Vertical boundary at (formula presented) in the structural phase diagram of the (formula presented) system (formula presented)

The structural phase diagram of the (formula presented) system in the compositional range (formula presented) has been investigated by high-resolution synchrotron x-ray powder diffraction techniques between (formula presented) 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 (formula presented) 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 (formula presented) 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 (formula presented) region with a near-vertical phase boundary just below (formula presented) The new phases indicate the presence of some novel type of orbital ordering unlike that found in (formula presented)