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2025
ACL
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R.Valois, L.Cornet, V.Rodriguez, O.Toulemonde, A.Fargues, E.Véron, M.Vigier, A.Demourgues, R.Boulesteix, A.Maître, J.-M.Heintz, M.Allix, M.J.Pitcher, V.Jubera, 'Suppression of long-range weberite-type ordering in high temperature processed Gd3+xNb1-xO7-x (x = 0 and 0.25) synthesised by co-precipitation and direct crystallization of the melt', J. Solid State Chem. 350 125508 (2025) doi:10.1016/j.jssc.2025.125508
The synthesis of cubic derivatives of rare-earth pyrochlores such as Gd3NbO7 is of interest for the development of new optical ceramics with high transparency (low birefringence) in visible to mid-IR wavelengths and high refractive index. At ambient temperature, Gd3NbO7 adopts an orthorhombic weberite-type structure with long range ordering of Nb and Gd on a fluorite-type sublattice. Here, we explore ways of suppressing this long-range ordering to achieve a cubic phase, by (i) using co-precipitation and direct crystallization of the undercooled melt as nonequilibrium synthesis routes, and (ii) tuning the gadolinium content to disrupt the local cation ordering directly, according to the formula Gd3+xNb1-xO7-x. PXRD analysis confirmed the orthorhombic weberite-type structure of Gd3NbO7 (x = 0) prepared by co-precipitation, but the long-range weberite-type ordering was absent in Gd3.25Nb0.75O6.75 (x = 0.25) even after a high temperature heat-treatment. Strikingly, weberite-type superstructure peaks were absent from both compositions prepared by melt-quenching, with both indexed to cubic defect-fluorite structures. The melt-quenched Gd-rich (x = 0.25) fully-crystallised beads had a transparent appearance. The local environment of the RE3+ ion was probed by spectroscopic characterizations including second harmonic generation measurements which confirmed at the microscale the centric symmetry and the associated disordered RE environment obtained at high temperatures. These results open new pathways to visible and IR-transparent optical ceramics based on Gd3+xNb1-xO7-x.
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