LiPr₂P₄N₇O₃: Structural Diversity of Oxonitridophosphates Accessed by High-Pressure Metathesis

The structural diversity of tetrahedra networks of phosphates can greatly be enhanced by introduction of mixed N/O anion positions. LiPr₂P₄N₇O₃ exemplifies the benefits of N/O mixed anion positions as it is the first rare-earth (oxo)nitridophosphate with a single-layered structure and a degree of condensation (atomic ratio of tetrahedra centers (P) to tetrahedra corners (N/O atoms)) of 2/5. The compound was prepared through high-pressure metathesis starting from PrF₃, LiPN₂, Li₂O, and PON using a hydraulic 1000t press and the multianvil technique. LiPr₂P₄N₇O₃ crystallizes as pale-green single-crystals, from which its structure was determined (space group P2₁/ c (no. 14), a = 4.927(1), b = 7.848(2), c = 10.122(2) Å, β = 91.55(3)°, Z = 2, R₁ = 0.020, wR₂ = 0.045). The structure consists of single-layers of vertex-sharing Q³-type P(N/O)₄ tetrahedra forming four- and eight-membered rings arranged in the fashion of the Archimedean fes net. UV-vis spectroscopy revealed the typical Pr³⁺ f -f transitions, leading to a pale-green color of the crystals. Moreover, the optical band gap was determined to 4.1(1) eV, assuming a direct transition. High-temperature powder X-ray diffraction showed the beginning of a gradual decomposition starting at ca. 500 °C.