Mg_5.23Sm_0.77Sb₄: an ordered superstructure derived from the Mg₃Sb₂ structure type

Abstract

The ternary polar intermetallic phase Mg_5.231(8)Sm_0.769(8)Sb₄ has been obtained from solid-state reactions at 700-850 °C in sealed Ta or Nb containers when the synthetic conditions took into account its characteristic incongruent melting point. The compound crystallizes in the trigonal space group P₃ (Z = 1) with a = 4.618(1) Å and c = 14.902(6) Å in a structure that derives from that of Mg₃Sb₂ (anti-La₂O₃ type). This composition appears to be near the lower limit of Sm content, and solutions with appreciably higher Sm contents are also stable [Mg_6−xSm_xSb₄, x ≤ 1]. The result provides the first example of a superstructure of a Mg₃Sb₂-like structure with a doubled c axis induced by ordering a mixture of the larger divalent Sm and Mg ions separately within alternate layers of octahedral sites. Still larger proportions of Sm also give rise to a second solid solution region in the parent Mg₃Sb₂ type structure (P₃m1), Mg_3−ySm_ySb₄, 0 ≤ y ≤ 1. Retention of the same 3e⁻ valence electron counts per antimony in all of these phases suggests that the compounds remain electron precise and Zintl phases. Analogous compounds with Ca, Sr, or Ba substitution have evidently not been investigated.