbBi4Se8, and 0.40 for CdSnBi4Se8 at 850 K.