Ultrasonic study of Jahn–Teller effect in charge‐ordered Nd1−xCax MnO3 (x = 0.4–0.8) manganites

Abstract The ultrasonic, magnetic and transport properties of charge‐ordered manganites Nd1−x Cax MnO3 (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 (T CO) and dramatic stiffening below T CO were observed for all samples. This feature gives clear evidence for strong electron–phonon coupling in these compounds. The Jahn–Teller coupling energy E JT increases with increasing Ca concentration x, and reaches the maximum at around x = 0.6, which is similar in characteristics to the T CO. The results imply that the charge‐ordering in Nd1−x Cax MnO3 is dominated by the Jahn–Teller effect. Our analyses suggested that the type of cooperative Jahn–Teller distortion be Q 2‐mode for x ≤ 0.6, and change to Q 3 mode at around x = 0.6 in the CO state. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)