Structural phase transition and superlattice misfit strain of RFeAsO (R = La, Pr, Nd and Sm)

TL;DR

This study investigates the structural phase transition in RFeAsO compounds, revealing differences between single crystals and polycrystalline samples related to superlattice misfit strain and surface effects, which may influence superconducting properties.

Contribution

It provides new insights into how surface to volume ratio affects lattice fluctuations and phase transition behavior in 1111 systems, linking these effects to superconducting transition temperatures.

Findings

Sharp SPT in single crystals versus broad in polycrystals

Different correlation length critical exponents between sample types

Surface reconstruction influences lattice fluctuations and superconductivity

Abstract

The tetragonal-to-orthorhombic structural phase transition (SPT) in LaFeAsO (La-1111) and SmFeAsO (Sm-1111) single crystals measured by high resolution x-ray diffraction is found to be sharp while the RFeAsO (R=La, Nd, Pr, Sm) polycrystalline samples show a broad continuous SPT. Comparing the polycrystalline and the single crystal 1111 samples, the critical exponents of the SPT are found to be the same while the correlation length critical exponents are found to be very different. These results imply that the lattice fluctuations in 1111 systems change in samples with different surface to volume ratio that is assigned to the relieve of the temperature dependent superlattice misfit strain between active iron layers and the spacer layers in 1111 systems. This phenomenon that is missing in the AFe2As2 (A=Ca, Sr, Ba) "122" systems, with the same electronic structure but different for the thickness and the elastic constant of the spacer layers, is related with the different maximum superconducting transition temperature in the 1111 (55 K) versus 122 (35 K) systems and implies the surface reconstruction in 1111 single crystals.