Study of bulk and grain-boundary conductivity of Ln2+xHf2−xO7−δ (Ln = Sm-Gd; x = 0, 0.096) pyrochlores

Abstract

The electrical conductivity of new solid electrolytes Eu 2.096 Hf 1.904 O 6.952 and Gd 2 Hf 2 O 7 have been compared with those for different pyrochlores including titanates and zirconates Ln 2+x М 2−x O 7−δ (Ln = Sm-Lu; M = Ti, Zr; x= 0−0.81). Impedance spectroscopy data demonstrate that Eu 2.096 Hf 1.904 O 6.952 and Gd 2 Hf 2 O 7 synthesized from mechanically activated oxides have high ionic conductivity, comparable to that of their zirconate analogues. The bulk and grain-boundary components of conductivity in Sm 2.096 Hf 1.904 O 6.952 (Т synth =1600ºС), Eu 2.096 Hf 1.904 O 6.952 and Gd 2 Hf 2 O 7 (Т synth =1670ºС) have been determined. The highest bulk conductivity is offered by the disordered pyrochlores prepared at 1600ºC and 1670ºC:~1.5 × 10 −4 S/cm for Sm 2.096 Hf 1.904 O 6.952 , 5×10 −3 S/cm for Eu 2.096 Hf 1.904 O 6.952 and 3×10 −3 S/cm for Gd 2 Hf 2 O 7 at780ºС, respectively. The conductivity of the fluorite-like phases at the phase boundaries of the Ln 2+x М 2−x O 7−δ (Ln = Eu, Gd; M = Zr, Hf; x~0.286) solid solutions, as well as that of the high-temperature fluorite-like phases Ln 2+x М 2−x O 7−δ (Ln = Eu, Gd; M = Zr, Hf; x = 0−0.286), is lower than the conductivity of the disordered pyrochlores Ln 2+x М 2−x O 7−δ (Ln = Eu, Gd; M = Zr, Hf; x=0−0.096).

The organic residue and synthesis of LaFeO3 by combustion of citrate and nitrate precursors

“…This is depicted in the shift of the relaxation peak towards higher frequencies in our case. It is generally found that the specific resistance associated with the grain interiors increases with decrease in grain size [49,50]. Grain boundaries can be visualized as positively/negatively charged core structures sandwiched between oppositely charged space charge layers extending into the adjacent grain interiors.…”

Evolution of Oxygen–Ion and Proton Conductivity in Ca-Doped Ln2Zr2O7 (Ln = Sm, Gd), Located Near Pyrochlore–Fluorite Phase Boundary

“…Therefore, an important task is to find alternative materials, which have higher or similar proton conductivity compared with well-known BaZr 0.8 Y 0.2 O 3−δ perovskite proton conductor, but the problem of low grain-boundary conductivity, which limits the total conductivity of BaZr 0.8 Y 0.2 O 3−δ , should disappear in these new materials. Although there are numerous experimental studies discussing the effect of chemistry and/or disorder on ionic conductivity of pyrochlores [2,3,4,5,6,7,8,9,10,11,12,13,14], there are only a few that examine the impact of GBs (grain boundaries) on oxygen and proton diffusion in pyrochlores [15,16,17,18].…”

Electrical and thermophysical properties of mechanochemically obtained lanthanide hafnates

“…Therefore, they are under consideration as host phases for the immobilization of high level radioactive waste . In contrast, the information available about the structural peculiarities and physical properties of similar hafnates Ln 2 Hf 2 O 7 , is rather limited . Hf is considered in many aspects, chemically identical to Zr; similarities also extend to the chemistry of their oxides, HfO 2 and ZrO 2 .…”

“…8,9 In contrast, the information available about the structural peculiarities and physical properties of similar hafnates Ln 2 Hf 2 O 7 , is rather limited. [10][11][12][13][14][15] Hf is considered in many aspects, chemically identical to Zr; similarities also extend to the chemistry of their oxides, HfO 2 and ZrO 2 . However, they present significant differences in physical properties since both Hf 4+ and Zr 4+ , have similar ionic radii (0.71 A vs 0.72 A for sixfold coordination, respectively 16 ), but very different electronic density (atomic numbers 72 vs 40): for example, hafnium has a high absorption cross section for thermal neutrons, more than 500 times that of zirconium.…”