Electrical properties and high-temperature stability of high-entropy pyrochlore ceramics (La0.2Nd0.2Sm0.2Gd0.2M0.2)2Sn2O7 (M = Eu, Dy, Ho, or Yb) for negative temperature coefficient thermistor applications

Understanding electrical transport mechanisms and key thermal stability factors in high-temperature thermosensitive ceramics is essential for developing highly stable temperature sensors under extreme thermal conditions. This study develops the high-entropy pyrochlore stannates (La_0.2Nd_0.2Sm_0.2Gd_0.2M_0.2)₂Sn₂O₇ (M = Eu, Dy, Ho, or Yb) via solid-state synthesis, demonstrating exceptional negative temperature coefficient (NTC) behavior across 300–1400 °C. Hall measurements and X-ray photoelectron spectroscopy (XPS) analyses reveal a dual redox mechanism for these ceramics: partial reduction of Sn⁴⁺ to Sn²⁺ at B-sites and rare-earth ions (Sm/Eu/Yb) to RE²⁺ at A-sites, generating hole-dominated p-type conductivity. The materials exhibit outstanding thermal stability, with resistance drift below 1.96 % after 500 h at 1400 °C, attributed to the entropy-stabilized structure that effectively suppresses lattice disorder transformation. Aging studies further demonstrate that high oxygen vacancy concentrations lead to resistance drifts, as oxygen filling at elevated temperatures oxidizes RE²⁺ to RE³⁺, reducing charge carriers and ultimately increasing resistivity. This study elucidates for the first time the hole-dominated conduction mechanism arising from dual-site cation valence changes (Sn/RE) and the regulation effect of oxygen vacancies on high-temperature stability, which provides an important theoretical basis and material design strategy for the development of the next-generation high-temperature NTC sensors with excellent stability.