Kinetics of Cr3+ to Cr4+ ion valence transformations and intra-lattice cation exchange of Cr4+ in Cr,Ca:YAG ceramics used as laser gain and passive Q-switching media

Abstract

This paper focuses on the kinetics of Cr4+ formation in Cr,Ca:YAG ceramics prepared by solid-state reaction sintering. The kinetics of Cr4+ formation was studied by annealing of Cr,Ca:YAG ceramics in ambient air under different temperatures at different times, resulting in the transformation of Cr3+ to Cr4+. The activation energy (Ea) of Cr3+ oxidation determined by the Jander model was 2.7 ± 0.2 eV, which is in good correlation with the activation energy of innergrain oxygen diffusion in the YAG lattice. It is concluded that Cr3+ to Cr4+ transformation in YAG ceramics is limited by oxygen diffusion through the grain body. It was established that in Cr,Ca:YAG ceramics, the intralattice cation exchange, in which the Cr4+ ions exchange positions with the Al3+ ions, switching from “A” to “D” sites, is faster than Cr3+ to Cr4+ oxidation. In the temperature range of 900–1300 °C, the reaction enthalpy of Al3+/Cr4+ ion exchange between octahedral “A” and tetrahedral “D” lattice sites is close to zero, and this exchange ratio is thermodynamically driven by entropy.

Effects of divalent dopants on the microstructure and conversion efficiency of Cr4+ ions in Cr,Me:YAG (Me - Ca, Mg, Ca/Mg) transparent ceramics

“…Some discrepancies with our results can be explained by the presence of Cr additives, not using TEOS, different concentration of Ca 2+ and Mg 2+ , and peculiarity of the sintering conditions. As it has been established earlier, the Cr 3+ additive changed the sintering process of YAG ceramics [18] and redistributed divalent additives in the grain volume [29].…”

“…It means that there is no mutual effect of Ca 2+ and Mg 2+ on the morphology of the ceramics. Regarding the influence of calcium ion, it was mentioned [29] that when Ca 2+ concentration was increased to 0.5 at.%, the inhibition of the grain growth by Ca-rich layer took place.…”

Size-Dependent Persistent Luminescence of YAGG:Cr3+ Nanophosphors

“…Direct dependence of the average grain size on temperature is clearly seen, increasing the temperature from 800 °C to 1300 °C led to a growth in the average crystallite size from 26(2) nm to 110(2) nm. Meanwhile, the lattice parameters decrease from 12.1806( 16) Å to 12.1640 (26) Å for an annealing temperature of 1100 °C , and a further rise in the temperature barely changes the lattice parameter.…”

“…Unlike Al 3+ and Ga 3+ ions, Cr 3+ can occupy only octahedral sites, because the crystal field stabilization energy of tetravalent chromium ions in tetrahedral coordination is about a third of the value in octahedral coordination [25]. Otherwise, Cr 4+ can occupy both octahedral and tetrahedral sites [26]. Accordingly, the only variable independent parameter is the occupancy of octahedral and tetrahedral sites by Al 3+ and Ga 3+ ions.…”

Crystallographic control for Cr4+ activators toward efficient NIR-II luminescence

“…33 Chaika et al proved that the oxidation of Cr 3+ to Cr 4+ was achieved by air annealing in YAG:Cr,Ca. 34 By changing the synthesis conditions, Liu et al found that high-temperature sintering tends to facilitate the transformation from Cr 3+ to Cr 4+ in the Mg 2 GeO 4 host. 35 However, in these reports the luminescence from Cr 3+ could still be traced.…”