Microstructural evolution throughout the structural transition in 1111 oxypnictides

The microstructural evolution throughout the first-order tetragonal-to-orthorhombic structural transition is analyzed by powder diffraction analysis for two different systems belonging to the class of compounds referred to as 1111 oxypnictides: (La1-yYy) FeAsO and SmFeAs(O1-xFx). Both systems are characterized by similar behavior: On cooling, microstrain along the tetragonal hh0 direction takes place and increases as the temperature is decreased. Just above the structural transition, microstrain reaches its maximum value and then is abruptly suppressed by symmetry breaking. No volume discontinuity throughout the first-order transition is observed, and a group-subgroup relationship holds between the tetragonal and the orthorhombic structures, thus suggesting that orbital ordering drives symmetry breaking. Microstrain reflects a distribution of lattice parameters in the tetragonal phase and explains the occurrence of anisotropic properties commonly attributed to nematic correlations; in this scenario, the nematic behavior is induced by the tendency towards ordering of Fe orbitals.