Hydrothermal Syntheses and Crystal Structure of NH4Ln3F10 (Ln=Dy, Ho, Y, Er, Tm)

ZrO2–SiO2 sol–gel powders were produced using tetraethoxysilane (TEOS) and zirconium propoxide. After gellation, the ZrO2 crystallization process was investigated using X-ray diffraction (XRD), thermal analysis (DTA/TGA), and scanning electron microscopy (SEM). Fresh gels were amorphous. Thermal treatments were carried out from 100 to 1400°C for a total annealing time of 182 h. Tetragonal zirconia, (Z(t)) was the first phase to crystallize, between 300 and 500°C. Crystallization temperature was lower for zirconia-rich compositions, increasing as silica content was raised. DTA analysis showed that Z(t) crystallization occurred in two stages. Complete tetragonal–monoclinic zirconia transformation occurred near 1000°C, and was clearly observed only in ZrO2-rich compositions (>80%). Silica remains amorphous until 1200°C, when ZrSiO4 formation took place. A metastable sol-gel phase diagram was proposed to show the crystallization process between 100 and 1400°C.