Transformers: the changing phases of low-dimensional vanadium oxide bronzes

Abstract

In this feature article, we explore the electronic and structural phase transformations of ternary vanadium oxides with the composition MxV2O5 where M is an intercalated cation. The periodic arrays of intercalated cations ordered along quasi-1D tunnels or layered between 2D sheets of the V2O5 framework induce partial reduction of the framework vanadium atoms giving rise to charge ordering patterns that are specific to the metal M and stoichiometry x. This periodic charge ordering makes these materials remarkably versatile platforms for studying electron correlation and underpins the manifestation of phenomena such as colossal metal-insulator transitions, quantized charge corrals, and superconductivity. We describe current mechanistic understanding of these emergent phenomena with a particular emphasis on the benefits derived from scaling these materials to nanostructured dimensions wherein precise ordering of cations can be obtained and phase relationships can be derived that are entirely inaccessible in the bulk. In particular, structural transformations induced by intercalation are dramatically accelerated due to the shorter diffusion path lengths at nanometer-sized dimensions, which cause a dramatic reduction of kinetic barriers to phase transformations and facilitate interconversion between the different frameworks. We conclude by summarizing numerous technological applications that have become feasible due to recent advances in controlling the structural chemistry and both electronic and structural phase transitions in these versatile frameworks.

Review of flash sintering: materials, mechanisms and modelling

“…To give some idea of the complexity involved, the room temperature electrical conductivity of oxides spans over an impressive 22 orders of magnitude encompassing insulating, semiconducting, and metallic behaviour [31]. A simplified schematic showing the typical temperature-dependence of electrical conductivity (normalised to their melting/decomposition temperatures) of electronic conductors (metals, insulators, semiconductors) and ionic conductors is given in Figure 3 (for dense materials).…”

Structure‐Induced Switching of the Band Gap, Charge Order, and Correlation Strength in Ternary Vanadium Oxide Bronzes

“…It is clear that there is potential in these systems for MITs and other changes in electronic properties that could be induced by structural strain, temperature, and cation concentration. Changes of this kind have been seen in other studies and bronzes and derive from alterations in the coupling of half‐filled V 3d states . To summarize the effects of structural relaxation: when present intercalants are drawn deeper into layers of octahedra, if intercalants are removed from between layers the nearest V−O v bonds compress and acquire greater vanadyl character, there are minimal changes seen in the bond lengths along the conducting planes.…”

“…Ternary vanadium oxide bronzes exhibit many unique structural and electronic properties . Their layered and tunnel structures facilitate the intercalation of a variety of cations, making them candidates for the next generation of Li‐ and multivalent‐ion battery cathodes .…”

Stabilization of a Metastable Tunnel‐Structured Orthorhombic Phase of VO2 upon Iridium Doping

“…In addition, several other polymorphs are known such as the M­ 2 polymorph, which is stabilized by strain or chemical pressure; the M 3 phase; metastable VO 2 (A) and VO 2 (B) phases, often accessible from hydrothermal synthesis; and a mineral phase paramontroseite or VO 2 (P) . The ability to access a rich palette of structural motifs with varying connectivity of V–O bonds, tunable linkages of vanadium‐centered polyhedra, and reconfigurable stacking of extended sheets has profound implications for the relative extent of ionicity or covalency and allows for broad tunability of transport properties from insulating to semiconducting and metallic regimes …”