Emergence of A-Site Cation Order in the Small Rare-Earth Melilites SrREGa3O7 (RE = Dy-Lu, Y)

SrREGa3O7 melilite ceramics with large rare-earth elements (RE = La to Y) are famous materials especially known for their luminescence properties. Using an innovative approach, the full and congruent crystallization from glass process, SrREGa3O7 transparent polycrystalline ceramics with small rare earth elements (RE = Dy-Lu and Y) have been successfully synthesized and characterized. Interestingly, compared to the classic tetragonal (P (4) over bar2(1)m) melilite structure composed of mixed Sr/RE cationic sites, these compositions can crystallize in a 3 X 1 X 1 orthorhombic (P2(1)2(1)2) superstructure. A detailed study of the superstructure, investigated using different techniques (synchrotron and neutron powder diffraction, STEM-HAADF imaging, and EDS mapping), highlights the existence of a Sr/RE cation ordering favored by a large Sr/RE size mismatch and a sufficiently small appropriate control of the synthesis conditions through glass crystallization enables the formation of the desired polymorphs, ordered or disordered. The influence of this tailored cationic ordering/disordering on the RE luminescent spectroscopic properties have been investigated. A stronger structuration of the RE emission band is observed in the ordered ceramic compared to the disordered ceramic and the glass, whose band shapes are very similar, indicating that the RE environments in the glass and disordered ceramic are close.

Automated summary

By employing a unique approach of full and congruent crystallization from glass, SrREGa3O7 transparent polycrystalline ceramics have been successfully synthesized with small rare earth elements (RE = Dy-Lu and Y). This innovative process results in the formation of an orthorhombic superstructure, as opposed to the classic tetragonal melilite structure, leading to a distinctive cationic ordering of the Sr/RE ions. The tailored cationic ordering in these ceramics impacts the luminescent spectroscopic properties of the rare earth elements, with a stronger structuration of the emission band observed in the ordered ceramic compared to the disordered ceramic and glass.