Kinetic arrest of the first-order R 3-bar c to Pbnm phase transition in supercooled LaxMnO3+δ (x = 1 and 0.9)

We report the occurrence of kinetic arrest of the first-order phase transition from R 3-bar c to Pbnm in supercooled LaxMnO3±δ (x = 1 and 0.9, i.e. δ > 0.125). Structural studies have been done, employing low temperature transmission electron microscopy (LT-TEM) and low temperature x-ray diffraction (LT-XRD) techniques. No phase transformation was observed even in LaxMnO3±δ aged for ∼12 h at 98 K. The evidence of the occurrence of kinetic arrest was realized at low temperatures through in situ electron beam triggered nucleation and perpetual devitrification of the R 3-bar c phase into a Pbnm phase. It was clearly evidenced that the R 3-bar c structure of LaxMnO3±δ, below its ferromagnetic transition temperature, is metastable and prone to be transformed to a Pbnm orthorhombic structure following initiation by an electron beam trigger. The electron beam transformed Pbnm phase was found to transform back to the R 3-bar c phase through a first-order phase transition occurring close to the ferromagnetic to paramagnetic transition (Tc) during heating. The glass-like kinetics of the arrested R 3-bar c phase has been investigated through resistance relaxation measurements, showing a decreasing logarithmic rate of decay of the arrested R 3-bar c phase towards the stable Pbnm phase with decreasing temperature, down to 5 K. On the basis of the correlations observed in the resistance-versus-temperature, magnetization-versus-temperature, magnetization-versus-field, resistance relaxation and LT-XRD measurements, the occurrence of kinetic arrest has been attributed to the suppression of Jahn-Teller distortion by double exchange across the insulator-metal transition. (paper)