Synthesis, crystal and electronic structure of the Zintl phase Ba16Sb11. A case study uncovering greater structural complexity via monoclinic distortion of the tetragonal Ca16Sb11 structure type

The binary Zintl phase Ba16Sb11 has been synthesized and structurally characterized. Detailed studies via single‐crystal X‐ray diffraction methods indicate that although Ba16Sb11 appears to crystallize in the tetragonal Ca16Sb11 structure type (space group P4‾${\overline 4 }$21m with a=13.5647(9) Å, c=12.4124(12) Å, Z=2, R1=3.14 %; wR2=4.77 %), there exists an extensive structural disorder. Some Ba16Sb11 crystals were found to be monoclinic and the structure was solved and refined in space group P21 (a=18.3929(12) Å, b=13.5233(8) Å, c=18.3978(12) Å, β=94.6600(10)°; Z=4, R1=5.84 %; wR2=9.58 %). The latter corresponds to a 2‐fold superstructure of the tetragonal one, which provides a disorder‐free structural model. In both descriptions, the disordered tetragonal and the ordered monoclinic superstructure, the basic building units that make up the structure of this Ba‐rich compound are pairs of face‐shared square antiprisms of Ba atoms, which are centered by Sb atoms. The dimerized antiprisms are linked into parallel chains via square prisms of Ba atoms, which are also centered by Sb atoms. The Zintl concept can be applied in a straightforward manner and as result, the structure of Ba32Sb22 (=2×Ba16Sb11) can be rationalized as (Ba2+)32(Sb3−)20[Sb2]4−. The partitioning of the valence electrons is done taking into an account the homoatomic Sb−Sb contacts (d=3.01 Å), which can be clearly distinguished in the lower symmetry space group. Electronic structure calculations of Ba16Sb11 are in good accordance with the Zintl rationalization and predict a semiconductor with a band gap of 0.77 eV.In appreciation of the scientific oeuvre of Eduard Zintl on the occasion of his 125th birthday.