High‐entropy effects enhance atomic displacements to manipulate thermal/oxygen ionic transport properties of ordered RE ₃ <i>M</i> O ₇ ceramics

Abstract The high‐entropy engineering has emerged as a novel method for manipulating various properties of ceramics. Herein, the enhancement of atomic displacements was shown to effectively tailor the thermal/oxygen ionic conductivities of high‐entropy ceramics (HECs). Although RE 3 M O 7 (RE represents rare‐earth elements; M is Ta or Nb) ceramics have been widely studied for use in thermal barrier coatings and solid oxide fuel cells, their application scope can be broadened through the effective manipulation of the thermal/oxygen ionic conductivities. Herein, (Sm 1/5 Eu 1/5 Gd 1/5 Dy 1/5 Ho 1/5 ) 3 Ta 1/2 Nb 1/2 O 7 HECs are designed and synthesized. These ceramics exhibit lower thermal conductivity and higher oxygen ionic conductivity than pristine ordered RE 3 M O 7 ceramics. Characterization results prove that high‐entropy effects in the prepared material enhance atomic displacement parameters (ADPs), causing an increase in the oxygen ionic conductivity by a factor of 10 and enhancement in the phonon scattering rate, thereby reducing thermal conductivity by increasing ADPs. The ADPs can act as an indicator of the anharmonic vibration strength of the lattice, affecting the thermal/oxygen ionic conductivity and thermal expansion coefficient. Furthermore, in nano‐indentation tests, (Sm 1/5 Eu 1/5 Gd 1/5 Dy 1/5 Ho 1/5 ) 3 Ta 1/2 Nb 1/2 O 7 HECs show high hardness (11.8 GPa), toughness (2.0 MPa·m 1/2 ), and modulus (215.0 GPa), which are beneficial for structural/functional applications. Overall, the obtained results indicate that thermal/oxygen ionic transport mechanisms can be used to further improve the properties and thereby expand the application scope of various HECs.