Polymorphism In Zintl Phases Acd(4)Pn(3): Modulated Structures Of Nacd(4)Pn(3) 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 beta-polymorphs adopt the RbCd4As3 structure type which has been recently reported for alkali metal (A)-d(10) transition metal (T)-pnictides (Pn). The title compounds crystallize rhombohedrally in space group R (3) over barm at room temperature and show reversible phase transitions to incommensurately modulated alpha-polymorphs at lower temperatures. The low-temperature phases are monoclinic and can be described in space group Cm(alpha 0 gamma)s with q vectors of q = (-0.04,0,0.34) for alpha-NaCd4P3 and q(1) = (-0.02,0,0.34) for alpha-NaCd4As3. Thermal properties, Raman spectroscopy, and electronic structures have been determined. Both compounds are Zintl phases with band gaps of 1.05 eV for beta-NaCd4P3 and similar to 0.4 eV for beta-NaCd4As3.