Evidence for negative thermal expansion in the superconducting precursor phase SmFeAsO

Area of Science:

• Condensed Matter Physics
• Materials Science
• Solid-State Chemistry

Background:

• Rare-earth iron oxypnictides, such as SmFeAsO$_{1-x}$F$_{x}$, are superconductors with complex phase diagrams.
• Understanding their structural and magnetic properties is crucial for materials science and condensed matter physics.

Purpose of the Study:

• To investigate the structural and magnetic phase transitions in SmFeAsO$_{1-x}$F$_{x}$ using high-resolution powder x-ray scattering.
• To elucidate the relationship between structural distortions, spin-density wave (SDW) instability, magnetic ordering, and superconductivity.

Main Methods:

• High-resolution powder x-ray scattering to analyze crystal structure and phase transitions.
• Analysis of thermal expansion coefficient to identify phase boundaries.
• Correlation with previous spectroscopic and magnetic measurements.

Main Results:

• Observed tetragonal-to-orthorhombic structural distortion suppressed by fluorine doping.
• Identified a magnetoelastic distortion at 120 K.
• Revealed a rich phase diagram with a spin-density wave instability at 125 K and magnetic ordering at 110 K.
• Observed significant negative thermal expansion in SmFeAsO, distinct from other rare-earth and actinide oxypnictides.

Conclusions:

• The negative thermal expansion is interpreted as evidence for itinerant electron condensation accompanying the opening of an SDW gap.
• Fluorine doping suppresses structural distortions and influences the superconducting dome.
• The study provides insights into the interplay of structural, magnetic, and electronic properties in iron-based superconductors.