Geometrical Spin Frustration of Unusually High Valence Fe5+in the Double Perovskite La2LiFeO6.

A double perovskite-structure oxide La2LiFeO6 with unusually high-valence Fe5+ was synthesized using a high-pressure technique. The Li+ and Fe5+ ions at the B site in the rhombohedral R3 perovskite structure are ordered in a rock salt manner, and the resultant tetrahedral network of Fe5+ gives geometrical spin frustration, which is consistent with a large frustration index f (|θ|/TN) ≈ 10. Mg2+ substitution for Li+ produces Fe4+ from some Fe5+ and changes the magnetic properties. The Weiss temperature is increased from −119 to 21 K by the substitution of only 1%, significantly decreasing the frustration index. The geometrical frustration of the Fe5+ spin sublattice cannot be tolerant for even a very small amount of Fe4+ disturbance.