Ionic exchange and the local structure in the HfO2/Ho2O3 system studied by PAC spectroscopy

• Suitability of PAC spectroscopy to study inter-diffusion processes.
• High level of ionic exchange obtained by ball-milling and thermal treatments.
• EFG characterization of 181Ta impurities at defect-free cation sites of Ho2O3.
• Excellent agreement with other EFG results in bixbyites and ab initio calculations.
• A new Ho-doped m-HfO2 phase was characterized.

The ionic exchange of Hf and Ho atoms in the HfO2/Ho2O3 system was studied at the atomic level applying the nuclear solid-state Time-Differential γ–γ Perturbed-Angular-Correlation (PAC) spectroscopy. This exchange was promoted by a ball-milling-assisted solid-state reaction between Ho2O3 and m-HfO2 initial powders. In order to follow and to elucidate the effect of different variables (milling time, temperature, pressure) on the exchange process and the appearance of new phases, 181Hf(→181Ta) ions were used as local probes in the PAC experiments. The measured hyperfine interactions enabled the electric-field gradient tensor (EFG) characterization at Hf sites at each step of the process. At the final stages of the solid-state reaction, 75–90% Hf-doping at both substitutional defect-free cation sites of Ho2O3 was achieved, being the EFG measured at these sites in excellent agreement with those determined in 181Hf-implanted Ho2O3 samples and to those predicted by the EFG systematics established in rare-earth bixbyites doped by ion-implantation of 181Hf(→181Ta) ions. Ab initio electronic structure calculations of the EFG at Ta impurities localized at both cation sites in Ho2O3 also confirm the 181Hf cationic substitution in both PAC experiments. Additional ab initio calculations at Hf impurity sites in Ho2O3 and Tm2O3 were performed to study the relative Hf preference for the symmetric site of the structure. We showed that high-energy milling plus high temperature treatments are both necessary to achieve a high degree of Hf substitution in the cation sublattice of the Ho2O3 structure. Also, we found that the pressure effect on the crystal structure favors the impurity substitution at cationic sites closer to a homogenous distribution of the probes and with much less local and far disorder. Additional spurious hyperfine interactions that were always present in 181Hf-implanted Ho2O3 samples were not observed when using this solid-state reaction method. The appearance of a modified m-HfO2 phase produced after heavy Ho-doping was also discussed.