The elaboration of new highly energetic compact LASER sources, in the mid-infrared optical window, is expected to improve further the efficiency of opto-electronic devices for many applications in telecommunication, medical or defence sector. However, few materials are available for these fields of optical applications and remain limited in their power performances. Since the elaboration of the first LASER Nd:YAG ceramic in 1995 by Ikesue et al. [1], this class of materials has demonstrated an excellent alternative to LASER single crystals. The recent development of transparent ceramics of chemical compositions Ho:YAG, Ho:LuAG and Ho:Lu2O3 [2] gave rise to the production of gain materials with LASER emissions generation around 2µm.
Previous works dedicated to the elaboration of rare-earth niobates-based transparent ceramics (Lu3NbO7 and Y3NbO7 [3]) have shown promising spectroscopic properties as potential LASER gain media. Thanks to its disordered crystalline structure (lacunar fluorite) when doped by rare earth and its large solid solution domain, these niobates ceramic phases are of interest in widening the range of accessible wavelengths. Beside their unusual chemical and physical properties, rare-niobates possess a crystal structure depending on the rare-earth cations size and the synthesis route. The niobate Gd3NbO7, well-known for its application as thermal barrier coating, has been extensively studied regarding its structural characteristics. More precisely, a weberite-like structure is reported under conventional solid state synthesis while a cubic lacunar fluorine is observed when a liquid phase synthesis is used. In this later case, a phase transition toward weberite-like structure occurs at high temperature (between 1100°C and 1500°C [4]).
In this context, Gd3NbO7 composition was tuned to follow the structural stability. Eu3+ ion spectroscopy has been used to characterize the evolution of crystalline structure and the purity of the prepared sample by photoluminescence. Relying on the chemical composition variation, the effects of the phase stabilisation and transition on the ceramics microstructure has been studied while proceeding pressure-less sintering process to obtain an optical ceramic.

References
[1] Ikesue, A., K. Kamata, et K. Yoshida. « Synthesis of Nd3+,Cr3+-Codoped YAG Ceramics for High-Efficiency Solid-State Lasers ». Journal of the American Ceramic Society 78, no 9 (septembre 1995): 2545?47.
[2] Viers, L., F. Delaunay, R. Boulesteix, M. Vandenhende, G. Antou, et A. Maître. « Study of Densification Mechanisms during Spark Plasma Sintering of Co-Precipitated Ho:Lu2O3 Nanopowders: Application to Transparent Ceramics for Lasers ». Journal of the European Ceramic Society 41, no 14 (novembre 2021): 7199?7207.
[3] « Ho3+-doped Y3NbO7 transparent ceramic as potential gain materials at 2 microns ». (2023) To be published
[4] Chesnaud, A., M.-D. Braida, S. Estradé, F. Peiró, A. Tarancón, A. Morata, et G. Dezanneau. « High-Temperature Anion and Proton Conduction in RE3NbO7 (RE = La, Gd, Y, Yb, Lu) Compounds ». Journal of the European Ceramic Society 35, no 11 (octobre 2015): 3051?61.