Regulating the metal-insulator transition of REBaCo2O5+δ combining RE-substitution and anion control

Abstract

Although an intriguing metal-insulator transition (MIT) stemming from spin-state transition was discovered for cobaltite double-perovskites (REBaCo₂O_5+δ), its regulatory mechanism is yet unclear owing to the intertwined dual-determinants from cationic and anionic perspectives. Herein, we demonstrate that the occurrence of abrupt MIT for REBaCo₂O_5+δ relies on the synergistic coordination between the RE ionic-radius-induced CoO₆ octahedral distortion and the oxygen-vacancy-modulated Co valence-state for low structural symmetry. A gradual shift in the transportation behavior from metal to insulator is observed for REBaCo₂O_5+δ synthesized in air with REs from Pr to Ho, while the MIT emerges for middle REs (e.g., Sm, Eu, Gd and Tb). Nevertheless, the MIT was could be further extended to more RE compositions from an anionic perspective by annealing the insulating (RE: Dy and Ho) or metallic (RE: Pr and Nd) REBaCo₂O_5+δ at MPa-high oxygen or nitrogen pressure, respectively. Further estimation the oxygen composition via their thermopowers indicates that their MIT behaviors were coincide with a narrow distribution of δ around 0.5 and descendent structural symmetry towards Pmmm from P4/mmm. This unveils the distinctiveness in spin-state transition-driven MIT compared to the Bloch–Wilson, Mott, or Peierls transitions, providing a versatile platform for fundamental explorations beyond conventional correlated electron systems.