Synthesis, Crystal Structure, and Properties of La₄Zn₇P₁₀ and La₄Mg_1.5Zn_8.5P₁₂

Two new zinc phosphides, La₄Zn₇P₁₀ and La₄Mg_1.5Zn_8.5P₁₂, were synthesized via transport reactions, and their crystal structures were determined by single crystal X-ray diffraction. La₄Zn₇P₁₀ and La₄Mg_1.5Zn_8.5P₁₂ 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 La₄Mg_1.5Zn_8.5P₁₂. 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 La₄Mg_1.5Zn_8.5P₁₂. 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 ∼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.