The Gd2−xMgxZr2O7−x/2 Solid Solution: Ionic Conductivity and Chemical Stability in the Melt of LiCl-Li2O

Abstract

Materials with pyrochlore structure A2B2O7 have attracted considerable attention owing to their various applications as catalysts, sensors, electrolytes, electrodes, and magnets due to the unique crystal structure and thermal stability. At the same time, the possibility of using such materials for electrochemical applications in salt melts has not been studied. This paper presents the new results of obtaining high-density Mg2+-doped ceramics based on Gd2Zr2O7 with pyrochlore structure and comprehensive investigation of the electrical properties and chemical stability in a lithium chloride melt with additives of various concentrations of lithium oxide, performed for the first time. The solid solution of Gd2−xMgxZr2O7−x/2 (0 ≤ x ≤ 0.10) with the pyrochlore structure was obtained by mechanically milling stoichiometric mixtures of the corresponding oxides, followed by annealing at 1500 °C. The lattice parameter changed non-linearly as a result of different mechanisms of Mg2+ incorporation into the Gd2Zr2O7 structure. At low dopant concentrations (x ≤ 0.03) some interstitial positions can be substituted by Mg2+, with further increasing Mg2+-content, the decrease in the lattice parameter occurred due to the substitution of host-ion sites with smaller dopant-ion. High-density ceramics 99% was prepared at T = 1500 °C. According to the results of the measurements of electrical conductivity as a function of oxygen partial pressure, all investigated samples were characterized by the dominant ionic type of conductivity over a wide range of pO2 (1 × 10–18 ≤ pO2 ≤ 0.21 atm) and T < 800 °C. The sample with the composition of x = 0.03 had the highest oxygen-ion conductivity (10−3 S·cm−1 at 600 °C). The investigation of chemical stability of ceramics in the melt of LiCl with 2.5 mas.% Li2O showed that the sample did not react with the melt during the exposed time of one week at the temperature of 650 °C. This result makes it possible to use these materials as oxygen activity sensors in halide melts.

Equilibrium of intrinsic and impurity point defects in Ca-doped Sm2Zr2O7

“…A small effect of pO 2 on the conductivity is seen at all temperatures studied. Analogous dependencies can be found in the literature for Gd 2−x M x Zr 2 O 7−δ (M = Li, Mg) systems [8,9]. Taken into account the specifics of disorder in the studied pyrochlore-like [46] data are also given.…”

“…Due to their good thermomechanical characteristics, compositions such as Gd 2 Zr 2 O 7 , and (La 1−x Gd x ) 2 Zr 2 O 7 are recommended as thermal barrier coating materials [1][2][3][4][5][6], which is one of the promising applications of pyrochlores. Having high chemical stability, pyrochlores are used as electrolytes for oxygen sensors for aggressive environments, such as Li-based melts [7][8][9]. High radiation resistance allows pyrochlores to be used as materials for the encapsulation of nuclear waste [10,11].…”

Electrical Properties and Chemical Resistance of the Composites (1-x)Gd2Zr2O7·xMgO in Li-Containing Chloride Melts

“…The introduction of magnesium is accompanied by an increase in the degree of order of 0.95Gd 2 Zr 2 O 7 -0.05MgO by 11.4% compared to the undoped sample. In our previous work [31], it was shown that the intro-duction of magnesium into the structure could be achieved at an amount of 0.05 wt.%, and the calculations were conducted using the Rietveld method. The calculations performed for the two different models of 0.95Gd 2 Zr 2 O 7 -0.05MgO sample presented in Tables 1 and 2 show close results regarding the obtained calculation errors.…”

“…It has been established [31] that the homogeneous magnesium doping of Gd 2 Zr 2 O 7 (solid-solution Gd 2-x Mg x Zr 2 O 7-x/2 ) can prevent the ion exchange of high-density ceramics with LiCl-xLi 2 O melts up to a Li 2 O concentration of 4 wt.%. Thus, a small concentration of magnesium is assumed to be able to protect the material from interaction with the melt [32].…”