HfO2-based refractory compounds and solid solutions

Energetics of rare-earth-doped hafnia

“…For HfO 2 -SmO 1.5 and HfO 2 -GdO 1.5 solid solutions, this is consistent with the existence of an ordered pyrochlore Hf 0.5 Sm 0.5 O 1.75 and Hf 0.5 Gd 0.5 O 1.75 structure. 6 No ordered structure is reported for HfO 2 -DyO 1.5 and HfO 2 -YbO 1.5 . However, the strongly negative interaction parameter suggests the possibility of an ordered or at least partly ordered structure.…”

“…The A site is usually occupied by large, low valence cations, and the B site by any transition metal element capable of octahedral coordination. Phase stabilities of HfO 2 -REO 1.5 systems from 1000°C to their melting temperature were discussed and reviewed extensively by Glushkova et al 6 For Hf 1−x RE x O 2−x/2 between x ‫ס‬ 0.18 and 0.66 (10−50 mol% RE 2 O 3 , as reported in the phase diagram), fluorite and pyrochlore stability fields are mainly controlled by the radius of the RE dopant. A large RE 3+ cation (La-Nd) leads to the presence of pyrochlore and pyrochlore/monoclinic phase fields, whereas pure fluorite phase is stable only at temperatures above 1800°C.…”

“…4,5 Quenchable HfO 2 -REO 1.5 solid solutions with fluorite structure are reported for the RE sequence from Sm to Yb. 6 The disordered fluorite-type structure is related to the pyrochlore structure (A 2 B 2 O 7 ), characterized by an ordered cation sublattice and ordered oxygen vacancies. The A site is usually occupied by large, low valence cations, and the B site by any transition metal element capable of octahedral coordination.…”

“…Compositions were chosen depending on the reported phase stabilities of the HfO 2 -REO 1.5 solid solutions. 6 Enthalpy of formation with respect to the oxide end members was determined using oxide melt solution calorimetry. 15,16 Samples with a composition of x ‫ס‬ 0.4 were further characterized by high-resolution synchrotron powder xray diffraction (XRD) and differential scanning calorimetry (DSC).…”