Ammonothermal Synthesis, Optical Properties, and DFT Calculations of Mg₂PN₃ and Zn₂PN₃

Abstract

The phosphorus nitrides, Mg₂ PN₃ and Zn₂ PN₃ , are wide band gap semiconductor materials with potential for application in (opto)electronics or photovoltaics. For the first time, both compounds were synthesized ammonothermally in custom-built high-temperature, high-pressure autoclaves starting from P₃ N₅ and the corresponding metals (Mg or Zn). Alkali amides (NaNH₂ , KNH₂ ) were employed as ammonobasic mineralizers to increase solubility of the starting materials in supercritical ammonia through formation of reactive intermediates. Single crystals of Mg₂ PN₃ , with length up to 30 μm, were synthesized at 1070 K and 140 MPa. Zn₂ PN₃ already decomposes at these conditions and was obtained as submicron-sized crystallites at 800 K and 200 MPa. Both compounds crystallize in a wurtzite-type superstructure in orthorhombic space group Cmc2₁ , which was confirmed by powder X-ray diffraction. In addition, single-crystal X-ray diffraction measurements of Mg₂ PN₃ were carried out for the first time. To our knowledge, this is the first single-crystal X-ray study of ternary nitrides synthesized by the ammonothermal method. The band gaps of both nitrides were estimated to be 5.0 eV for Mg₂ PN₃ and 3.7 eV for Zn₂ PN₃ by diffuse reflectance spectroscopy. DFT calculations were carried out to verify the experimental values. Furthermore, a dissolution experiment was conducted to obtain insights into the crystallization behavior of Mg₂ PN₃ .