Discovery of Two Polymorphs of TiP₄ N₈ Synthesized from Binary Nitrides

TiP₄ N₈ was obtained from the binary nitrides TiN and P₃ N₅ upon addition of NH₄ F as a mineralizer at 8 GPa and 1400 °C. An intricate interplay of disorder and polymorphism was elucidated by in situ temperature-dependent single-crystal X-ray diffraction, STEM-HAADF, and the investigation of annealed samples. This revealed two polymorphs, which consist of dense networks of PN₄ tetrahedra (degree of condensation κ=0.5) and either augmented triangular TiN₇ prisms or triangular TiN₆ prisms for α- and β-TiP₄ N₈ , respectively. The structures of TiP₄ N₈ exhibit body-centered tetragonal (bct) framework topology. DFT calculations confirm the measured band gaps of α- and β-TiP₄ N₈ (1.6-1.8 eV) and predict the thermochemistry of the polymorphs in agreement with the experiments.