Ultrasonic study of the charge mismatch effect in charge-ordered (Nd(0.75)Na(0.25))(x)(Nd(0.5)Ca(0.5))(1-x)MnO(3)

The resistivity, magnetization and ultrasonic properties of charge-ordered polycrystalline (Nd(0.75)Na(0.25))(x)(Nd(0.5)Ca(0.5))(1-x)MnO(3) have been investigated from 50 to 300 K. A considerable velocity softening accompanied by an attenuation peak was observed around the charge-ordering transition temperature (T(CO)) upon cooling. The simultaneous occurrence of the charge ordering (CO) and the ultrasonic anomaly implies strong electron-phonon coupling, which originates from the cooperative Jahn-Teller effect. At very low temperature, another broad attenuation peak was observed, which is attributed to the phase separation (PS) and gives a direct evidence of spin-phonon coupling in the compound. With increasing x, T(CO) shifts to lower temperature, the magnetization of the system is strengthened and the PS is enhanced. The temperature dependence of the longitudinal modulus shows that the Jahn-Teller coupling energy E(JT) decreases with increasing Na content. The analysis suggests that the charge mismatch effect may be the main reason for the suppression of the CO and enhancement of the PS.