Effect of isotopic composition and microstructure on the crystalline and magnetic phase states in R0.5Sr0.5MnO3

The results of structural neutron experiments on determining crystal and magnetic phase states of perovskite-like manganites R0.5Sr0.5MnO3 (R = 152Sm, Nd0.772Tb0.228, and Nd0.544Tb0.456) are reported. Experiments are carried out for revealing microscopic factors responsible for the giant oxygen isotope effect that was discovered recently in Sm1-xSrxMnO3 for x ∼ 0.5. It is shown that separation into two crystal phases P1 and P2 with the same spatial symmetry but different types of Jahn-Teller distortions in MnO6 octahedra and magnetic ordering of Mn atoms takes place in all studied compounds at low temperatures. Structural analysis has been carried out successfully owing to exceptionally large differences in the unit cell parameters of the coexisting phases. The P1 phase is ferromagnetic and MnO6 octahedra are distorted only slightly. The P2 phase is antiferromagnetic (A-type ordering) and MnO6 octahedra are strongly compressed in the apical direction. The relative volumes occupied by the P1 and P2 phases depend on the mean radius of the A cation, and the replacement of 16O by 18O results in their redistribution in favor of the P2 phase. The results unambiguously point to the percolation nature of the metal-insulator transition in a Sm-containing compound upon isotopic substitution of oxygen due to a sharp decrease (from 65 to 13%) in the fraction of ferromagnetic phase P1. In all investigated compounds, the ordered magnetic moment of manganese Mn in the P1 and P2 phases varies from 1.7μB to 3.5μB. The data on the evolution of the miscrostructure parameters during a phase transition to the stratified state indicate that the initial spread in the A cation radii, as well as the internal microstrains, produce a critical effect on the formation of mesoscopic phase separation