Blue-Emitting Sr₃Si_8-xAl_xO_7+xN_8-x:Eu²⁺ Discovered by a Single-Particle-Diagnosis Approach: Crystal Structure, Luminescence, Scale-Up Synthesis, and Its Abnormal Thermal Quenching Behavior

The single-particle-diagnosis approach allows for the fast discovery of novel luminescent materials using powdered samples. This paper reports a new blue-emitting Sr₃Si_8-xAl_xO_7+xN_8-x:Eu²⁺ phosphor for solid state lighting and its scale-up synthesis. The structure-, composition-, and temperature-dependent luminescence were investigated and discussed by means of various analytic techniques including single-crystal XRD diffractometer, single-particle fluorescence spectroscopy, FTIR spectra, decay time, low-temperature luminescence, and computed energy level scheme. Sr₃Si_8-xAl_xO_7+xN_8-x crystallizes in the monoclinic system (space group C2/c, no. 15) with a = 18.1828 (13) Å, b = 4.9721 (4) Å, c = 15.9557 (12) Å, β = 115.994 (10)^o, and Z = 2. The Sr atoms are coordinated to 8 and 6 O/N atoms and located in the voids along [010] formed by vertex-sharing (Si,Al)-(O,N)₄ tetrahedra. Phase-pure powder samples of Sr₃Si_8-xAl_xO_7+xN_8-x:Eu²⁺ were synthesized from the chemical composition of the single particle by controlling the x value. Luminescence of both a single particle and powders show a broad Eu²⁺ emission band centered at ∼465 nm and a fwhm of ∼70 nm, under the UV light irradiation. The title phosphor has a band gap of 5.39 eV determined from the UV-vis spectrum, absorption efficiency of 83%, internal quantum efficiency of 44.9%, and external quantum efficiency of 37.4% under the 355 nm excitation. An abnormal thermal quenching behavior is observed in Sr₃Si_8-xAl_xO_7+xN_8-x:Eu²⁺ that has a high activation energy for thermal quenching (0.294 eV) but a low thermal quenching temperature (∼370 K), which is ascribed to the partial overlap between the Eu²⁺ excited energy level and the conduction band of the host.