Hidden Charge Order in an Iron Oxide Square-Lattice Compound

Area of Science:

• Condensed Matter Physics
• Materials Science
• Crystallography

Background:

• Charge disproportionation in Sr3Fe2O7 was discovered over 50 years ago.
• The spatial ordering of charges in FeO2 planes remained elusive to conventional diffraction techniques.
• Previous studies utilized Mössbauer spectroscopy and scattering methods without fully resolving the charge order.

Purpose of the Study:

• To elucidate the hidden charge ordering pattern in the FeO2 planes of Sr3Fe2O7.
• To determine the nature of the phase transition associated with the charge order.
• To explain the challenges in detecting this order using conventional probes.

Main Methods:

• Neutron Larmor diffraction
• Fe K-edge resonant X-ray scattering

Main Results:

• Demonstrated a checkerboard charge order in the FeO2 planes.
• Observed the charge order vanishes at a sharp second-order phase transition above 332 K.
• Identified stacking disorder due to frustrated interlayer interactions as the cause for broadened and weakened superstructure reflections.

Conclusions:

• The checkerboard charge order in Sr3Fe2O7 is now experimentally confirmed.
• The findings explain the historical difficulty in observing this 'hidden order'.
• Provides insights into 'hidden order' phenomena in other materials.