Two new zinc phosphides, La4Zn7P10 and La4Mg1.5Zn8.5P12, were synthesized via transport reactions, and their crystal structures were determined by single crystal X-ray diffraction. La4Zn7P10 and La4Mg1.5Zn8.5P12 are built from three-dimensional Zn-P and Zn-Mg-P anionic frameworks that encapsulate lanthanum atoms. The anionic framework of La4Zn7P10 is constructed from one-dimensional Zn4P6, Zn2P4, and ZnP4 chains. The Zn4P6 chains are also the main building units in La4Mg1.5Zn8.5P12. In La4Zn7P10, 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 La4Mg1.5Zn8.5P12. Investigation of the transport properties of a densified polycrystalline sample of La4Zn7P10 demonstrates that it is an n-type semiconductor with a small bandgap of similar to 0.04 eV at 300 K. La4Zn7P10 also exhibits low thermal conductivity, 1.3 Wm(-1) K-1 at 300 K, which mainly originates from the lattice thermal conductivity. La4Zn7P10 is stable in a sealed evacuated ampule up to 1123 K as revealed by differential scanning calorimetry.