Theoretical investigation of intrinsic point defects and the oxygen migration behavior in rare earth hafnates

In this work, the composition-dependent point defect types and formation energies of RE₂Hf₂O₇ (RE = La, Ce, Pr, Nd, Pm, Sm, Eu and Gd) as well as the oxygen diffusion behavior are systematically investigated by first-principles calculations. The possible defect reactions and dominant defect complexes under stoichiometric and non-stoichiometric conditions are revealed. It is found that O Frenkel pairs are the predominant defect in stoichiometric pyrochlore hafnates. Hf-RE cation anti-site defects, accompanied by RE vacancies and/or oxygen interstitials, are stable in the non-stoichiometric case of HfO₂ excess. On the other hand, RE-Hf anti-site defects together with oxygen vacancies and/or RE interstitials are preferable in the case of RE₂O₃ excess. The energy barriers for the migration along the V_O48f - V_O48f pathway of pyrochlore hafnates were calculated to be between 0.81 eV and 0.89 eV. Based on these results, a defect engineering strategy is proposed and the pyrochlore hafnates investigated here are predicted to exhibit potential oxygen ionic conductivity.     

Thermal diffusivity characterization of europium zirconate,cerate and hafnate

The following paper presents study on synthesis and thermal diffusivity of europium zirconate, cerate and hafnate as a new types of materials with pyrochlore or fluorite type of lattice, dedicated to replacement of conventional 8YSZ (yttria stabilized zirconia) in TBC (thermal barrier coatings) systems for aircraft gas turbines.All materials were synthesized via solid-state reaction (SSR) preceded preparation of powder mixtures (mechanical milling in ethanol). The feedstock powders were characterized by analysis of crystallite size, particle size distribution and phase composition. On the base of DSC measurements, the parameters of high temperature sintering were selected (1350?°C/2?h/15?MPa). Structural characterization of obtained materials inclusive analysis of morphology, chemical and phase composition of sinters was performed. Thermal diffusivity was measured in temperature range 25÷1400?°C by laser flash analysis (LFA). The obtained materials with pyrochlore and fluorite type of lattice exhibit lower thermal diffusivity compared to that of yttria-stabilized zirconia at high temperatures. In particular, the most promising material in view of insulating properties is Eu₂Ce₂O₇, which exhibit much lower thermal diffusivity than Eu₂Zr₂O₇ and Eu₂Hf₂O₇, especially at temperature higher than 1000?°C. However, in temperature range 25–700?°C, europium hafnate and zirconate exhibit lower thermal diffusivity compared to that of europium cerate and 8YSZ.