Revealing the role of calcium codoping on optical and scintillation homogeneity in Lu₂SiO₅:Ce single crystals

Abstract

The presence of impurities and dopants (or codopants) can influence solute transport at the solid-liquid interface and its stability during bulk crystal growth, which ultimately affect the crystal uniformity. Codoping with Ca2+ can significantly improve the performance of Lu2SiO5:Ce single crystals, well-known scintillators for nuclear medical imaging, albeit at the expense of growth stability due to surface tension reduction. However, an understanding of the correlation between crystal growth properties and radial performance uniformity in Czochralski-grown LSO:Ce, Ca single crystals is still lacking. In this work, we reveal this essential correlation by studying the roles of Ca2+ concentration on the radial distribution of stable Ce3+ ions and oxygen vacancy (VO) related defects as well as solute transport at the solid-liquid interface. Through mapping of optical and scintillation properties across cross-sections of 0.1 at% and 0.4 at% Ca codoped boules and the defect formation energies derived from density functional theory (DFT) calculations, a direct link between Ca2+ ions, stable Ce3+ or Ce4+ ions, and {CaLu + VO} complex defects is established. The light yield enhancement in Ca2+ codoped LSO:Ce single crystals is attributed to the dissociation of spatially correlated Ce ions and oxygen vacancies. Based on the quantitative comparison of photoluminescence (PL) and radioluminescence (RL) emissions from Ce1 (seven-coordinated) and Ce2 (six-coordinated) sites, the preferential occupation of stable Ce4+ is believed to depend on the concentration of stable Ce4+. The underlying causes of symmetry and variation in the radial distribution of optical and scintillation properties, as consequences of Ca2+ distribution, are discussed from the perspective of crystal growth.