High-Pressure Phase Transformations in TiPO4: A Route to Pentacoordinated Phosphorus

Modular Principle for Complex Disordered Tetrahedral Frameworks in Quenched High-Pressure Phases of Phosphorus Oxide Nitrides

The crystal structures of the new phosphorus oxide nitrides P₄₀ O₃₁ N₄₆ and P₇₄ O₅₉ N₈₄ , which were synthesized from amorphous phosphorus oxide nitride imide, exhibit complex frameworks built up from P(O,N)₄ tetrahedra. The latter form various chain-like building units with various degrees of branching. These modular units can be combined and arranged in different ways, which leads to closely related structures and several disordered configurations in each compound. As the material was obtained by high-pressure high-temperature synthesis, the disorder is most likely a consequence of quenching a high-pressure phase with P(O,N)₅ trigonal bipyramids. Under ambient conditions, P atoms are expected to relax by moving to the centers of the face-sharing tetrahedra that constitute the bipyramid. Diffraction patterns acquired with microfocused synchrotron radiation reveal that domains of both compounds are intergrown with H₃ P₈ O₈ N₉ , whose tetrahedral framework represents a cutout of the structures of both P₄₀ O₃₁ N₄₆ and P₇₄ O₅₉ N₈₄ . Powder diffraction patterns do not indicate any further phases.

Boron Phosphorus Nitride at Extremes: PN6 Octahedra in the High-Pressure Polymorph β-BP3 N6

The high-pressure behavior of non-metal nitrides is of special interest for inorganic and theoretical chemistry as well as materials science, as these compounds feature intriguing elastic properties. The double nitride α-BP₃ N₆ was investigated by in situ single-crystal X-ray diffraction (XRD) upon cold compression to a maximum pressure of about 42 GPa, and its isothermal bulk modulus at ambient conditions was determined to be 146(6) GPa. At maximum pressure the sample was laser-heated, which resulted in the formation of an unprecedented high-pressure polymorph, β-BP₃ N₆ . Its structure was elucidated by single-crystal XRD, and can be described as a decoration of a distorted hexagonal close packing of N with B in tetrahedral and P in octahedral voids. Hence, β-BP₃ N₆ is the first nitride to contain PN₆ octahedra, representing the much sought-after proof of principle for sixfold N-coordinated P that has been predicted for numerous high-pressure phases of nitrides.

Stishovite's Relative: A Post-Coesite Form of Phosphorus Oxonitride

Phosphorus oxonitride (PON) is isoelectronic with SiO₂ and may exhibit a similar broad spectrum of intriguing properties as silica. However, PON has only been sparsely investigated under high-pressure conditions and there has been no evidence on a PON polymorph with a coordination number of P greater than 4. Herein, we report a post-coesite (pc) PON polymorph exhibiting a stishovite-related structure with P in a (5+1) coordination. The pc-PON was synthesized using the multianvil technique and characterized by powder X-ray diffraction, solid-state NMR spectroscopy, TEM measurements and in situ synchrotron X-ray diffraction in diamond anvil cells. The structure model was verified by single-crystal X-ray diffraction at 1.8 GPa and the isothermal bulk modulus of pc-PON was determined to K₀ =163(2) GPa. Moreover, an orthorhombic PON polymorph (o-PON) was observed under high-pressure conditions and corroborated as the stable modification at pressures above 17 GPa by DFT calculations.