Synthesis, crystal structure, electronic structure and electrical conductivity of La3GeSb0.31Se7 and La3SnFe0.61Se7

• Two new lanthanum selenides are introduced.
• These selenides are isostructural, crystallizing in a polar structure type.
• La3GeSb0.31Se7 is a red and La3SnFe0.61Se7 a black semiconductor.
• The Fe-containing one exhibits the smaller band gap because of the d-orbitals of Fe.

The selenides La3EM1−xSe7 (La6E2M2−xSe14) adopt the Ce6Al3.33S14 structure type. La3GeSb0.31Se7 and La3SnFe0.61Se7 crystallize in the non-centrosymmetric space group P63 with La replacing Ce in the 6c site, E = Ge or Sn replacing Al in the 2b site and M = Fe or Sb replacing the other, deficient Al site (2a). The structure contains La atoms in square antiprisms of Se atoms, isolated distorted [ESe4] tetrahedra, and face sharing distorted [MSe6] octahedra forming a linear chain along the c-axis with short M–M distances. Band structure calculations predict semiconducting character with different gaps, which was demonstrated by electrical conductivity measurements and reflected in their different colors.

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