Thermal decomposition of priceite Ca2B5O7(OH)5·H2O
摘要
Borate minerals hold broad industrial significance, particularly in the production of high-performance ceramics and specialty glasses, where precise control over thermal and structural properties is critical. Detailed understanding of the thermal decomposition pathways of borate phases is essential for optimizing these applications. Priceite, calcium borate hydrate Ca2B5O7(OH)5·H2O, is a borate mineral occurring in many regions. In this study, we investigated its thermal decomposition via thermogravimetric (TG) analysis and ex situ high-temperature synchrotron powder X-ray diffraction (XRD), revealing three dehydration steps, ultimately forming an amorphous anhydrous phase and subsequently crystallizing into CaB2O4 (I) at 700 °C. The XRD pattern remained stable until 200 ℃, whereas it changed significantly after heating at 250 ℃ for 60 min, forming the dehydrated phase Ca2B5O7(OH)5. Heating at 300 ℃ for 60 min significantly increased crystallinity and produced a second dehydrated phase Ca2B5O8(OH)3, which persisted until 400 ℃. Heating at 450 ℃ for 60 min fully converted Ca2B5O8(OH)3 to the amorphous phase Ca2B5O9, consistent with the TG/DTA measurements. CaB2O4 (I) appeared at 700 ℃, with its crystallinity improving up to 1000 ℃. Therefore, through several dehydration, amorphization, and decomposition steps, the fundamental building block of priceite 〈3▢⟩–〈∆2▢⟩ finally transformed into the one-dimensional infinite chain of corner-sharing BO3 triangles in CaB2O4 (I). The recent identification of Ca(BO2)2 as an outstanding deep-ultraviolet (DUV) birefringent material further highlights the importance of elucidating its formation mechanism from hydrated precursors. This study provides insights into the structural evolution leading to CaB2O4 (I), thereby advancing future material design and industrial applications in optics and thermal-resistant materials.