Polymorphism in Zintl Phases ACd4Pn3: Modulated Structures of NaCd4Pn3 with Pn = P, As

NaCd4P3 and NaCd4As3 were synthesized via short-way transport using the corresponding elements and CdI2 as mineralizer. At room temperature, the two β-polymorphs adopt the RbCd4As3 structure type which has been recently reported for alkali metal (A)–d10 transition metal (T)–pnictides (Pn). The title compounds crystallize rhombohedrally in space group R3̅m at room temperature and show reversible phase transitions to incommensurately modulated α-polymorphs at lower temperatures. The low-temperature phases are monoclinic and can be described in space group Cm(α0γ)s with q vectors of q = (−0.04,0,0.34) for α-NaCd4P3 and q1 = (−0.02,0,0.34) for α-NaCd4As3. Thermal properties, Raman spectroscopy, and electronic structures have been determined. Both compounds are Zintl phases with band gaps of 1.05 eV for β-NaCd4P3 and ∼0.4 eV for β-NaCd4As3.