We have synthesized single-crystalline and pure light rare earth oxides REO (RE = La–Sm) with rocksalt structure for the first time (Fig. 1 right). However, it was very difficult to synthesize pure heavy REO (RE = Gd and heavier lanthanoids), because a significant amount of thermodynamically stable RE2O3 (Fig. 1 left) was also formed owing to a large lattice mismatch between the REO film and the single crystal substrate (Fig. 2 left). In order to reduce the lattice mismatch, we developed a lattice-constant-tunable rocksalt (Ca,Sr)O buffer layer, where the lattice constant is a function of Ca/Sr ratio. As a result, we are able to synthesize single crystalline and pure heavy REO for the first time (Fig. 2 right). These heavy REO possess significantly high electrical conduction and ferromagnetic transition temperatures among rare earth simple compounds (Fig. 3).
Fig. 1. The crystal structure of RE2O3 (left) and REO (right).
Fig. 2. The previous synthesis method of heavy REO directly on a substrate (left) and the improved synthesis method of REO on (Ca,Sr)O buffer layer (right).
Fig. 3. The ferromagnetic transition temperature of rocksalt-type REO (RE = Gd−Er) with the divalent RE ions (red) and rocksalt-type RE nitrides REN with the trivalent RE ions (blue).
