Ba₂AlSi₅N₉—A New Host Lattice for Eu²⁺-Doped Luminescent Materials Comprising a Nitridoalumosilicate Framework with Corner- and Edge-Sharing Tetrahedra

Ba 2 AlSi 5 N 9 was synthesized starting from Si 3 N 4, AlN, and Ba in a radio-frequency furnace at temperatures of about 1725 °C. The new nitridoalumosilicate crystallizes in the triclinic space group P 1 (no. 1), a = 9.860(1) Å, b = 10.320(1) Å, c = 10.346(1) Å, α = 90.37(2)°, β = 118.43(2)°; γ = 103.69(2)°, Z = 4, R 1 = 0.0314. All synthesized crystals were characteristically twinned by reticular pseudomerohedry with twin law (1 0 0, −0.5 −1 0, −1 0 −1). The crystal structure of Ba 2 AlSi 5 N 9 was determined from single-crystal X-ray diffraction data of a twinned crystal and confirmed by Rietveld refinement both on X-ray and on neutron powder diffraction data. Statistical distribution Si/Al is corroborated by lattice energy calculations (MAPLE). 29 Si and 27 Al solid-state NMR are in accordance with the crystallographic results. Ba 2 AlSi 5 N 9 represents a new type of network structure made up of TN 4 tetrahedra (T = Si, Al). Highly condensed layers of dreier rings with nitrogen connecting three neighboring tetrahedral centers occur which are further crosslinked by dreier rings and vierer rings. The dreier rings consist of corner-sharing tetrahedra, whereas some of the vierer rings exhibit two pairs of edge-sharing tetrahedra. In the resulting voids of the network there are eight different Ba 2+ sites with coordination numbers between 6 and 10. Thermogravimetric investigations confirmed a thermal stability of Ba 2 AlSi 5 N 9 up to about 1515 °C (He atmosphere). Luminescence measurements on Ba 2 AlSi 5 N 9:Eu 2+ (2 mol % Eu 2+ ) with an excitation wavelength of 450 nm revealed a broadband emission peaking at 584 nm (FWHM = 100 nm) originating from dipole-allowed 4f 6 ( 7 F)5d 1 → 4f 7 ( 8 S 7/2 ) transitions.