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

Two new compounds CdPbBi 4 Se 8 and CdSnBi 4 Se 8 adopt the pavonite structure type and crystallize in the monoclinic space group C 2/ m with a = 13.713(3) Å, b = 4.1665(8) Å, c = 15.228(3) Å, β = 115.56(3)° for CdPbBi 4 Se 8; a = 13.679 Å, b = 4.153 Å, c = 15.127 Å, β = 115.51° for CdSnBi 4 Se 8 . Their crystal structures are composed of two different types of polyhedral slabs, one containing a mixture of one octahedron [MSe 6 ] block and paired squared pyramids [MSe 5 ], while the other forms distorted galena-type (or NaCl-type) lattices with three [MSe 6 ] octahedral chains (M = Pb, Cd, Bi, Sn). Both CdPbBi 4 Se 8 and CdSnBi 4 Se 8 are stable up to ∼970 K. Density functional theory (DFT) calculations show that both CdPbBi 4 Se 8 and CdSnBi 4 Se 8 are indirect band gap semiconductors. DFT phonon dispersion calculations performed on CdSnBi 4 Se 8 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 CdPbBi 4 Se 8, and 0.40 for CdSnBi 4 Se 8 at 850 K.