Observation of charge–transfer–driven antiferroelectricity in 3d-pyrochlore multiferroic Cu₂OCl₂

Very recently, multiferroic behavior close to the boiling temperature of liquid nitrogen was reported in oxohalide-based Cu₂OCl₂. In this study, we have established yet another novel mechanism for the origin of charge ordering in multiferroic Cu₂OCl₂. The direct current (DC) magnetization, specific heat, and neutron diffraction measurements confirm the antiferromagnetic (AFM) ordering (T_N) of Cu₂OCl₂ near 70 K. The collinear AFM spin structure in Cu₂OCl₂ was obtained from neutron powder diffraction experiments based on the fitting to the combination of two irreducible representations—Г₁⊕Г_2. Electrical properties, such as dielectric constant, pyroelectric current, and polarization-electric field hysteresis-loop measurements, together suggest a long-range antiferroelectric ordering near T_E ≈ 75 K. The charge transfer from Cl to O along the c-axis was found to induce antiferroelectricity near T_E and is supported from ab initio calculations. This finding largely reduces the possibility of well-known Dzyaloshinskii–Moriya interaction in the observed multiferroic behaviors and provides insight into the various origins of high-T_C multiferroics.