Scintillation properties and increased vacancy formation in cerium and calcium co-doped yttrium aluminum garnet

Synergetic Li–Ca Co‐Doping for Ce Valence Conversion in Yttrium Aluminum Garnet

“…The rise time is noticeably shorter in samples with high cerium content and containing Ca 2+ and Li + (Figure 4b). It confirms the higher Ce 3+ to Ce 4+ conversion, leading to a reduced rise time, as shown in Lucchini et al and Dickens et al [ 13,17 ] As a reference, a typical decay under excitation directly in the 5d state (at 445 nm) is shown for one sample in Figure 4. The decays obtained on all the samples are completely similar (62.8 ns), showing that co‐doping does not affect the emission properties of existing Ce 3+ centers.…”

“…It is commonly accepted that the positive charge deficiency in garnets co‐doped with Ca 2+ ions is compensated in part by Ce 3+ → Ce 4+ conversion and O − center formation (the latter were observed in electron spin resonance studies of LuAG:Ce,Mg [ 26 ] ). When Ca 2+ is substituting for Y 3+ sites, the positive charge deficiency is compensated also by anion vacancies, [ 27,28 ] leading to an increase of charge carrier traps. Location of Li + in interstitial spaces in YAG:Ce,Ca,Li decreases the concentration of vacancies, including those created by Ca 2+ .…”

“…The presence of stable Ce 4+ centers was confirmed by direct X‐ray spectroscopy methods [X‐ray absorption near edge structure (XANES) or X‐ray absorption spectroscopy (XAS)] in LYSO:Ce,Ca(Mg), [ 9 ] GAGG:Ce,Mg, [ 14,16 ] LuAG:Ce,Mg, [ 14 ] and YAG:Ce,Ca. [ 17 ] In XANES spectra of GGAG:Ce,Ca, [ 16 ] only some traces of the band corresponding to Ce 4+ could be tentatively resolved and explained as possibly due to a low concentration of Ca 2+ . For applicative purposes, the overlap between the charge transfer (CT) absorption of Ce 4+ with onset below 350 nm and the emission of Ce 3+ has to be minimal, which is the case for most garnets.…”

Characterization of GAGG Doped with Extremely Low Levels of Chromium and Exhibiting Exceptional Intensity of Emission in NIR Region

“…It can be seen that the samples have emission decay times showing the characteristics of Cr 3+ ions, because characteristic decay times for Ce 3+ ions are much shorter and reach about 50-60 ns [5,52]. Cr 3+ ions electron transition are considered forbidden, which usually results in longer decay times in a rage of milliseconds [42,53]. However, for the Ce 3+ ions, electron transitions are allowed, and the decay time is in the range of nanoseconds [5].…”

“…Since calcium has a 2+ charge and it is introduced instead of 3+ gadolinium ion, oxygen vacancy defects are most likely created, due to the aliovalent nature of substitution. It is commonly known that such defects reduce the luminescence intensity of most compounds [41,42]. In this case, the compound with the lowest Ca 2+ amount (Sample 3) has the highest luminescence intensity, whereas Sample 1 and Sample 2 have similar amounts Ca 2+ ions and show lower emission intensities.…”