Vertical boundary atx≈0.11in the structural phase diagram of theLa1−xSrxMnO3system(0.08<~x<~0.125)

Abstract

The structural phase diagram of the ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3}$ system in the compositional range $0.08<~x<~0.125$ has been investigated by high-resolution synchrotron x-ray powder diffraction techniques between $20\ensuremath{-}600 \mathrm{K}.$ Recent studies have reported that there is an unusual reentrant-type phase transition in this range involving an abrupt change in lattice parameters but no change in the crystal symmetry, which remains orthorhombic $\mathrm{Pbnm}.$ The transition to the reentrant phase is from a ferromagnetic metallic to a ferromagnetic insulating phase with some unusual properties. Our results demonstrate that for samples with $x=0.11\ensuremath{-}0.125$ there exist two lower-symmetry structural regions having monoclinic and triclinic symmetry, respectively. There is a sharp first-order transition from the monoclinic to the triclinic phase coinciding with the transition to the ferromagnetic insulating phase, and an abrupt crossover from the orthorhombic $\mathrm{Pbnm}$ region with a near-vertical phase boundary just below $x=0.11.$ The new phases indicate the presence of some novel type of orbital ordering unlike that found in ${\mathrm{LaMnO}}_{3}.$