Ultrasonic signature of Jahn–Teller effect in La1/3Sr2/3Co1−xFexO3 perovskites

The longitudinal ultrasonic velocity (V), as well as resistivity has been measured in single phase polycrystalline La1/3Sr2/3Co1−xFexO3 (x=0, 0.10, 0.3) at a frequency of 10 MHz, from 20 to 300 K. For x=0, the resistivity shows metallic behavior in the whole temperature range and a kink at 235 K was observed, which coincides with the ferromagnetic (FM) transition temperature (TC). With the Fe doping, the resistivity increases and a metal–insulator transition occurs at about 100 K for x=0.3, which is attributed to the weakening of the FM correlation and the development of antiferromagnetic interactions. For all samples, the V softens conspicuously as the temperature decreasing and then dramatically stiffens, which exhibits a valley at low temperature. The analysis suggests that this ultrasonic anomaly may correspond to the local lattice distortions via the Jahn–Teller effect of intermediate spin Co3+. Since the Fe substitution decreases Co3+ concentration and weakens this effect, the temperature of minimum sound velocity shifts to lower temperature.