The pH-dependent reactions in the sonochemical synthesis of luminescent fluorides: The quest for the formation of KY3F10 crystal phases

In this study Eu3+-doped yttrium fluorides were designed by ultrasound-assisted processes at different pH values (4.0–9.0). This novel strategy has enabled to obtain materials with intriguing morphologies and modulated crystal structures: α-KY3F10, δ–KY3F10·xH2O, and Y(OH)3–xFx. To date, the literature has primarily focused only on the α-phase of KY3F10. Yet, explaining the formation of the mostly uncharted δ-phase of KY3F10 remains a challenge. Thus, this paper offers the key to synthesizing both the α and the δ-phases of KY3F10 and also reports the first ultrasound-assisted process for the preparation of yttrium hydroxyfluorides. It is also unraveled the connection between the different pH-dependent reactions and the formation mechanisms of the compounds. In addition to this, the unique features of the Eu3+ ion have allowed to conduct a thorough study of the different materials and have endowed the compounds with photoluminescent properties. The results underscore a highly tunable optical response, with a wide gamut of color emissions (from orangish to red hues), lifetimes (from 7.9 ms to 1.1 ms) and quantum efficiencies (98–28%). The study unveils the importance of sonochemistry in obtaining luminescent fluorides with controlled crystal structures that can open up new avenues in the synthesis and design of inorganic materials.