Elusive Valence Transition in Mixed-Valence Sesquioxide Cs₄O₆

Area of Science:

• Solid State Chemistry
• Materials Science
• Crystallography

Background:

• Cesium suboxide (Cs4O6) is a mixed-valence molecular oxide.
• Its properties are highly sensitive to cooling protocols.
• The material features valency-delocalized O2(4/3-) units in a cubic structure.

Purpose of the Study:

• To investigate the structural and electronic properties of Cs4O6 under different cooling conditions.
• To elucidate the nature of the phase transition and the resulting mixed-valence state.
• To understand the relationship between crystal structure, charge/orbital order, and magnetic properties.

Main Methods:

• Neutron powder diffraction was employed to analyze the crystal structure.
• Variable-temperature studies were conducted to observe phase transitions and thermal response.
• Analysis of anion species and their ordering was performed.

Main Results:

• Deep quenching kinetically arrests the cubic phase down to cryogenic temperatures.
• Ultraslow cooling induces an incomplete transition to a tetragonal phase.
• The tetragonal phase exhibits a 1:2 ratio of O2(2-) and O2(-) anions, indicating charge and orbital order (Robin-Day Class II).
• The phase transition is martensitic, with complex interconversions upon heating and reentrant kinetic arrest near 260 K.
• The S=1/2 O2(-) units form a geometrically frustrated pyrochlore lattice, explaining the absence of magnetic order.

Conclusions:

• Cooling protocols critically determine the low-temperature phase of Cs4O6.
• The material exhibits complex charge, orbital, and magnetic ordering phenomena.
• The interplay between crystal structure and magnetic frustration is key to understanding its properties.