Tuning of structure and host dynamics via yttrium doping in Bi2O3 to enhance oxygen ion diffusion

We present a comprehensive study of the structure and dynamics of ${\mathrm{Bi}}₂{\mathrm{O}}₃$ with different Y-doping concentrations $\mathrm{[}{({\mathrm{Bi}}_1--x{\mathrm{Y}}_x)}₂{\mathrm{O}}₃\phantom{\rule{4pt}{0ex}}(x=0$, 0.2, and 0.4)] using inelastic neutron scattering (INS) and Raman scattering experiments supplemented by ab initio molecular dynamics (AIMD) and lattice dynamics simulations. Our INS and Raman experiments show a significant change in the vibrational spectrum between normal $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Bi}}₂{\mathrm{O}}₃$ and superionic $\ensuremath{\delta}\text{\ensuremath{-}}{\mathrm{Bi}}₂{\mathrm{O}}₃$, and a pronounced change with temperature in Y-doped compounds, which reveal the presence of soft anharmonic phonons and the role of host dynamics in fast oxygen diffusion. The results corroborate very well the AIMD simulations. Further, the AIMD simulations show a faster oxygen diffusion with 20% Y-doping than with 40%. Our analysis reveals that the ${\mathrm{Bi}}₄\mathrm{O}$ tetrahedral volume distribution plays a crucial role in O diffusion; a larger mean volume of ${\mathrm{Bi}}₄\mathrm{O}$ leads to faster oxygen diffusion.