Chemical disorder effect on orthorhombic-rhombohedral structural transition in La_0.7Ca_0.3MnO₃

High temperature X-ray powder diffraction technique was employed to study the orthorhombic to rhombohedral (O-R) structural transition (ST) in single crystal and ceramics of La_0.7Ca_0.3MnO₃. The data obtained evidence that such ST in both samples occurs in the vicinity of temperature T ∼ 700 K and demonstrates O and R phases coexistence within the T-intervals of about 20 K in crystal and ∼60 K in ceramics. This first order transition is hysteretic on heating and cooling procedures and it is shifted on ΔT ∼ 10 K to higher temperatures in crystal as compared to the ceramics of the same composition. We have described these features in the terms of the difference between "technologically driven" chemical disorder characteristic for considered samples. Thus, it seems that spatial (up to the micron size scale) fluctuations of the Ca-dopant content in crystal and "mesoscopic scale disorder" within grain boundary regions in ceramics are responsible for both: above noted difference of ST parameters and reported earlier difference in magnetic ordering of these samples.