M₂PO₃N (M = Ca, Sr): ortho-Oxonitridophosphates with β-K₂SO₄ Structure Type

Abstract

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 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) Å, b = 5.5778(3) Å, c = 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 Pnma(α00)0ss [Z = 4, a = 6.7942(7) Å, b = 5.4392(6) Å, c = 9.4158(11) Å, 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 Eu(2+), Ca2PO3N exhibits luminescence in the green range (λem = 525 nm) of the visible spectrum if excited by near-UV light (λexc = 400 nm).

Ammonothermal Synthesis of Ba2PO3N – An Oxonitridophosphate with Non‐Condensed PO3N Tetrahedra

“…The P–O [1.573(2)–1.602(2) Å] and P–N distances [1.578(2) Å] are in good agreement with bond lengths of other alkaline earth oxonitridophosphates known from literature (e.g. Ca 2 PO 3 N, Sr 2 PO 3 N, SrP 3 N 5 O, Sr 3 P 6 O 6 N 8 , Ba 3 P 6 O 6 N 8 ) , . The corresponding O/N–P–O/N angles vary between 107.81(9) and 112.6(2)° and deviate only slightly from the regular tetrahedron angle and are also in good agreement with values known from literature.…”

“…Wyckoff positions and atomic coordinates, anisotropic displacement parameters, as well as interatomic distances and angles are given in Tables S1–S3 (Supporting Information). Ba 2 PO 3 N crystallizes in the β‐K 2 SO 4 structure type and is isotypic to its lighter homologues Ca 2 PO 3 N and Sr 2 PO 3 N . As expected, the lattice parameters of the three EA 2 PO 3 N compounds increase linearly with increasing size of the alkaline earth ions (see Figure S1, Supporting Information).…”

“…The crystal structure is built up from non‐condensed [PO 3 N] 4– tetrahedra (see Figure ) and therefore exhibits a degree of condensation of κ = n (P)/ n (O,N) = 1/4, which expands the range of ammonothermally accessible degrees of condensation for (oxo)nitridophosphates to 1/4 ≤ κ ≤ 4/7. The assignment of O and N atoms was carried out in accordance with the structure model of EA 2 PO 3 N ( EA = Ca, Sr) . Lattice energy calculations (MAPLE), bond‐valance sums (BVS), as well as charge distribution (CHARDI) calculations support the ordering and show only slight deviations from expected values (see Tables S4–S6, Supporting Information).…”

“…However, there is no ammonothermally synthesized ortho ‐(oxo)nitridophosphate showing non‐condensed [PO 4– x N x ] (3+ x )– tetrahedra, up to now. Such structural features were only observed in Li 7 PN 4 , Li 14 [PON 3 ] 2 O, EA 2 PO 3 N ( EA = Ca, Sr) and Ho 3 [PN 4 ]O, which were synthesized in ampoules or via high‐pressure synthesis, respectively . Especially, the EA 2 PO 3 N compounds are of special interest, as they crystallize isotypically to β‐K 2 SO 4 .…”

“…Accompanied with a plethora of compounds crystallizing in this structure type, a great diversity of materials properties is observed. Especially, the potential as host lattices for luminescent materials is impressive, covering the whole visible spectrum from red (La EA SiO 3 N:Eu 2+ with EA = Sr, Ba) over green (Ca 2 PO 3 N:Eu 2+ ) to blue (KSrPO 4 :Eu 2+ ) emission , , …”