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 (P4̅21m) melilite structure composed of mixed Sr/RE cationic sites, these compositions can crystallize in a 3 × 1 × 1 orthorhombic (P21212) 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 RE cation. An appropriate control of the synthesis conditions through glass crystallization enables the formation of the desired polymorphs, either 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.

Insight into the structure-elastic property relationship of calcium silicate glasses: a multi-length scale approach.

Based on a combination of molecular dynamics simulations, and Raman and Brillouin light scattering spectroscopies, we investigate the structure and elastic properties relationship in an archetypical calcium silicate glass system. From molecular dynamics and Raman spectroscopy, we show that the atomic structure at the short and intermediate length scales is made up of long polymerized silicate chains, which adjusts itself by closing the Si-O-Si angles and leaving more space to [CaO]n edge shared polyhedra to strengthen the glass. Using Brillouin spectroscopy, we observe an increase of elastic constants of the glass with the calcium content, as the cohesion of the glass structure is enhanced through an increase of the binding between the cross-linked calcium-silicate frameworks. This result, albeit being simple in its nature, illustrates for the first time the implication of the calcium framework in the elastic behavior of the glass and will contribute substantially to the understanding of the composition-structure-property relationships in multi-component industrial glasses.