Transition-metal pnictides (group 15 compounds) are potentially interesting electronic materials, forming superconductors, thermoelectric compounds, and layered structures. As a part of a study to identify new solid ion conductors, Tom Nilges and colleagues, Technical University Munich, Germany, explored the isostructural Zintl phases (polyanionic compounds) NaCd₄P₃ and NaCd₄As₃.

The researchers synthesized the compounds from the elements under an argon atmosphere, with CdI₂ as a mineralizer. NaCd₄P₃ (pictured) forms platelike crystals with a metallic luster. This phosphide is a new member of the RbCdAs₃ structure family, and it undergoes a reversible rhombohedral-to-monoclinic phase transition at 260 K, driven by secondary attractive interactions within the Cd-P polyanionic substructure. The low-temperature (monoclinic) polymorph has an incommensurate modulated structure, which was more pronounced for the phosphide than for the arsenide.

The thermoelectric properties of the phosphide are typical of an n-type semiconductor, while the arsenide is a semimetal. Quantum chemical calculations determined a band gap of 1.05 eV for the β phase of NaCd₄P₃ and 0.43 eV for β-NaCd₄As₃.

• Polymorphism in Zintl Phases ACd₄Pn₃: Modulated Structures of NaCd₄Pn₃ with Pn = P, As,
  Carolin Grotz, Maximilian Baumgartner, Katharina M. Freitag, Franziska Baumer, Tom Nilges,
  Inorg. Chem. 2016.
  DOI: 10.1021/acs.inorgchem.6b01233