Structural properties and phase diagram of the La(Fe1-xRux)AsO 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 D 0:10, possibly to tricritical for x D 0:20, up to second order for x D 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.