Structural studies on single crystals of Chevrel phase selenides REMo6Se8 (RE = La, Ce, Pr, Nd, or Sm) at 298 K.

We report on structural studies at room temperature of rare-earth based Chevrel phase selenides of the formula RExMo6Se8, where RE stands for a light rare-earth La (1), Ce (2), Pr (3), Nd (4), or Sm (5). The single crystals were grown at 1650 degrees C < T < 1690 degrees C from off-stoichiometric starting compositions, with the exception of 3, which was grown at 1710 degrees C from a stoichiometric charge (congruently melting material). The crystal structures were solved in space group R3 (No. 148; Z = 1) and found to be isostructural with the well-known Chevrel phases having large cations (e.g., PbMo6S8, REMo6S8). The structures are based on Mo6Se8 metallic clusters that are slightly rotated inside a pseudocubic rare-earth sublattice. Structural refinements revealed that the origin site, occupied by the RE atoms, exhibits slight deficiencies, leading to a RExMo6Se8 composition, with x ranging between approximately 0.82 and approximately 0.92: 1 La0.88Mo6Se8, arh = 6.7577(9) A, alpha rh = 88.62(2) degrees; 2a Ce0.82Mo6Se8, arh = 6.7407(6) A, alpha rh = 88.83(2) degrees; 2b Ce0.92Mo6Se8, arh = 6.7473(9) A, alpha rh = 88.69(2) degrees; 3 Pr0.86Mo6Se8, arh = 6.7385(6) A, alpha rh = 88.81(2) degrees; 4 Nd0.85Mo6Se8, arh = 6.7286(5) A, alpha rh = 88.85(1) degrees; and 5 Sm0.87Mo6Se8, arh = 6.7182(2) A, alpha rh = 88.956(3) degrees. All of the structural data presented in this work (lattice constants, positional parameters and interatomic distances) concern an average RE content of x approximately 0.87. In this way, any influence due to electronic effects (VEC number) can be discarded, and exact correlations between these parameters and the ionic radius of the rare-earth atoms can then be established.