Defect structure of yttria-stabilized hafnia nanoparticles.

High-temperature polymorphs of hafnia (HfO2) are of significant interest in electronics and fuel-cell applications, and stabilization at ambient conditions can be achieved by aliovalent substitution and nanosize effects. Y3+ stabilization of hafnia (YSH) introduces local cation disorder around charge-compensating oxygen-ion vacancies, and here we establish both the average and local structure of YSH nanoparticles using synchrotron powder X-ray diffraction (PXRD) and pair distribution function (PDF) analysis. A range of phase-pure crystalline nanoparticles of Hf1-xYxO2-x/2 were prepared via continuous flow solvothermal synthesis and subsequent high-temperature annealing, and full stabilization of the cubic phase is achieved already at 13 at% Y3+. The average structure conforms to the cubic fluorite phase of HfO2, but local displacive disorder caused by electrostatic attraction of neighbouring oxygen ions and repulsion of neighbouring metal ions by the net-positive oxygen-ion vacancies is established. The well-known Zr3Y4O12 structure, which incorporates such relaxation motifs, provides a good proxy description of YSH. In situ X-ray total scattering experiments provide insight into the formation mechanism of the YSH nanoparticles and initial precipitation of an atomically mixed amorphous phase is followed by crystallization over several minutes. The crystallization rate increases with higher reaction temperature, whereas an increased doping level results in slower crystallization.