In situsynchrotronpowder X-ray diffraction study of formation and growth of yttrium and ytterbium aluminum garnet nanoparticles in sub- and supercritical water

Abstract

The formation and growth of yttrium and ytterbium aluminium garnet (Y_3−xYb_xAl₅O₁₂, x = 0.0–3.0) nanoparticles in sub- and supercritical water have been followed by in situ synchrotron powder X-ray diffraction. At a pressure of 256 bar the formation of crystalline garnet nanoparticles occurs within seconds when the temperature is above 300 °C, and it takes place through an intermediate phase of (Y/Yb)₄O(NO₃)(OH)₉. Below the critical point of water (T_c = 374 °C, P_c = 221 bar), AlOOH crystallizes simultaneously with the garnet phase, and it is observed that both very rapid heating of the reactants and supercritical conditions are necessary to achieve phase pure YAG. Thus, under supercritical conditions YAG forms directly within the time resolution of the experiments (5 s). The particle growth is dependent on the Yb content, the Yb source and the synthesis temperature, and the crystallite size decreases from 35 nm for pure Y₃Al₅O₁₂ to 20 nm for Y_2.4Yb_0.6Al₅O₁₂. After initial particle formation, growth is much slower in sub-critical water than in supercritical water.