Pressure effect on iron-based superconductor LaFeAsO_1-xH_x: Peculiar response of 1111-type structure

Area of Science:

• Condensed Matter Physics
• Materials Science
• Crystallography

Background:

• Layered iron oxypnictides, such as LaFeAsO$_{1-x}$H$_x$, are known for their complex phase diagrams.
• Superconductivity in these materials is often linked to specific structural motifs, like the FeAs$_4$ tetrahedron.

Purpose of the Study:

• To systematically investigate the crystal structure of LaFeAsO$_{1-x}$H$_x$ under varying pressure.
• To understand the relationship between structural changes, phase transitions, and superconductivity in this material.

Main Methods:

• Synchrotron X-ray diffraction was employed to analyze the crystal structure.
• Systematic variation of hydrostatic pressure was applied to the samples.

Main Results:

• A unique phase diagram emerged, featuring two superconducting and two parent phases.
• The As-Fe-As angle within the FeAs$_4$ tetrahedron widened under pressure, deviating from a regular tetrahedral geometry.
• Superconducting transition temperature (T$_c$) increased from 18 K to 52 K at 6 GPa for x=0.2.
• The second parent phase (x ~ 0.5) was suppressed by pressure (~1.5 GPa), while the first parent phase (x ~ 0) remained robust.

Conclusions:

• The observed structural deviation from a regular tetrahedron challenges existing models linking superconductivity to this geometry.
• Pressure-induced spin fluctuations from the suppressed parent phase are suggested to be crucial for high-T$_c$ superconductivity.