Robust UnusuallyHigh Valence Fe⁵⁺ Stateand Large Magnetic Interaction Change in the Double Perovskites La_2–<i>x</i>Ca<i>_<i>x</i></i>LiFeO_{6–0.5<i>x</i>}

We successfully synthesized Ca-substituted <i>B</i>-site-ordered double perovskites La_2–<i>x</i>Ca<i>_<i>x</i></i>LiFeO₆ by using high-pressure techniques and investigated site-substitution effects on the stability of unusually high valence Fe ions and their magnetic properties. Our research revealed the robust Fe⁵⁺ state against the aliovalent substitution at the <i>A</i> site, as seen in the actual composition La_2–<i>x</i>Ca<i>_<i>x</i></i>LiFeO_{6–0.5<i>x</i>}. Introducing oxygen vacancies is a key factor in this discovery, as it significantly weakens the strong antiferromagnetic interaction for <i>x</i> = 0, transforming it into a weak ferromagnetic one for <i>x</i> > 0.3. This significant interaction change, coupled with the partial randomness of the magnetic interactions, easily releases the geometrical spin frustration of Fe⁵⁺ located on a face-centered-cubic lattice with just 5% Ca substitution. These results deepen our understanding of the site-substitution effects in geometrically frustrated <i>B</i>-site-ordered double perovskites.