High‐Temperature Thermoelectric Behavior of Alkaline Earth Metal–Doped Spinel Zinc Ferrites

Developing robust ceramic thermoelectric materials that maintain stable performance at high temperatures remains a critical challenge. This article reports the first systematic investigation of the structural, morphological, and thermoelectric properties of alkaline earth metal–doped spinel ferrites, ZnFe 1.975 M 0.025 O 4 (M = Mg, Ca, Sr, and Ba), synthesized via the sol–gel method. Among the compositions studied, Mg proved the most effective dopant, with ZnFe 1.975 Mg 0.025 O 4 exhibiting the highest electrical conductivity of 525 S/m and a maximum power factor of 20 µW/mK 2 at 850 K. An enhanced figure of merit ( z T) of 3.52 × 10 − 3 was achieved at 750 K. This improvement is attributed to the substitution of lower valent dopants at higher valent cation sites, which generates oxygen vacancies and substitutional point defects. The key novelty of this work lies not only in providing a comprehensive comparison of alkaline earth metal dopants in spinel zinc ferrite under identical synthesis conditions, but also in showing that ionic radii–driven site occupancy, rather than aliovalent charge compensation alone, governs oxygen vacancy formation and carrier generation across the dopant series in ZnFe 2 O 4 . These findings offer valuable insights into dopant chemistry, defect engineering design rules, and charge transport, supporting the optimization of high‐performance spinel zinc ferrite oxide ceramics.