KCu₇P₃: A Two-Dimensional Noncentrosymmetric Metallic Pnictide

We report a 2D material, KCu₇P₃, with a noncentrosymmetric structure (trigonal space group P31m, a = 6.9637(2) Å, c = 24.1338 (10) Å), which forms both from a molten potassium polyphosphide flux and from the elements. This phase consists of infinite [Cu₇P₃]^- layers with hexagonal P sheets separated by K⁺ ions. The structure of the layers is unique but related to both Cu₃P and the CaCu₄P₂ structure-types. Single-crystal refinement reveals extensive disorder within the Cu₃P-like slabs. KCu₇P₃ is paramagnetic and exhibits a room temperature resistivity of ∼335 μω cm with a metal-like temperature dependence. The metallic character is supported by density functional theory electronic structure calculations. Hall and Seebeck effect measurements yield p-type behavior with a hole mobility of ∼15 cm² V^-1 s^-1 at 300 K and a carrier concentration on the order of 10²¹ cm^-3. KCu₇P₃ is chemically stable in ambient conditions, as well as in aqueous neutral and acidic solutions.