Synthesis, Crystal Structure, and Properties of La 4 Zn 7 P 10 and La 4 Mg 1.5 Zn 8.5 P 12 .

Two new zinc phosphides, La4 Zn7 P10 and La4 Mg1.5 Zn8.5 P12 , were synthesized via transport reactions, and their crystal structures were determined by single crystal X-ray diffraction. La4 Zn7 P10 and La4 Mg1.5 Zn8.5 P12 are built from three-dimensional Zn-P and Zn-Mg-P anionic frameworks that encapsulate lanthanum atoms. The anionic framework of La4 Zn7 P10 is constructed from one-dimensional Zn4 P6 , Zn2 P4 , and ZnP4 chains. The Zn4 P6 chains are also the main building units in La4 Mg1.5 Zn8.5 P12 . In La4 Zn7 P10 , the displacement of a zinc atom from the origin of the unit cell causes the Zn4 position to split into two equivalent atomic sites, each with 50% occupancy. The splitting of the atomic position substantially modifies the electronic properties, as suggested by theoretical calculations. The necessity of splitting can be overcome by replacement of zinc with magnesium in La4 Mg1.5 Zn8.5 P12 . Investigation of the transport properties of a densified polycrystalline sample of La4 Zn7 P10 demonstrates that it is an n-type semiconductor with a small bandgap of ∼0.04 eV at 300 K. La4 Zn7 P10 also exhibits low thermal conductivity, 1.3 Wm-1 K-1 at 300 K, which mainly originates from the lattice thermal conductivity. La4 Zn7 P10 is stable in a sealed evacuated ampule up to 1123 K as revealed by differential scanning calorimetry.