Spectroscopic determination of Cr3+ sites in sol gel synthesized lithium tantalate
Pham, Hoang Long Minh
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This study focuses on the spectroscopic characteristics of novel optical impurity sites of lithium tantalate synthesized via the sol gel process. The intrinsic Li+ de¯cit of the congruent crystal, due to the thermodynamic phase boundaries of the melt, results in the occupation of these defect sites by the Cr3+ dopant ions. The phase boundaries are avoided with sol gel processing. As a result, crystals produced by the sol gel process have a composition closer to stoichiometry. Like the congruently grown crystals, these crystals also show a preferential substitution of the Li+ sites for the dopant Cr3+ ions. The spectroscopic characteristics of sol gel processed ceramic are similar to the congruently grown crystal. The indication of the sol gel synthesized samples' higher quality crystals are given by their longer radiative decay. The assignment of the majority impurity center in the singly doped congruent and sol gel samples is the Cr3+ ion residing in a Li+ vacancy, Cr[Li]. Codoping lithium tantalate with a secondary ion, Mg2+, is found to change the spec- troscopy center from the Cr[Li]. At a Mg2+ 1% concentration and greater, two novel centers are found. An emission from Cr3+ in the high crystal ¯eld strength environment is observed in the R line region and assigned to Cr3+ occupying the octahedral void, Cr[Void]. The radiative lifetime of this emission is 1.2 ms. A second main center is observed emitting a broad NIR band centered at 840 nm. The lifetime of this emission is 40.8 ¹s, and is assigned to the low crystal ¯eld center of Cr3+ substituting for Ta5+, Cr[Ta].