Pressure-induced suppression of Jahn–Teller distortions and enhanced electronic properties in high-entropy oxide (Mg0.2Ni0.2Co0.2Zn0.2Cu0.2)O

Abstract

Jahn-Teller distortion commonly exists in Cu-containing complex oxides and remarkably changes the electronic property. However, how it functions to pressure in high-entropy oxides (HEOs) remains unknown. Here we studied pressure engineering on quenching the Jahn-Teller distortion in the (Mg 0.2 Ni 0.2 Co 0.2 Zn 0.2 Cu 0.2 )O HEO and its effect on the electronic structure. Synchrotron X-ray diffraction demonstrate that the local structural distortion of the CuO 6 octahedral sublattice in the rocksalt-type (Mg 0.2 Ni 0.2 Co 0.2 Zn 0.2 Cu 0.2 )O HEO is progressively reduced and the distorted structure evolves into a nearly ideal form with increasing pressure up to 40 GPa. Alternating current impedance and ultraviolet-visible absorption reveal a dramatic resistance drop by more than 3 orders of magnitude and an obvious bandgap decrease of ~0.1 eV, accompanied by the pressure-induced quenching of the Jahn-Teller distortion in the (Mg 0.2 Ni 0.2 Co 0.2 Zn 0.2 Cu 0.2 )O HEO. Our study presents a high-pressure route for tuning the local structural distortion and electronic structure of Cu-containing HEOs for optimizing the materials functionality.

Electrical Conductivities and Conduction Mechanism of Lithium-Doped High-Entropy Oxides at Different Temperature and Pressure Conditions

“…The resistance of the undoped HEO decreases by ∼3 orders of magnitude upon compression from ∼0 to 43 GPa . By comparison, the resistance/resistivity of the Li x -HEO ( x = 0.065, 0.117, and 0.200) drops only by ∼1 order of magnitude upon compression from ∼0 to 50 GPa (Figure a–c).…”

“…In a prior work, we determined the electrical resistance of an undoped HEO from ∼2 to 43 GPa . It was found that the resistance remains almost unchanged below ∼20 GPa, while it drops markedly above ∼20 GPa.…”

“…The resistance of the undoped HEO decreases by ∼3 orders of magnitude upon compression from ∼0 to 43 GPa. 21 By comparison, the resistance/resistivity of the Li x -HEO (x = 0.065, 0.117, and 0.200) drops only by ∼1 order of magnitude upon compression from ∼0 to 50 GPa (Figure 4a−c). This is likely due to an enhanced carrier concentration due to a decrease in the band gap after Li doping (Figure 2b), which makes the electrical conduction in a Li x -HEO less sensitive to pressure than that in an undoped HEO.…”

“…In these prior works, the electrical conductivities were measured at ambient pressure and at room temperature or in a relatively narrow temperature range (e.g., −40 to 100 °C). Recently, we studied the electrical conductivity of undoped (MgCoNiCuZn)O at pressures up to ∼45 GPa at room temperature . It was found that above ∼22 GPa, the electrical conductivity could be significantly increased by external compression.…”

Nanoscale chemical and structural investigation of solid solution polyelemental transition metal oxide nanoparticles

“… 9 , 10 Yan et al. 11 showed that copper occupied at octahedral sites (CuO 6 ) in the high entropy oxide (HEO) nanoparticles can reduce local structural distortions on application of compressive force (∼40 GPa), allowing the delocalization of the higher energy state (e g ) electrons into the stable state in the d-orbitals. In addition, the rapid heating and cooling rates promoting the formation of entropy stabilized nanomaterials, facilitate the chemical homogeneity in the high entropy structures by tuning the thermodynamic mixing kinetics leading to the single-phase solid solution emergence.…”