Successful synthesis of proton-conducting high-entropy (La0.2Nd0.2Ho0.2Lu0.2Y0.2)2ZrO5 ceramics

Abstract

Using mechanical activation of an oxide mixture containing 0.2 wt% hexagonal boron nitride, followed by high-temperature firing (1400–1500 °C), we prepared single-phase (La0.2Nd0.2Ho0.2Lu0.2Y0.2)2ZrO5 ceramic, a high-entropy oxide (HEO) analog of Gd2ZrO5, whereas we failed to obtain single-phase Gd2ZrO5 under similar conditions: the material consisted of two phases, with the fluorite and bixbyite structures, like in the case of conventional synthesis or coprecipitation in previous work. Thus, the use of the HEOs allowed us to obtain a phase-pure compound with the fluorite structure at a markedly lower synthesis temperature. An important role was played by 0.2 wt% hyperstoichiometric hexagonal BN additions, which ensured considerable amorphization of the starting oxides. The start powders and resulting ceramics were studied by X-ray diffraction, SEM analyses and conductivity was measured by impedance spectroscopy method in dry and wet air.The highest proton conductivity, ∼2.5 × 10−5 S/cm at 630 °C, was offered by the (La0.2Nd0.2Ho0.2Lu0.2Y0.2)2ZrO5 HEO ceramic. The use of HEO analogs of various compounds can be helpful for not only assessing thermodynamic properties of ceramics (reduction in synthesis and polymorphic transformation temperatures), but also preparing proton conductors when it is necessary to enhance hydrating properties of the cation sublattice.