The effect of internal pressure on the tetragonal to monoclinic structural phase transition in ReOFeAs: the case of NdOFeAs

TL;DR

This study investigates how internal chemical pressure influences the structural phase transition in NdOFeAs, revealing that increased pressure lowers the transition temperature and correlates with higher superconducting critical temperatures.

Contribution

It demonstrates that internal chemical pressure reduces the structural transition temperature in NdOFeAs, suggesting a pathway to enhance superconducting properties by controlling pressure.

Findings

The structural phase transition temperature is reduced by ~30K with increased chemical pressure.

Superconducting critical temperature increases from 27K to 51K across the series.

The orthorhombic phase competes with the superconducting tetragonal phase.

Abstract

We report the temperature dependent x-ray powder diffraction of the quaternary compound NdOFeAs (also called NdFeAsO) in the range between 300 K and 95 K. We have detected the structural phase transition from the tetragonal phase, with P4/nmm space group, to the orthorhombic or monoclinic phase, with Cmma or P112/a1 (or P2/c) space group, over a broad temperature range from 150 K to 120 K, centered at T0 ~137 K. Therefore the temperature of this structural phase transition is strongly reduced, by about ~30K, by increasing the internal chemical pressure going from LaOFeAs to NdOFeAs. In contrast the superconducting critical temperature increases from 27 K to 51 K going from LaOFeAs to NdOFeAs doped samples. This result shows that the normal striped orthorhombic Cmma phase competes with the superconducting tetragonal phase. Therefore by controlling the internal chemical pressure in new materials it should be possible to push toward zero the critical temperature T0 of the structural phase transition, giving the striped phase, in order to get superconductors with higher Tc.