Effects of Cr substitution and oxygen reduction on elastic anomaly and ultrasonic velocity in charge-ordered Nd0.5Ca0.5Mn1−xCrxO3−δ ceramics

Charge-ordered Nd0.5Ca0.5Mn1−xCrxO3−δ ceramics have been investigated by electrical resistivity, AC susceptibility and ultrasonic velocity measurements to elucidate the effects of Cr substitution and oxygen reduction on charge ordering (CO). Resistivity and susceptibility measurements showed that the x=0 sample exhibits insulating behavior and an anti-ferromagnetic transition at 230 K as well as a CO transition at 280 K. The substitution of Cr induces a ferromagnetic–paramagnetic and metal–insulator (MI) transition as well as gradually suppressing the CO state due to weakening of the Jahn–Teller (JT) effect. Quenching to reduce the oxygen content of the x=0.05 sample caused the MI transition temperature to shift to lower temperatures most likely due to oxygen reduction. On the other hand, both longitudinal and shear velocities at 100 K increased significantly with Cr substitution indicating improvement in elastic properties. However, quenching the x=0.05 sample slightly deceased both velocities and related elastic moduli. A step-like longitudinal velocity anomaly characterized by a slope change suggests the existence of CO state for x=0, 0.02 and 0.05 samples. The step-like anomaly shifts to lower temperatures from 266 K (x=0) to 222 K (x=0.05) with Cr substitution indicating a weakening of the CO state. Absence of the step-like anomaly for the quenched x=0.05 sample suggests suppression of the CO state due to oxygen reduction. Analysis of the step-like anomaly using the mean-field theory suggests involvement of the JT effect which transforms from dynamic to static type with decreasing temperature. Cr substitution weakened the CO state as a result of weakening the JT effect while reducing the oxygen content suppresses the CO state as a result of oxygen reduction.