Two novel oxonitridophosphates M2PO3N with M = Ca and Sr were synthesized under high-pressure high-temperature (7 GPa and 1100 degrees C) using the multianvil technique or by solid-state reaction in the silica ampules (1100 degrees 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 ortho-oxonitridophosphates containing noncondensed [PO3N](4-) ions. The crystal structures were elucidated by single-crystal X-ray diffraction. Sr2PO3N [space group Pnma (No. 62), Z = 4, a = 7.1519(5) angstrom, b = 5.5778(3) angstrom, c = 9.8132(7) angstrom, R1 = 0.020, wR2 = 0.047] crystallizes in the beta-K2SO4 structure type. The structure of Ca2PO3N was solved and refined in the (3 + 1)D superspace group Pnma(a00)0ss [Z = 4, a = 6.7942(7) angstrom, b = 5.4392(6) angstrom, c = 9.4158(11) angstrom, R1 = 0.041, wR2 = 0.067]. It exhibits an incommensurate modulation along [100] with a modulation vector q = [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 (lambda(em) = 525 nm) of the visible spectrum if excited by near-UV light (lambda(exc) = 400 nm).