Dissimilar behavior of YAG:Ce and LuAG:Ce scintillator garnets regarding Li⁺ co-doping

Abstract

Using a combination of experimental methods, the substitution tendencies of Li+ and involved charge compensation mechanisms are determined and compared in two important similar scintillators, LuAG:Ce and YAG:Ce.

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

“…The slightly lower γ-ray-induced absorption in YAG:Ce,Ca,Li, in comparison to YAG:Ce,Ca, reflects a lower concentration of charge carrier traps, which is in line with the location of Li þ in interstitial spaces (as it was observed in YAG:Ce,Li [25] ). It is commonly accepted that the positive charge deficiency in garnets co-doped with Ca 2þ ions is compensated in part by Ce 3þ !…”

“…[24] Ce 4þ states were not detected in X-ray absorption spectra in LuAG:Ce,Li [14] and YAG:Ce,Li. [22] The XAS detection limit of Ce 4þ was estimated to be of the order of 13 ppm in Dickens et al [24] In our earlier article, [25] considering the difference of behavior between LuAG and YAG regarding Li þ co-doping, it was suggested that Li þ ions in YAG:Ce,Li are situated in interstitial spaces and do not interact with Ce 3þ ions, while the charge compensation is attained by reduction of anion vacancies followed by reduction of related absorbing F-type centers. Under X-ray excitation, Li co-doping, in the range of concentrations investigated, does not change the rise time noticeably, while it slightly increases the amount of delayed luminescence.…”

Interstitial Li+ Controls the UV Transmission and the Radiation Hardness in YAG

“…As expected, absorption in YAG:Ca is significantly higher due to a larger amount of anion vacancies present in the lattice, since Ca 2+ ions substitute for Y 3+ sites due to the small (≈10%) size difference (Ca 2+ − 1.12 Å, Y 3+ ‐ 1.02 Å, in eight‐fold coordination) and the charge misfit of only unity. Increase of absorption in the UV range was observed also in LuAG:Li (in comparison to LuAG), in which Li + ions are incorporated at Lu 3+ sites (Li + − 0.92 Å, Lu 3+ − 0.977 Å; size difference ≈6 %) . Thus, the preference for site substitution by Li + becomes less favorable, when going to larger cations in place of Lu 3+ .…”

“…In comparison to un‐doped YAG, absorption in the 200–270 nm range in YAG:Li (25 ppm) and YAG:Li (50 ppm) is remarkably lower. Following, Li + ions, when taken in small amounts, do not substitute for any lattice site but are accomodated in interstitials. The process is accompanied by a charge compensation via reduction of anion vacancies and of associated defect centers (mainly of F‐type), which are responsible for absorption in this range.…”

“…Methods of control of anion vacancy concentration in as‐grown YAG crystals remain thus a critical aspect for most of applications mentioned above. Incorporation mechanisms and the functional role of Li + impurity in different garnets have been examined in the recent paper . It was found that, in contrast to isostructural LuAG (Lu 3 Al 5 O 12 ), Li + ions in the YAG host do not substitute for any lattice site but reside preferentially at interstitials decreasing the amount of anion vacancies.…”