Al-doped apatite-type nanocrystalline lanthanum silicates prepared by mechanochemical synthesis: Phase, structural and microstructural study

摘要

Al-doped La9.33+x/3+ySi6-xAlxO26+3y/2 (x = 0-1.5, y = 0-0.67) apatite-type lanthanum silicates (ATLS) were synthesized with a high-power (1200 rpm) planetary ball mill that allows the formation of the lanthanum silicate phase after milling for 20-35 min at room temperature. The apatite phase already appears after milling for 5 min. Phase, structural and microstructural studies on the nanocrystalline lanthanum silicates were carried out by using XRD, TEM, and IR, Al-27 and Si-29 MAS NMR spectroscopy to clarify the mechanism of ATLS formation. The cluster-topotactic mechanism of the formation of apatite by mechanochemical treatment involving hydration of the starting compounds and acid-base reactions is suggested, Al-doping results in promoting the formation of apatite. However, a secondary LaAlO3 phase may be present in the sample depending on its stoichiometry. This suggests the existence of a biphase domain in the La2O3-SiO2-Al2O3 phase diagram at room temperature, in which Al-doped apatite and LaAlO3 phases coexist. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008).