C-type Nd(2)Se(3)

The title compound, neodymium sesquiselenide, is isotypic with the other known rare-earth metal(III) selenides M (2)Se(3) (M = La–Pr and Sm–Lu) with the cubic C-type structure. It adopts a cation-defective Th(3)P(4)-type arrangement with close to 8/9 of the unique neodymium-cation site occupied, leading to the composition Nd(2.667)Se(4) (Z = 4) or Nd(2)Se(3) (Z = 5.333), respectively. The Nd(3+) cations are thus surrounded by eight selenide anions, forming trigonal [NdSe(8)](13−) dodeca­hedra, whereas the Se(2−) anions exhibit a sixfold coordination, but due to the under-occupation of neodymium, each one is statistically surrounded by only 5.333 cations. The crystal studied was a merohedral twin with a 0.31 (6):0.69 (6) domain ratio.