Cr6+ as the hidden key to low-temperature Pechini synthesis of YAG:Cr4+

Cr 4+ -doped Y 3 Al 5 O 12 (YAG:Cr 4+ ) is widely employed in photonic devices as a saturable absorber and has recently attracted attention as an infrared gain medium. However, its synthesis typically requires annealing at high temperature (>1573 K) to achieve the oxidation of Cr 3+ into Cr 4+ , thus limiting cost efficiency and scalability. Here, we report the first successful preparation of YAG:Cr 4+ nanopowders via the Pechini process at a temperature as low as 1173 K. Structural and morphological analyses (XRD, SEM, TEM) confirm the formation of nanocrystalline YAG with homogeneous chromium incorporation. Optical spectroscopy combined with XANES unambiguously demonstrates the presence of both Cr 4+ and Cr 6+ ions, the latter arising from the oxidative decomposition of organics during calcination—an unprecedented observation in YAG materials. To maximize Cr 4+ concentration, we explored oxidation strategies including well-known Ca 2+ co-doping and more original H 2 O 2 -assisted synthesis; both of which significantly enhance tetrahedral Cr 4+ luminescence. Finally, we investigated the potential of heat treatment in a reducing atmosphere for obtaining Cr 4+ ions, with the aim of better understanding the possible pathways for achieving the Cr 4+ oxidation state in the YAG matrix at low temperatures, leading to very promising results. The low-temperature route studied here not only reduces synthesis costs but also provides new opportunities to tailor the oxidation state of chromium in garnet-type nanomaterials, paving the way for their integration in energy-efficient optical devices.