Theoretical Study on the Negative Thermal Expansion Perovskite LaCuFeO: Pressure-Triggered Transition of Magnetism, Charge, and Spin State.

The A-site ordered negative thermal expansion material LaCuFeO (LaCFO) was comprehensively investigated by using first-principles calculations. A pressure-triggered crystal structural phase transition from space group Im3̅ (No. 204) to Pn3̅ (No. 201) and magnetic transformation from a G-type antiferromagnetic (G_AFM) ground state to ferrimagnetic (FerriM) coupling were observed in LaCFO via gradual compression of the equilibrium volume. Correspondingly, the Fe-Cu intersite charge transfer from Fe to Cu 3d orbital, expressed as 4Fe + 3Cu → 4Fe + 3Cu, was simulated along with the magnetic phase transformation from the G_AFM configuration to the FerriM state. Intriguingly, the Fe charge disproportionation, formulated as 8Fe → 5Fe + 3Fe, appeared and was attributed to the strong hybridization between Fe 3d and O 2p orbitals in the FerriM state when the volumes were substantially compressed up to less than or equal to 80%V. Meanwhile, the external hydrostatic pressure also leads to a spin flip from a high-spin Fe antiferromagnetically arranged LaCuFeO Mott insulator at low pressure and goes through a FerriM LaCuFeO half-metal to a low-spin FerriM coupled LaCuFeFeO metal at high pressure. Therefore, the crossover from high spin to low spin is responsible for the charge disproportionation in LaCFO. Essentially, the charge transfer and spin flip originate from the discontinuous changes of metal-oxygen bond lengths and angles in the compressed atomic structure. Finally, the negative thermal expansion behavior and mechanism of LaCFO were theoretically examined and clearly revealed.