On-line phase transitions of bulk and nanocrystalline La1-xPbxMnO3 (x=0.3, 0.4, and 0.5) perovskite manganite materials using ultrasonic measurements

Bulk and nanostructured La1-xPbxMnO3 perovskite manganite samples for different Pb contents (x=0.30, 0.40 and 0.50) were synthesised using solid-state reaction and sonochemical methods, respectively. XRD, FTIR, SEM, EDAX, BET and TEM studies were carried out to characterise the prepared bulk and nanosamples. The XRD characterization studies confirmed that the prepared bulk and nano perovskite manganite samples were crystalline in nature. The SEM and TEM studies revealed that the particle sizes of bulk perovskite samples is in the range of 0.75-85 mu m, whereas it ranges from 50 to 90 nm in the case of nano perovskite samples. Further, it has been confirmed that there is an increase in particle size with an increase in Pb content both in bulk and nano perovskite samples. In addition, ultrasonic velocities and attenuations of bulk and nanosamples were measured using the on-line ultrasonic through transmission method. The temperature-dependent ultrasonic parameters showed an interesting anomaly in both bulk and nano perovskite samples. The observed marked softening and hardening in sound velocities and attenuation were related to ferromagnetic to paramagnetic phase transition temperature (T-C). In nano perovskites samples, the existence of diffused phase transition took place at a lower temperature compared with the corresponding bulk samples. The observed broad ferromagnetic (FM)-paramagnetic (PM) transition in nano perovskites confirms the absence of sharp transition, which is attributed to the coexistence of orbitally, ordered and disordered phases. The spin-phonon and electron-phonon couplings are known to exist because of single ion magnetostriction and dynamic Jahn-Teller effect, respectively, are evidenced by the observed anomalies in both velocities and attenuations. (C) 2012 Elsevier B.V. All rights reserved.