Twisted Kelvin Cells and Truncated Octahedral Cages in the Crystal Structures of Unconventional Clathrates, AM₂P₄ (A = Sr, Ba; M = Cu, Ni)

A new strontium nickel polyphosphide, SrNi₂P₄, was synthesized from elements and structurally characterized by single-crystal X-ray diffraction. It crystallizes in the orthorhombic space group Fddd (No. 70), with Z = 8. The crystal structure is that of a clathrate type, composed of Ni₈P₁₆, 14-faced polyhedral cages that encapsulate Sr atoms. Together with the previously reported but unrecognized clathrate VII, BaNi₂P₄, and another previously reported clathrate, BaCu₂P₄, which is isostructural to SrNi₂P₄, a family of transition metal–phosphorus clathrates is represented. The crystal structures of each of the discussed transition metal-based clathrates are composed of unique polyhedra containing square faces. These structural fragments were predicted to be unstable for the conventional clathrates based on Si, Ge, and Sn. In this work, we report the crystal and electronic structures, chemical bonding, as well as the thermoelectric properties of this novel class of unconventional clathrates.