Probing the Local Site Disorder and Distortion in Pyrochlore High-Entropy Oxides

Area of Science:

• Materials Science
• Solid State Chemistry
• Crystallography

Background:

• High-entropy oxides (HEOs) are materials with tunable properties due to their complex compositions.
• Controlling local atomic order and disorder is crucial for unlocking the full potential of HEOs.
• Pyrochlore HEOs with multiple M-site cations present a unique platform for studying local structure-property relationships.

Purpose of the Study:

• To synthesize and characterize novel pyrochlore HEOs with five M-site cations (Nd2M2O7).
• To investigate the local order/disorder of M-site cations using advanced experimental and computational techniques.
• To establish structure-property correlations by understanding how local atomic arrangements influence material behavior.

Main Methods:

• Neutron diffraction and pair distribution function (PDF) analysis to probe local atomic structure.
• Density functional theory (DFT) calculations with special quasirandom structure (SQS) models to compute theoretical PDFs.
• Reverse Monte Carlo (RMC), ab initio molecular dynamics (AIMD), and Metropolis Monte Carlo (MMC) simulations to analyze M-site order/disorder.

Main Results:

• The average structure of the synthesized pyrochlores is orthorhombic Imma.
• Nd2(Ta0.2Sc0.2Sn0.2Hf0.2Zr0.2)2O7 exhibits highly randomized/disordered M-site cations at the local to nanoscale.
• Nd2(Ti0.2Nb0.2Sn0.2Hf0.2Zr0.2)2O7+ shows significant TiO6 octahedron distortion and short-range order (SRO) of Ti on the subnanometer scale, potentially intrinsic.

Conclusions:

• Engineered variation of M-site ions in HEOs, even with similar radii, allows precise control over local order/disorder motifs.
• Understanding and controlling local atomic arrangements are key to tailoring HEO properties for specific applications.
• Advanced computational and experimental analyses are essential for guiding structure-property tuning in HEOs.