Structural properties and phase diagram of the LaFe1−xRuxAsO system

Structural refinement, lattice micro-strain and spontaneous strain analyses have been carried out on selected members of the La(Fe1−xRux)AsO system using high-resolution neutron and synchrotron powder diffraction data. The obtained results indicate that the character of the tetragonal to orthorhombic structural transition changes from first order for x = 0.10, possibly to tricritical for x = 0.20, up to second order for x = 0.30; for x ≥ 0.40 symmetry breaking is suppressed, even though a notable increase of the lattice micro-strain develops at low temperature. By combining structural findings with previous muon spin rotation data, a phase diagram of the La(Fe1−xRux)AsO system has been drawn. Long-range ordered magnetism occurs within the orthorhombic phase (x ≤ 0.30), whereas short-range magnetism appears to be confined within the lattice strained region of the tetragonal phase up to x < 0.60. Direct comparison between the magnetic and structural properties indicates that the magnetic transition is always associated with structural symmetry breaking, although confined to a local scale at high Ru contents.