Crystal structure, magnetism, and thermoelectric properties of Nd1−xSrxFeO3−δ (0.1 ≤ x ≤ 0.9)

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Polycrystalline Nd1−xSrxFeO3−δ (0.1 ≤ x ≤ 0.9) samples exhibit a single-phase perovskite-type crystal structure and G-type antiferromagnetism with small ferromagnetic order. Assuming that the Fe site is in the mixed-valence state, the spin state of Fe changes from a low-spin (LS) or intermediate spin Fe3+-dominant state at x ≤ 0.5 to a LS Fe4+ dominant state at x ≥ 0.6. This strongly suggests that the charge carriers change from holes to electrons. In fact, at temperatures less than 500 K, these samples show a p-type Seebeck coefficient for 0.1 ≤ x ≤ 0.5 and an n-type Seebeck coefficient for 0.6 ≤ x ≤ 0.9. However, for 0.6 ≤ x ≤ 0.9, Fe4+ is reduced to Fe3+ because of an oxygen deficiency in the 500–600 K temperature range. In particular, Nd0.7Sr0.3FeO2.99(1) shows the largest p-type ZT = 0.025 at 765 K, whereas compositions with high n-type ZT could not be identified.

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