High-pressure polymorph of phosphorus nitride imide HP₄N₇ representing a new framework topology.

A new polymorph of phosphorus nitride imide HP4N7 has been synthesized under high-pressure/high-temperature conditions from P3N5 and NH4Cl at 6 GPa and temperatures between 800 and 1300 °C. Its crystal structure was elucidated using single-crystal X-ray diffraction data. β-HP4N7 (space group C2/c, no. 15, Z = 4, a = 12.873(2) Å, b = 4.6587(4) Å, c = 8.3222(8) Å, β = 102.351(3)°, R1 = 0.0485, wR2 = 0.1083) crystallizes in a new framework structure type that is made up of all-side vertex-sharing PN4 tetrahedra. The topology of the network is represented by the point symbol (3(2).4(2).5(2).6(3).7)(3(4).4(4).5(4).6(3)), and it has not been identified in other compounds so far. Structural differences between the two polymorphs of HP4N7 as well as the topological relationship to the recently discovered high-pressure polymorph β-HPN2 are discussed. Additionally, FTIR and solid-state NMR spectroscopy are used to corroborate the results of the structure determination.