Spin-state transition in La1−xSmxCoO3 perovskites

Abstract

Structural and magnetic properties of La1−xSmxCoO3 with x=0, 0.02, 0.04, 0.06, 0.08, 0.12, 0.16, and 0.2 have been systematically studied. LaCoO3 exhibits a rhombohedral structure of R3̄C symmetry. The lattice parameters decrease progressively with the incorporation of Sm, and a second orthorhombic phase of Pbnm symmetry appears for x⩾0.12. The volume fraction of the orthorhombic phase is a sensitive function of the Sm content in the compounds, and increases from 16% for x=0.12 to 75% for x=0.2. Accordingly, the low-spin-to-high-spin transition of the trivalent Co ions, characterized by a steep rise of the magnetization, shifts to higher temperatures. A molecular field analysis indicates an increased energy gap, from 317 K for x=0 to 500 K for x=0.2, between the low-spin and the high-spin state, implying a stabilization of the low-spin trivalent Co ions by the presence of Sm. The enhanced compressive chemical pressure due to the introduction of Sm is suggested to be responsible for the present observations.