Ternary rare-earth selenides with the U₃ScS₆ structure type: Synthesis, characterization, and some magnetic properties of Ln₃TSe₆ (Ln = Sm, Gd; T = In, Cr) and Tb₃CrSe₆

The new compounds Ln₃TSe₆ (Ln = Sm, Gd; T = In, Cr) and Tb₃CrSe₆ have been synthesized by the solid-state reactions of the elements at 850 °C. A KBr flux was used to promote crystal growth. These isostructural compounds crystallize with the U₃ScS₆ structure type. The crystal structure is built from LnSe₇ pseudo-octahedra or LnSe₈ bicapped trigonal prisms and TSe₆ octahedra. Magnetic measurements show that Sm₃TSe₆ (T = In, Cr) and Tb₃CrSe₆ are paramagnetic down to 5 K whereas Gd₃CrSe₆ undergoes an antiferromagnetic transition at 10 K. Crystal data: orthorhombic, Pnnm, Z = 4, T = -120 °C, (compound, a (Å), b (Å), c (Å))Sm₃InSe₆, 14, 177(3), 17.352(4), 4.0625(8); Gd₃InSe₆, 14.071(3), 17.286(4), 4.0202(8); Sm₃CrSe₆, 14.032(3), 16.782(3), 3.9841(8); Gd₃CrSe₆, 13.938(3), 16.780(3), 3.9559(8); Tb₃CrSe₆, 13.885(3), 16.672(3), 3.9215(8).