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Discovery of Two Polymorphs of TiP₄ N₈ Synthesized from Binary Nitrides.

ABSTRACT: 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.

SUBMITTER: Eisenburger L

PROVIDER: S-EPMC9310718 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Here we present a detailed ab initio study of two experimentally synthesized bismuth niobate BiNbO4 (BNO) polymorphs within the framework of density functional theory (DFT). We synthesized orthorhombic α-BNO and triclinic β-BNO using a solid-state reaction technique. The underlying Pnna and P1̄ crystal symmetries along with their respective phase purity have been confirmed from Rietveld refinement of the powdered X-ray diffraction measurements in combination with generalized gradient approximation of Perdew-Burke-Ernzerhof (GGA-PBE) based DFT simulations. The scanning electron micrographs revealed average grain sizes to be 500 nm and 1 μm for α-BNO and β-BNO respectively. The energy-dispersive X-ray spectroscopy identified the Bi, Nb, and O with proper stoichiometry. The phase purity of the as-synthesized samples was further confirmed by comparing the local density approximation (LDA) norm-conserving pseudo-potential based DFT-simulated Raman peaks with that of experimentally measured ones. The relevant bond vibrations detected in Fourier transform infrared spectroscopy were matched with GGA-PBE derived phonon density of states simulation for both polymorphs. The structural stability and the charge dynamics of the polymorphs were verified from elastic stress and born charge tensor simulations respectively. The dynamical stability of the α-BNO was confirmed from phonon band structure simulation using density functional perturbation theory with Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional. The electronic band gaps of 3.08 and 3.36 eV for α-BNO and β-BNO measured from UV-Vis diffuse reflectance measurements were matched with the sophisticated HSE06 band structure simulation by adjusting the Hartree-Fock exchange parameter. Both GGA-PBE and HSE06 functional were used to simulate complex dielectric function and its derivatives with the help of Fermi's golden rule to define the optical properties in the linear regime. All these may have provided a rigorous theoretical analysis for the experimentally synthesized α-BNO and β-BNO polymorphs.

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Discovery of Two Polymorphs of TiP₄ N₈ Synthesized from Binary Nitrides.

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Discovery of Two Polymorphs of TiP₄ N₈ Synthesized from Binary Nitrides.