Luminescence of an Oxonitridoberyllate: A Study of Narrow-Band Cyan-Emitting Sr[Be₆ON₄]:Eu²⁺

Oxo- and (oxo)­nitridoberyllates show exceptional potential as host lattices for application in illumination grade phosphor converted (pc)­LEDs due to their remarkable electronic and structural characteristics, allowing highly efficient narrow-band emission upon doping with Eu²⁺. Sr­[Be₆ON₄]:Eu²⁺, the first example of an oxonitridoberyllate phosphor, exhibits narrow-band cyan emission (λ_em = 495 nm; full width at half-maximum, fwhm = 35 nm; ≈1400 cm^–1), comparable to the emission of the oxonitridosilicate BaSi₂O₂N₂:Eu²⁺ (fwhm = 35 nm) or a cyan-emitting primary LED (fwhm = 27 nm). Sr­[Be₆ON₄]:Eu²⁺ reveals a highly condensed rigid 3D network with a remarkably large degree of condensation [i.e., atomic ratio Be:(O,N)] of κ = 1.2 that is achieved by interconnection of highly condensed layers of BeN₄ tetrahedra by Be₂ON₆ units via common edges. The crystal structure of Sr­[Be₆ON₄]:Eu²⁺ was solved on the basis of single-crystal and powder XRD data (C2/c, no. 15, a = 13.9283(14), b = 5.7582(6), c = 4.9908(5) Å, β = 90.195(1)°, Z = 4, R₁ = 0.033, wR₂ = 0.065, GoF = 1.046). Sr­[Be₆ON₄]:Eu²⁺ shows a close structural relationship to other nitride as well as oxide compounds, and therefore closes a structural gap helping to understand relations in Be-containing solid-state materials. The electronic structure of Sr­[Be₆ON₄]:Eu²⁺ was characterized by X-ray spectroscopy measurements, supported by density functional theory (DFT) calculations. Due to its excellent emission properties, large band gap, rigid 3D network, as well as chemical and thermal stability, Sr­[Be₆ON₄]:Eu²⁺ is a promising phosphor to close the cyan gap in efficient high-CRI pcLEDs (CRI, color rendering index).