A new sialon Eu_3.60LiSi_13.78Al_6.03O_6.82N_22.59 has been discovered via the single-particle diagnosis approach. Its crystal structure (space group P3m1) was solved and refined from single-crystal X-ray diffraction data. It has the interesting feature of two types of disorder at the Eu2 site: positional disorder (Eu2a/Eu2b) and substitutional disorder with (Si/Al)₂(O/N). The structure is generalized to the formula A_4-mB_nC_19+2mX_29+m (A = Sr, La, Eu, Ce; B = Li; C = Si, Al; X = O, N; 0 ≤ m ≤ 1; 0 ≤ n ≤ 1), of which Sr_3.61LiSi_14.27Al_5.61O_6.19N_23.25 (Sr-sialon, m = 0.41, n = 1) and La_2.85Sr_0.76LiSi_14.86Al_4.93O_2.89N_26.51 (LaSr-sialon, m = 0.40, n = 1) are two examples that have been obtained as a single-phase powder. Sr-sialon:Eu and LaSr-sialon:Eu both show blue to yellow emission, depending on the Eu concentration, whereas Sr-sialon:1% Ce shows a deep-blue emission band centered at 422 nm with a full width at half-maximum of 80 nm and an internal quantum efficiency of 80% (λ_ex = 355 nm). The latter phosphor has very good thermal stability of both emission intensity and color. A white light-emitting diode (LED) containing the newly discovered Sr-sialon:5% Ce as the blue phosphor component shows excellent color-rendering indices (R_a = 96 and R₁₂ = 97) with a correlated color temperature of 4255 K. This indicates that Sr-sialon:Ce is a highly promising deep-blue phosphor for illumination grade white LEDs.