Abstract
The longitudinal ultrasonic velocity (V), as well as resistivity and magnetization has been measured in polycrystalline R1/3Sr2/3FeO3 (R=La, Pr, Gd). When the R-site ion is changed from La to smaller-size R ion, the charge ordering (CO) transition shifts to lower temperature and finally disappears for R=Gd. Dramatic anomalies in V for the R=La, Pr compounds are observed near the CO transition temperature, which implies the electron-phonon coupling arising from the Jahn-Teller effect of Fe4+. And the theory calculation result shows that this coupling is weakened for Pr, which may lead to the shift of TCO. In the case of Gd1/3Sr2/3FeO3 with no CO transition, the anomaly in V is small, and may be attributed to the antiferromagnetic spin fluctuations. Below TCO, another unexpected softening in V is discerned in La1/3Sr2/3FeO3, while only a slope change is observed in Pr1/3Sr2/3FeO3. This feature is attributed to the breathing-type distortion of Fe-O octahedron via the charge disproportionation, which is weakened in Pr1/3Sr2/3FeO3.
Published Version
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