Comparative Analysis of the Crystal Structures and Physical Properties of the Complex Group 14 Selenides Sr 8 Ge 4 Se 17 and Ba 8 Sn 4 Se 17 .

During our group's continued exploration of group 14 chalcogenides, we discovered two new compounds, Sr 8 Ge 4 Se 17 and Ba 8 Sn 4 Se 17 . Both compounds have an 8:4:17 stoichiometric ratio but adopt different centrosymmetric crystal structures. Sr 8 Ge 4 Se 17 crystallizes in the triclinic P 1̅ space group with a = 11.8429(18) Å, b = 12.172(3) Å, c = 13.624(3) Å, α = 114.472(5) 0 , β = 97.396(5) 0 , γ = 107.040(5) 0 , and Z = 2. Ba 8 Sn 4 Se 17 crystallizes in the monoclinic C 2/ c space group with a = 47.286(3) Å, b = 12.6294(5) Å, c = 25.7303(15) Å, β = 104.585(5) 0 , and Z = 16. The unit cell of Ba 8 Sn 4 Se 17 is approximately eight times larger than the unit cell of Sr 8 Ge 4 Se 17 , which is a consequence of the differently aligned tetrahedra, resulting in a quadrupled a axis, unchanged b axis, and doubled c axis. The lattice parameters and atomic coordinates were finalized via Rietveld refinements on data collected at a high-energy synchrotron beamline. Both compounds are semiconductors with band gaps in the visible region. Sr 8 Ge 4 Se 17 and Ba 8 Sn 4 Se 17 have optical band gaps of 1.88 and 1.93 eV, respectively. Both compounds have remarkably low thermal conductivities owing to their low symmetries and large unit cells. The minimum experimental thermal conductivity values of Sr 8 Ge 4 Se 17 and Ba 8 Sn 4 Se 17 are 0.45 W m -1 K -1 at 321 K and 0.31 W m -1 K -1 at 324 K, respectively.