Charge transport mechanisms in layered perovskite ACr2O7 (A = Be, Mg, Ca, Ba) structures: DFT perspectives

Abstract

First-principles calculations were performed to analyze the effects of oxygen vacancies (V_o) and the combined effect of V_o and copper doping on layered perovskites ACr₂O₇ (A = Be, Mg, Ca, Ba), to enhance the performance of resistive switching (RS) materials in non-volatile and low-power memory devices using PBE-GGA and HSE06 functionals. This study examined the structural, thermodynamic, elastic, electronic and optical properties. Formation energy is calculated to check the stability of all studied structures. Negative values of formation energy confirm that all studied layered perovskites is stable. Formation energy of BaCr₂CuO₇+V_o layered perovskite is lowest (−2.72 eV) as compared to other studied structures. Moreover, phonon calculation is also performed to confirm the thermodynamic stability of all investigated layered perovskites. Absence of negative frequency at gamma point confirms the thermodynamic stability of all studied structures. Elastic constant are calculated to check mechanical stability of all structures. Result of mechanical properties show that all layered perovskite structures is ductile, making these structures promising candidate for the fabrication of nanodevice. Electronic properties are calculated using PBE-GGA and HSE06 functionals, and compare the results with each other. HSE06 functional gives more accurate results as compared to PBE-GGA. By using the HSE06 functional, band gaps of Be-based compounds are 2.31 eV (BeCr₂O₇), 0.51 eV (BeCr₂O₇+V_o), and 0.15 eV (BeCr₂CuO₇+V_o). In the case of Mg-based systems, the corresponding band gaps are 2.70 eV, 0.80 eV, and 0.60 eV, respectively. The Ca-based structures exhibit band gaps of 2.61 eV for pristine CaCr₂O₇, 2.52 eV for CaCr₂O₇+V_o, and 0.30 eV for CaCr₂CuO₇+V_o. Lastly, for Ba-based compounds, the calculated band gaps are 2.66 eV (BaCr₂O₇), 0.96 eV (BaCr₂O₇+V_o), and 0.098 eV (BaCr₂CuO₇+V_o). These results clearly demonstrate that the introduction of oxygen vacancies leads to a reduction in the band gap, while the combined effect of copper doping and oxygen vacancies causes a more pronounced decrease. Among all the studied configurations, the BaCr₂CuO₇+V_o layered perovskite exhibits the lowest band gap. Optical properties are carried out using PBE-GGA, and HSE06 functional. Among all investigated layered perovskites, BaCr₂CuO₇+V_o exhibits the lowest band gap, along with high structural, mechanical, and thermodynamic stability, as well as high conductivity and absorption coefficient. These combined features indicate that BaCr₂CuO₇+V_o is promising candidate material for Resistive Random Access Memory (RRAM) applications.