Structural and Microstructural Characterization of Zirconium Phosphates Prepared by Mechanochemical Synthesis

Layered α-type zirconium phosphate (ZrP) was prepared by mechanochemical synthesis by using a planetary ball mill, in the absence of solvents. The reagents, a zirconyl organic salt and phosphoric acid (H3PO4/Zr molar ratio = 4), are mixed in an agate jar for different times, from 15 to 60 min. Additional syntheses are performed by adding oxalic acid to the reagents' mixture. The powder samples obtained by ball milling (BM samples) are characterized by X-ray powder diffraction, thermogravimetric, scanning electron microscopy analyses, as well as 31P MAS NMR and FTIR spectroscopy. Structural and microstructural parameters (unit cell parameters, unit cell volume, crystallite size, and amount of amorphous fraction) of the BM samples are compared with those of ZrP samples prepared by sol–gel and hydrothermal methods. The best results were obtained with a grinding time of 45 min, in the presence of oxalic acid (oxalic acid/Zr molar ratio = 0.2). Ball milling promotes the formation of single nanocrystals with larger unit cell parameters, especially along the layer packing direction (c axis), with respect to the ZrP samples prepared by hydrothermal synthesis, which, on the contrary, favors the formation of micrometric multidomain crystals.