Local Chemical Strain in PtFe Alloy Nanoparticles

Area of Science:

• Materials Science
• Nanotechnology
• Solid-State Physics

Background:

• Understanding atomic-level chemical strain in nanosolids is crucial for correlating structure with properties.
• Nanosized magnetic materials often exhibit weakened Invar behavior, impacting thermal expansion.
• PtFe nanoalloys present a complex system for studying structure-property relationships.

Purpose of the Study:

• To investigate the cause of abnormally enhanced negative thermal expansion (NTE) along the c axis in a 3.5 nm PtFe magnetic L1₀ alloy.
• To reveal the atomic-level chemical strain and distribution within the nanoalloy.
• To correlate local chemical environments with macroscopic functional properties.

Main Methods:

• ⁵⁷Fe Mössbauer spectroscopy to probe local magnetic and chemical states.
• Extended X-ray absorption fine structure (EXAFS) to analyze chemical coordination.
• Pair distribution function (PDF) analysis combined with reverse Monte Carlo (RMC) simulations to determine atomic structure and strain.

Main Results:

• An abnormally enhanced NTE along the c axis was observed in the 3.5 nm PtFe L1₀ alloy.
• Local disorder in chemical coordination within the L1₀ phase was identified.
• A surface A1-phase coating and atomic-level tensile chemical strain on heteroatomic pairs along the c axis were revealed.
• Increased magnetic moments of surface Fe atoms contributed significantly to the enhanced NTE.

Conclusions:

• The study provides an atomic-level understanding of chemical strain in bimetallic nanomaterials.
• Localized tensile chemical strain and surface magnetic effects are key to the enhanced NTE in PtFe nanoalloys.
• Tailoring functional properties by controlling the local chemical environment in nanomaterials is a promising approach.