Ba2AlSi5N9 was synthesized starting from Si3N4, AlN, and Ba in a radio-frequency furnace at temperatures of about 1725°C. The new nitridoalumosilicate crystallizes in the triclinic space group P1 (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, R1=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 Ba2AlSi5N9 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). 29Si and 27Al solid-state NMR are in accordance with the crystallographic results. Ba2AlSi5N9 represents a new type of network structure made up of TN4 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 Ba2+ sites with coordination numbers between 6 and 10. Thermogravimetric investigations confirmed a thermal stability of Ba2AlSi5N9 up to about 1515°C (He atmosphere). Luminescence measurements on Ba2AlSi5N9:Eu2+ (2 mol % Eu2+) with an excitation wavelength of 450 nm revealed a broadband emission peaking at 584 nm (FWHM=100 nm) originating from dipole-allowed 4f6(7F)5d1 → 4f7(8S7/2) transitions
