Magnetic, structural, and transport properties at very high temperature in manganites

AbstractMagnetic, structural, and electric transport measurements at high temperatures were carried out on La1−xCaxMnO3; x=0.20, 0.25, 0.30, 0.34, 0.40, and 0.45. All samples show a first-order structural phase transition from orthorhombic Pnma to rhombohedral R3¯c space group at TR–O. Magnetic susceptibility measurements show that the Curie–Weiss law is strictly obeyed in the rhombohedral phase as opposed to the orthorhombic phase where the effective magnetic moment has a temperature dependence. The electrical resistivity is well described by the small polaron hopping mechanism in the samples up to x=0.34. As the charge carriers are introduced into the system (x=0.40 and 0.45), this mechanism of hopping ceases to be valid. The value of Grüneisen parameter obtained through analysis of high-resolution X-ray powder diffraction as a function of temperature increases abruptly for the sample with x=0.40. This is consistent with an increase in bending and stretchinglike frequency modes observed by Raman spectroscopy.