Nanometric Sr‐Doped LaPO4 Monazite: Synthesis by Mechanical Milling, Characterization, and Water Incorporation on its Structure

Monazite‐type La 1 − x Sr x PO 4 ( x =0.025 and 0.05) nanoparticles have been obtained, for the first time, by milling a mixture of La(NO 3 ) 3 ·6H 2 O, Sr(NO 3 ) 2 , and (NH 4 ) 2 HPO 4 for only 3 h in a planetary ball mill using a moderate rotating disc speed (350 rpm). The formation of the La 1− x Sr x PO 4 takes place via a metathesis solid‐state reaction in a process, which significantly reduces the time and the steps reportedly needed to obtain the same product using different processing methods such as chemical reactions in an aqueous solution. As prepared powders consist of round shaped nanometric particles with a high‐specific surface area (13 m 2 /g for x =0.025 and 6.7 m 2 /g for x =0.05, respectively). Washed and dried samples show X‐ray powder diffraction patterns containing all the characteristic reflections of monazite‐type LaPO 4 . In addition to PO 4 3− orthophosphate ions, Fourier transform infrared spectroscopy measurements revealed the presence of P 2 O 7 4− pyrophosphate ions and HPO 4 2− hydrogen phosphate groups in the Sr‐doped LaPO 4 , under dry and wet conditions, respectively. These observations are in accordance with the defect model for proton conductors in which La substitution with Sr leads to the formation of P 2 O 7 4− as intrinsic positive defects, and in wet atmosphere, protons are introduced into the phosphate forming hydrogen phosphate groups through the equilibrium between P 2 O 7 4− and water vapor. Furthermore, a weight increase of the doped samples at high temperatures during heating in a water containing atmosphere, confirms the water vapor uptake on Sr‐doped LaPO 4 .