Ultrasonic anomalies in La1/3Sr2/3Fe1−xCoxO3 perovskites

The longitudinal ultrasonic velocity (Vl), as well as resistivity has been measured in single-phase polycrystalline La1/3Sr2/3Fe1−xCoxO3 (x = 0, 0.025, 0.075, 0.10, 0.15) at a frequency of 10 MHz, from 20 to 300 K. It is found that with increasing Co content, the resistivity decreases and the charge ordering transition temperature TCO shifts to lower temperature. For all samples, upon cooling down from 300 K, a substantial softening in Vl above TCO and dramatic stiffening below TCO are observed, which implies extremely strong electron-phonon coupling in this system. Below TCO, another softening in Vl is observed for x = 0, which is attributed to the breathing-type distortion of Fe-O octahedron. With Co doping, this anomaly weakens and finally disappears for x = 0.15. This behavior suggests that the charge disproportionation (CD) transition is suppressed by the Co substitution.