Effect of isotopic composition and microstructure on the crystalline and magnetic phase states in R_0.5Sr_0.5MnO₃

Abstract

The results of structural neutron experiments on determining crystal and magnetic phase states of perovskite-like manganites R_0.5Sr_0.5MnO₃ (R = ¹⁵²Sm, Nd_0.772Tb_0.228, and Nd_0.544Tb_0.456) are reported. Experiments are carried out for revealing microscopic factors responsible for the giant oxygen isotope effect that was discovered recently in Sm_1−x Sr _x MnO₃ for x ≈ 0.5. It is shown that separation into two crystal phases P ₁ and P ₂ 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 P ₁ phase is ferromagnetic and MnO₆ octahedra are distorted only slightly. The P ₂ phase is antiferromagnetic (A-type ordering) and MnO₆ octahedra are strongly compressed in the apical direction. The relative volumes occupied by the P ₁ and P ₂ 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 P ₂ 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 P ₁. In all investigated compounds, the ordered magnetic moment of manganese Mn in the P ₁ and P ₂ phases varies from 1.7μ_B to 3.5μ_B. 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.