Wide-Range Tuning of the Mo Oxidation State in La1–xSrxFe2/3Mo1/3O3 Perovskites

La1-xSrxFe2/3Mo1/3O3 (0x1) perovskites allow exceptionally wide tuning of the Mo oxidation state from +3 (x = 0) to +6 (x = 1) with the Fe-III oxidation state virtually unchanged. The end members of this series show antiferromagnetic ordering in LaFe2/3Mo1/3O3 at T-N = 520 K and ferrimagnetic ordering in SrFe2/3Mo1/3O3 at T-C = 420 K. The crucial role of electron correlations in the Mo 4d shell suggests that LaFe2/3Mo1/3O3 with a half-filled band is a Mott insulator, whereas SrFe2/3Mo1/3O3 is a band insulator. In both cases, the magnetic structure is dictated by antiferromagnetic superexchange between localized magnetic moments. At intermediate compositions, an interplay of antiferromagnetic-superexchange and double-exchange interactions results in nonmonotonous variations of both the magnetic-ordering temperature and saturation magnetization.