Tunable Red Luminescence in Nitridomagnesoaluminates α-Sr₂[MgAl₅N₇]:Eu²⁺, β-Sr₂[MgAl₅N₇]:Eu²⁺, and Sr₈[LiMg₂Al₂₁N₂₈]:Eu²⁺

Red-emitting phosphors with the general sum formula Sr8[Li(2–0.5x)MgxAl(22–0.5x)N28]:Eu2+ for application in warm-white illumination grade light emitting diodes were obtained by reacting the metals, hydrides, and nitrides in molybdenum crucibles in a hot isostatic press. Upon irradiation with blue light (440 nm), the materials exhibit red luminescence (emission maximum: 633–665 nm; full width at half-maximum (fwhm): 1736–1964 cm–1, 78–82 nm) tunable by adjusting the compositional variable x (here, 2 and 4) as well as the activator concentration (here, 0.075–1 atom-% Eu2+). The materials show promising thermal behavior of the emission with relative quantum efficiencies, compared to room temperature, of 66% (x = 0) and 84% (x = 2) at 200 °C. α-Sr8[Li(2–0.5x)MgxAl(22–0.5x)N28]:Eu2+ (x = 2 and 4) crystallizes isotypically with Sr4[LiAl11N14]:Eu2+ in the orthorhombic space group Pnnm (no. 58) with unit cell parameters a = 10.51818(5)–10.54878(6), b = 10.44513(5)–10.48253(6), and c = 3.25704(1)–3.272752(15) Å. β-Sr2[MgAl5N7]:Eu2+ (x = 4) crystallizes in the monoclinic space group C2/m (no. 12) with unit cell parameters a = 11.12874(11), b = 3.27289(3), c = 10.54530(11) Å, and β = 109.8939(7)° and is obtained as side phase (≤40 wt %) in syntheses with x = 4. The crystal structure of β-Sr2[MgAl5N7]:Eu2+ consists of a network of vertex- and edge-sharing Al(/Mg)N4 tetrahedra with four- and five-membered ring channels along [010]. The Sr2+ ions are located within the five-membered ring channels and are coordinated cube-like by eight N atoms.