M₂PO₃N (M = Ca, Sr): <i>ortho</i>‑Oxonitridophosphates with β‑K₂SO₄ Structure Type

Two novel oxonitridophosphates M₂PO₃N with M = Ca and Sr were synthesized under high-pressure high-temperature (7 GPa and 1100 °C) using the multianvil technique or by solid-state reaction in the silica ampules (1100 °C) from amorphous phosphorus oxonitride (PON) and the respective alkaline earth oxides MO (M = Ca, Sr). The products represent the first examples of alkaline earth <i>ortho</i>-oxonitridophosphates containing noncondensed [PO₃N]^4– ions. The crystal structures were elucidated by single-crystal X-ray diffraction. Sr₂PO₃N [space group <i>Pnma</i> (No. 62), <i>Z</i> = 4, <i>a</i> = 7.1519(5) Å, <i>b</i> = 5.5778(3) Å, <i>c</i> = 9.8132(7) Å, R1 = 0.020, wR2 = 0.047] crystallizes in the β-K₂SO₄ structure type. The structure of Ca₂PO₃N was solved and refined in the (3 + 1)­D superspace group <i>Pnma</i>(α00)­0<i>ss</i> [<i>Z</i> = 4, <i>a</i> = 6.7942(7) Å, <i>b</i> = 5.4392(6) Å, <i>c</i> = 9.4158(11) Å, R1 = 0.041, wR2 = 0.067]. It exhibits an incommensurate modulation along [100] with a modulation vector <i>q</i> = [0.287(5), 0, 0]. Rietveld refinements support the structural models as well as the phase purity of the products. Upon doping with Eu²⁺, Ca₂PO₃N exhibits luminescence in the green range (λ_em = 525 nm) of the visible spectrum if excited by near-UV light (λ_exc = 400 nm).