Ultrasonic study of Jahn-Teller effect in charge-ordered Nd1-xCaxMnO3 (x=0.4-0.8) manganites

The ultrasonic, magnetic and transport properties of charge-ordered manganites Nd1-xCaxMnO3 (x=0.4, 0.5, 0.6, 0.7 and 0.8) were studied between 15 and 300 K. Resistivity and magnetization measurements show that all the samples undergo a charge-ordering transition at low temperature. Upon cooling from 300 K, an obvious softening of sound velocity above the charge-ordering transition temperature (TCO) and dramatic stiffening below TCO were observed for all samples. This feature gives clear evidence for strong electron-phonon coupling in these compounds. The Jahn-Teller coupling energy EJT increases with increasing Ca concentration x, and reaches the maximum at around x=0.6, which is similar in characteristics to the TCO. The results imply that the charge-ordering in Nd1-xCaxMnO3 is dominated by the Jahn-Teller effect. Our analyses suggested that the type of cooperative Jahn-Teller distortion be Q2-mode for x≤0.6, and change to Q3 mode at around x=0.6 in the CO state. (copyright 2006 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)