Anomalous lattice softening at X=0.19 and 0.82 in La1-xCaxMnO3

The longitudinal sound velocity nu (s)(T) has been measured for La1-xCaxMnO 3 crystals, and anomalous softenings of nu (s)(T) have been observed at the peculiar Ca concentrations of X = 0.19 and X = 0.82. For the X = 0.19 samp le, in which the ferromagnetic (FM) metallic phase competes with the FM ins ulating phase with a polaron ordering, the anomalous nu (s)(T) softening oc curs immediately below the FM transition temperature T-c. For the X = 0.82 sample, in which the charge-ordered (CO) phase competes with the canted ant iferromagnetic (CAF) phase, the deep local minimum of nu (s) (T) occurs aro und the CO transition temperature T-CO These anomalous nu (s) (T) softening s result from the giant lattice fluctuations which originate from phase com petitions and suggest strong charge-lattice and spin-lattice coupling in th is system.