Microstructural evolution throughout the structural transition in 1111 oxy-pnictides

TL;DR

This study investigates the microstructural changes during the tetragonal to orthorhombic transition in 1111 oxy-pnictides, revealing microstrain behavior linked to orbital ordering and nematic correlations without volume discontinuity.

Contribution

It provides detailed analysis of microstrain evolution and its relation to symmetry breaking and orbital ordering in 1111 oxy-pnictides during the structural transition.

Findings

Microstrain along the hh0 direction increases with cooling.

Microstrain peaks just above the transition and is abruptly suppressed.

No volume discontinuity observed during the transition.

Abstract

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 oxy-pnictides: (La1-yYy)FeAsO and SmFeAs(O1-xFx). Both systems are characterized by a similar behaviour: 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 groupsubgroup 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 behaviour is induced by the tendency towards ordering of Fe orbitals.