Discovery of Two Polymorphs of TiP 4 N 8 Synthesized from Binary Nitrides.

TiP 4 N 8 was obtained from the binary nitrides TiN and P 3 N 5 upon addition of NH 4 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 PN 4 tetrahedra (degree of condensation κ=0.5) and either augmented triangular TiN 7 prisms or triangular TiN 6 prisms for α- and β-TiP 4 N 8 , respectively. The structures of TiP 4 N 8 exhibit body-centered tetragonal (bct) framework topology. DFT calculations confirm the measured band gaps of α- and β-TiP 4 N 8 (1.6-1.8 eV) and predict the thermochemistry of the polymorphs in agreement with the experiments.