Crystal chemical peculiarities of rare earth (R) rich magnesium intermetallic compounds in R–T–Mg (T = transition element) systems

Chem. Met. Alloys 2 (2009) 39-48

https://doi.org/10.30970/cma2.0088

Pavlo SOLOKHA, Serena DE NEGRI, Volodymyr PAVLYUK, Adriana SACCONE

The rare earth rich compounds R₂₃Ni₇Mg₄ (R = Ce, Pr) and R₄NiMg (R = La, Tb, Dy, Ho, Er) were synthesized by slow thermal treatment in a resistance furnace, starting from element pieces enclosed in sealed Ta crucibles. SEM-EDXS analyses were performed for monitoring the phase composition. The crystal structures of the majority of these intermetallics were investigated by X-ray diffraction on single crystals. The R₂₃Ni₇Mg₄ compounds crystallize in the Pr₂₃Ir₇Mg₄ structure type: P6₃mc, hP68, Z = 2, for Ce a = 0.99742(1), c = 2.25206(4) nm, wR₂ = 0.0686, 1483 F² values, 74 variables; for Pr a = 0.99249(4), c = 2.24460(9)  nm, wR₂ = 0.0735, 1395 F² values, 74 variables; for La a = 1.01234(8), c = 2.2887(2) nm, R_B = 7.3% and R_F = 5.1%. The series of isostructural R₄NiMg compounds belongs to the Gd₄RhIn structure type, F4¯3m, cF96, Z = 16. Crystal chemical relations of R₂₃Ni₇Mg₄ with other rare earth rich intermetallics are discussed. The cubic R₄NiMg structures are described in terms of nested polyhedra units and relations with some other cubic structures are shown on the basis of group-subgroup relations.

Unit cell projections of the Ce₂₃Ni₇Mg₄ structure. The coordination polyhedra of the atoms are shown; symmetries of the central atoms are indicated in parentheses.

Keywords

Crystal structure / Group-subgroup relation / Mg-containing alloys