Al-Doped Apatite-Type Nanocrystalline Lanthanum Silicates Prepared by Mechanochemical Synthesis: Phase, Structural and Microstructural Study

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, 27Al and 29Si 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. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)