With the discovery of La12+xSb9-yS38-z, for simplicity called La12Sb9S38, we characterized the first lanthanum antimony polysulfide. Despite La/Sb mixed occupancies and S atom deficiencies, this material exhibits only a small...

The new lanthanum chalcoantimonate TlLa2Sb3Se9 has been synthesized and its crystal structure determined. TlLa2Sb3Se9 crystallizes in an ordered variant of the KLa2Sb3S9 type, space group P212121 with the lattice parameters a=4.2621(2) Å, b=15.155(5) Å, c=25.505(9) Å. The band gap of TlLa2Sb3Se9 was calculated to be 0.47 eV, and experimentally determined to be 0.68 eV. Its thermoelectric properties were optimized via doping with Ca2+; samples with the compositions TlLa2‐xCaxSb3Se9 (x=0.01, 0.03, 0.05) were synthesized. Despite ultralow thermal conductivity, the maximum thermoelectric figure‐of‐merit of the undoped sample was only zT=0.031 at 623 K, which was increased to 0.078 for the sample with the nominal composition of TlLa1.95Ca0.05Sb3Se9. These low values are a consequence of the uncompetitively low electrical conductivity.