Valence state of europium and samarium in Ln2Hf2O7 (Ln = Eu, Sm) based oxygen ion conductors

Proton Conductivity of La2(Hf2−xLax)O7−x/2 “Stuffed” Pyrochlores

“…The conductivity of La2(Hf1.9La0.1)O wet air is ~8 × 10 −5 S/cm at 700 °C (Figure 3), which differs very little from the conducti Nd2(Hf1.9Nd0.1)O6.95 under the same conditions [22]. The conductivity of Ln2(Hf1.9Ln0.1)O = Sm, Eu) is not higher in wet air [16], which indicates that Sm-and Eu-based "stu hafnate pyrochlores are purely oxygen ion conductors (Figure 3). The Ln2(Hf1.9Ln0 (Ln = La, Nd) "stuffed" pyrochlores, in which the major defects are oxygen vacanc position 48f, have proton conductivity due to interaction of the oxygen vacancies sition 48f with water according to the Equation (3):…”

“…Ther La2Hf2O7 is a dielectric, unlike Nd2Hf2O7, studied previously [22], whose conductiv wet air is ~1.25 × 10 −6 S/cm at 700 °C, approaching the proton conductivity of La2Zr2O The negligible conductivity of La2Hf2O7 is attributable to the high covalence of the bonds and the perfect structure of this pyrochlore phase. Figure 3 shows Arrhenius for a number of Ln2(Hf1.9Ln0.1)O6.95 ((1) Ln = La; (2) Nd [22]; (3) Sm [16]; (4) Eu) [16] "stu pyrochlores in dry and wet air. La2(Hf1.9La0.1)O6.95 has considerable proton conductivity range 400-750 °C.…”

“…The (Ln = La-Lu; M = Ti, Zr, Hf) systems with a pyrochlore structure can be used as potential oxygen ion-conducting electrolytes for solid oxide fuel cell (SOFC) applications [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]. The most promising electrolytes among pyrochlores are rare-element (RE)-based zirconates: the Li-doped gadolinium zirconate Gd 1.7 Li 0.3 Zr 2 O 6.7 , with a conductivity of 1 × 10 -3 S/cm at 450 • C (~8 × 10 -3 S/cm at 700 • C) [1], and (Nd 2−x Zr x )Zr 2 O 7+x/2 (x = 0.4) (1.78 × 10 −2 S/cm at 700 • C) [5].…”

“…These results were essentially confirmed by Sardar et al [9], who were able to separate the total conductivity onto the bulk (1.0 × 10 −4 S/cm at 700 • C) and grain boundary (4.66 × 10 −5 S/cm at 700 • C) components for the Ho 2 Hf 2 O 7 fluorite prepared by the conventional solid-state reaction technique. Recently, Shlyakhtina et al [16] synthesized Eu 2 (Hf 2−x Eu x )O 7−x/2 (x = 0.1) stuffed pyrochlores, which showed the highest oxygen ion conductivity among the hafnate family: ~7.5 × 10 −4 S/cm at 700 • C. This is, however, lower than the conductivity of the RE zirconates and titanates [16][17][18][19][20]. The authors of [21] investigated the electronic structure of La 2 Hf 2 O 7 based on first-principle calculations with a particular focus on the anion Frenkel (anti-Frenkel) defect pair (V •• O + O i ) in terms of defect structures with correlated formation energies.…”

“…Such anion Frenkel pair (V •• O + O i ) defects are the most stable ones in pure La 2 Hf 2 O 7 . It is worth noting that, whereas the oxygen ion conductivity of the RE hafnates and related solid solutions has been the subject of intense research [8][9][10][11][12][13][14][15][16], there are few data on their proton conductivity, except for the Nd 2 (Hf 2−x Nd x )O 7−x/2 (x = 0; 0.1) system [22].…”