Structural change and charge ordering correlated ultrasonic anomalies in La1-xCaxMnO3 (x=0.5,0.83) perovskite - art. no. 024427

Ultrasonic sound velocity and attenuation have been measured in polycrystal line manganese oxide La(1-x)Ca(x)MnO3 (x = 0.5,0.83,1.0) at a frequency of 10 MHz. For x = 0.5, on cooling down from high temperature, a slight soften ing of the sound velocity above the charge ordering transition temperature T-CO and dramatic stiffening below T-CO coincided with big attenuation peak s for both longitudinal and transverse waves were observed. It was found th at these ultrasonic anomalies near T-CO are correlated with the fine struct ure (i.e., the lattice parameters) change caused by the Jahn-Teller effect. For x=0.83, the sound velocity starts to soften dramatically with decreasi ng temperature from higher temperature to T-S (180 K), and stiffens dramati cally below T-S. The large softening and stiffening of the sound velocity a ccompanied by a big attenuation peak are strongly correlated with a cubic-t o-tetragonal structural phase transition at T-S, which is confirmed by the low-temperature powder x-ray diffraction measurements. It is suggested that this structural phase transition be due to the Jahn-Teller distortion of t he Mn3+O6 octahedra and related to the charge ordering transition. For CaMn O3, the anomaly in sound velocity is small.