Ultrasonic study of the Jahn–Teller effect in La1−xCaxCoO3 (0.1 ≤ x ≤ 0.2)

The resistivity, magnetization, longitudinal ultrasonic velocity and attenuation have been measured as a function of temperature from 20 to 300 K in the single-phase polycrystalline La1−xCaxCoO3 (0.1 ≤ x ≤ 0.2). It is found that with increasing Ca content, the resistivity decreases and the ferromagnetic ordering temperature shifts to higher temperature. For all samples, the velocity decreases smoothly with decreasing temperature, and then increases piercingly, accompanied with an attenuation peak. This abnormal elastic softening can be described well by the mean-field theory, which is attributed to the Jahn–Teller effect of IS Co3+ ions. Since the increasing ferromagnetic interaction makes the localized IS Co3+ ions unstable, the temperature of velocity minimum shifts to lower temperature, which suggests that the static Jahn–Teller effect is suppressed.