Role of BPO₄ flux in BaBPO₅ crystal growth characterized through high-temperature Raman spectroscopy

In this study, we explored the growth of incongruently melting crystals in a starting solution with flux to better understand the role of flux in crystal growth mechanisms. We recorded the spectrum of BaBPO 5 crystals across a 200–1400 cm −1 range for detailed Raman spectroscopic analysis. The observed Raman bands corresponded to the vibrations of PO 4 and BO 4 units. These units link with B–O–P bonds via common corners to form PBO 7 groups that serve as basic structural units of the crystal. Notably, these PBO 7 structures remain stable at high temperatures up to the crystal melting point, at which the isolated PO 4 groups predominantly exist in the melt, as revealed by high-temperature Raman spectroscopy. Further investigations into the Raman spectra of the starting solution enriched with BPO 4 self-flux indicate that PBO 7 groups are crucial for crystal growth. The addition of PBO 4 flux aids in the formation of these PBO 7 units. This research paves the way for expanding our understanding to other systems, which will help elucidate the mechanisms behind crystal growth.