Vanadium clusters formation in geometrically frustrated spinel oxide AlV₂O₄

Area of Science:

• Materials Science
• Solid State Physics
• Crystallography

Background:

• Spinel oxide AlV2O4 exhibits complex ordering phenomena, including atomic, charge, and orbital ordering.
• A rhombohedral lattice distortion accompanies cluster formation in AlV2O4.
• Understanding the structural phase transition is crucial for its material properties.

Purpose of the Study:

• To propose a theoretical framework for the structural phase transition in AlV2O4.
• To elucidate the mechanisms behind the atomic and orbital structure formation in the rhombohedral phase.
• To investigate the nature of clusters and bonding within the AlV2O4 structure.

Main Methods:

• Analysis of order-parameter symmetry based on irreducible representations (k9(τ4) of Fd3̅m).
• Thermodynamic analysis and crystal chemical studies.
• Density Functional Theory (DFT) calculations to study electron density and V-O bonding.

Main Results:

• The critical order parameter transforms according to irreducible representation k9(τ4).
• The rhombohedral structure results from atomic displacements and ordering of Al, V, and O atoms, alongside d-orbital ordering.
• DFT reveals V atoms form trimers in the Kagomé sublattice, with no metallic bonding between them; dimers and trimers are proposed as main clusters.

Conclusions:

• A comprehensive theory for the AlV2O4 structural phase transition is presented.
• The proposed trimer model explains the low-symmetry rhombohedral structure.
• Phase diagrams constructed using Landau theory indicate first-order transitions near tricritical points.