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.