Local Disorder and Tunable Luminescence in Sr1–x/2Al2–xSixO4 (0.2 ≤ x ≤ 0.5) Transparent Ceramics

Eu-doped Sr_1-x/2Al_2-xSi_xO₄ (x = 0.2, 0.4, and 0.5) transparent ceramics have been synthesized by full and congruent crystallization from glasses prepared by aerodynamic levitation and laser-heating method. Structural refinements from synchrotron and neutron powder diffraction data show that the ceramics adopt a 1 × 1 × 2 superstructure compared to the SrAl₂O₄ hexagonal polymorph. While the observed superstructure reflections indicate a long-range ordering of the Sr vacancies in the structure, ²⁹Si and ²⁷Al solid-state NMR measurements associated with DFT computations reveal a significant degree of disorder in the fully polymerized tetrahedral network. This is evidenced through the presence of Si-O-Si bonds, as well as Si(OAl)₄ units at remote distances of the Sr vacancies and Al(OAl)₄ units in the close vicinity of Sr vacancies departing from local charge compensation in the network. The transparent ceramics can be doped by europium to induce light emission arising from the volume under UV excitation. Luminescence measurements then reveal the coexistence of Eu²⁺ and Eu³⁺ in the samples, thereby allowing tuning the emission color depending on the excitation wavelength and suggesting possible applications such as solid state lighting.