Supercritical Synthesis of Phase-Pure GdxCe1-xO2-x/2 Nanoparticles (x ≤ 0.3) with a Random Defect Structure.

Gd-doped ceria (GDC) is a key functional oxide for applications in catalysis and solid oxide fuel cells, where control over phase purity, dopant incorporation, and defect chemistry at the nanoscale is critical. Here, we use in situ synchrotron powder X-ray diffraction to identify synthesis conditions to obtain GdxCe1-xO2-x/2 nanoparticles in a one-step process. GDC nanoparticles with 0.0 ≤ x ≤ 0.3 are subsequently synthesized by a continuous-flow hydrothermal method using simple cerium and gadolinium nitrates and urea as precursors. Systematic variation of synthesis temperature and pH demonstrates that supercritical conditions (400  °C) under basic pH are required for the formation of a homogeneous fluorite solid solution under hydrothermal conditions. Ex situ synchrotron PXRD with Rietveld refinement reveals a monotonic lattice expansion from 5.41370(2) to 5.42701(7)  Å with increasing Gd content, accompanied by increased microstrain and a reduction of crystallite size from 7 to 4 nm. Pair distribution function analysis and STEM-EDX mapping indicate limited local disorder, with no evidence of defect ordering or clustering. A statistically distributed arrangement of Gd ions and oxygen vacancy defects is favorable for obtaining high oxygen mobility at relatively low temperatures. This contrasts with the correlated defects known in oxygen conductors such as yttria-stabilized hafnia.