Ultrasonic study on charge ordering in Bi1-xCaxMnO3 (x=0.8, 0.85)

The charge ordering (CO) transition in polycrystalline Bi1-xCaxMnO3 (x=0, 0.8, 0.85) has been studied by measuring the resistivity and longitudinal ultrasonic velocity (Vl). It is found that with increasing the Ca content, the resistivity decreases and the charge ordering transition temperature TCO shifts to lower temperature. For both samples, upon cooling down from 300 K, an obvious softening in Vl above TCO and dramatic stiffening below TCO are observed. This abnormal elastic softening above TCO can be described well by the mean-field theory, which indicates that the Jahn-Teller effect plays an important role for the charge ordering transition in Bi1-xCaxMnO3. And this effect is weakened with increasing the Ca content, which may lead to the shift of TCO.