Robust Unusually High Valence Fe⁵⁺ State and 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 B-site-ordered double perovskites La2–xCaxLiFeO6 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 A site, as seen in the actual composition La2–xCaxLiFeO6–0.5x. Introducing oxygen vacancies is a key factor in this discovery, as it significantly weakens the strong antiferromagnetic interaction for x = 0, transforming it into a weak ferromagnetic one for x > 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 B-site-ordered double perovskites.