Birabar Ranjit Kumar Nanda

Ferroelectric oxides have gained research attention in the field of ferroelectric photovoltaics (PV) after the discovery of power conversion efficiency exceeding the Shockley-Queisser limit in BaTiO3 (BTO) crystals. However, advancement in this field is hindered by the wide bandgap (>3 eV) nature of ferroelectric oxides. In this work, a novel lead-free ferroelectric (1 − x)BTO − xBi(Ni2/3Nb1/3)O3 system was proposed and demonstrated to show bandgap reduction without compromising the polarization. Notably, the system displayed a bandgap reduction from 3.1 to 2.4 eV upon varying the composition from x = 0.0 to 0.05. Particularly, the optimal composition x = 0.02 showed enhancement in polarization (Pmax = 16 μC/cm2) and anomalous PV response with an open-circuit voltage of 6 V at 300 K. The origin of the bandgap reduction and polarization retention is explored experimentally by Raman spectroscopic measurements and analyzed theoretically by density functional theory. Our results revealed that the oxygen octahedral distortions and Ni2+ doping favor bandgap lowering, and Bi3+ ions stabilize the ferroelectric polarization. This study provides insight into the origin of bandgap tuning and paves the route for exploring new low-bandgap ferroelectric material with room temperature polarization.

A-site ordered perovskites, CaCu3B4O12, which are derivatives of conventional ABO3 perovskites, exhibit varying electronic and magnetic properties. With the objective of examining the role of Cu in this work, we have studied CaCu3Ti4O12 and CaCu3Zr4O12 and presented the cause of the crystallization of A-site ordered perovskite from conventional ABO3 perovskite and the underlying mechanism leading to the stabilization of nontrivial and experimentally established G-type antiferromagnetic (G-AFM) ordering in these systems. The first-principles electronic structure calculations supplemented with phonon studies show that the formation of A-site ordered perovskite is driven by Jahn-Teller distortion of the CuO12 icosahedron. The crystal orbital Hamiltonian population analysis and magnetic exchange interactions estimated using spin dimer analysis infer that the nearest and next-nearest-neighbor interactions (J1 and J2) are direct and weakly ferromagnetic, whereas the third-neighbor interaction (J3) is unusually strong and antiferromagnetic driven by an indirect superexchange mechanism. The structural geometry reveals that stabilization of G-AFM requires J1<2J2, J1<2J3. The experimental and theoretical values of Néel temperature agree well for U≈ 7 eV, highlighting the role of strong correlation. The magnetic ordering is found to be robust against pressure and strain.