Although phase transitions have long been a centerpiece of condensed matter materials science studies, a number of recent efforts focus on potentially exploiting the resulting functional property changes in novel electronics and photonics as well as understanding emergent phenomena. This is quite timely, given a grand challenge in twenty-first-century physical sciences is related to enabling continued advances in information processing and storage beyond conventional CMOS scaling. In this brief review, we discuss synthesis of strongly correlated oxides, mechanisms of metal-insulator transitions, and exploratory electron devices that are being studied. Particular emphasis is placed on vanadium dioxide, which undergoes a sharp metal-insulator transition near room temperature at ultrafast timescales. The article begins with an introduction to metal-insulator transition in oxides, followed by a brief discussion on the mechanisms leading to the phase transition. The role of materials synthesis in influencing functional properties is discussed briefly. Recent efforts on realizing novel devices such as field effect switches, optical detectors, nonlinear circuit components, and solid-state sensors are reviewed. The article concludes with a brief discussion on future research directions that may be worth consideration.

Oxide Electronics Utilizing Ultrafast Metal-Insulator Transitions

Two-dimensional transition metal dichalcogenides (TMDs), an emerging family of layered materials, have provided researchers a fertile ground for harvesting fundamental science and emergent applications. TMDs can contain a number of different structural defects in their crystal lattices which significantly alter their physico-chemical properties. Having structural defects can be either detrimental or beneficial, depending on the targeted application. Therefore, a comprehensive understanding of structural defects is required. Here we review different defects in semiconducting TMDs by summarizing: (i) the dimensionalities and atomic structures of defects; (ii) the pathways to generating structural defects during and after synthesis and, (iii) the effects of having defects on the physico-chemical properties and applications of TMDs. Thus far, significant progress has been made, although we are probably still witnessing the tip of the iceberg. A better understanding and control of defects is important in order to move forward the field of Defect Engineering in TMDs. Finally, we also provide our perspective on the challenges and opportunities in this emerging field.

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Defect engineering of two-dimensional transition metal dichalcogenides

4.3. Manganese Oxide-Based Compounds 1291 4.3.1. MnO2-Type Compounds 1291 4.3.2. Ternary Lithiated LixMnOy Compounds 1293 4.4. Vanadium Pentoxide V2O5 1298 4.4.1. V2O5 Structure 1298 4.4.2. LixV2O5 Bulk Phases 1300 4.4.3. LixV2O5 Crystallized Thin Films 1303 4.5. Titanium Oxide-Based Compounds 1305 4.5.1. Lithium Titanate Li4Ti5O12 1306 4.5.2. TiO2 Anatase 1308 5. Phospho-olivine LiFePO4 Compound 1312 6. General Conclusion 1314 7. Acknowledgments 1315 8. References 1315

Raman Microspectrometry Applied to the Study of Electrode Materials for Lithium Batteries

Platinum disulfide (PtS2 ), a new member of the group-10 transition-metal dichalcogenides, is studied experimentally and theoretically. The indirect bandgap of PtS2 can be drastically tuned from 1.6 eV (monolayer) to 0.25 eV (bulk counterpart), and the interlayer mechanical coupling is almost isotropic. It can be explained by strongly interlayer interaction from the pz orbital hybridization of S atoms.

Extraordinarily Strong Interlayer Interaction in 2D Layered PtS2.

Most metallic elements have a crystal structure that is either body-centered cubic (bcc), face-centered close packed, or hexagonal close packed. If the bcc lattice is the thermodynamically most stable structure, the close-packed structures usually are dynamically unstable, i.e., have elastic constants violating the Born stability conditions or, more generally, have phonons with imaginary frequencies. Conversely, the bcc lattice tends to be dynamically unstable if the equilibrium structure is close packed. This striking regularity essentially went unnoticed until ab initio total-energy calculations in the 1990s became accurate enough to model dynamical properties of solids in hypothetical lattice structures. After a review of stability criteria, thermodynamic functions in the vicinity of an instability, Bain paths, and how instabilities may arise or disappear when pressure, temperature, and/or chemical composition is varied are discussed. The role of dynamical instabilities in the ideal strength of solids and in metallurgical phase diagrams is then considered, and comments are made on amorphization, melting, and low-dimensional systems. The review concludes with extensive references to theoretical work on the stability properties of metallic elements.

https://perssongroup.lbl.gov/papers/rmp2012-instabilities.pdf

Lattice instabilities in metallic elements

A theory of the morphotropic transformations occurring between stoichiometric phases in the vanadium-oxide system is proposed. The interconnections between the different structures are shown to result from definite symmetry-breaking mechanisms from a common parent structure. The homologous series ${\mathrm{V}}_{n}{\mathrm{O}}_{2n\ensuremath{-}1}$ and ${\mathrm{V}}_{n}{\mathrm{O}}_{2n+1}$ are interpreted as sequences of lock-in commensurate phases. Identification of the order-parameter symmetries provides us with the form of the thermodynamic functions which allow for construction of the theoretical phase diagrams in which the different phases are inserted. The general procedure allowing for the description of morphotropic transformations is outlined.

Theory of morphotropic transformations in vanadium oxides

The different polytypic structures observed in transition metal dichalcogenides are described as the result of ordering processes from a common parent disordered polytype structure. Interpolytypic transitions are suggested to be favored by the intrinsic stacking faults induced in the ordering mechanisms. The influence of these mechanisms on the intralayer superstructures is discussed.

Phase transitions between polytypes and intralayer superstructures in transition metal dichalcogenides

Displacive mechanisms are proposed for the high-pressure structural transitions which take place in Si, Ge, Sn, and Pb The mechanisms are analyzed in the framework of the Landau theory of phase transitions It reveals a number of unifying features which are discussed in connection with the electronic structures of these elements, leading to a more precise understanding of the diversity of phase diagrams found for the elements of Group IVa

Theory of the high-pressure structural phase transitions in Si, Ge, Sn, and Pb

The high-pressure structural transitions observed in the Group Va elements P, As, Sb, and Bi are described by displacive atomic mechanisms. A theoretical analysis of the proposed mechanisms shows that two different structural paths are followed relating the low-pressure A7 structure to the highest-pressure bcc structure. One structural path gives rise to the host-guest structures found in As, Sb, and Bi, whereas another path yields the incommensurate P IV structure recently disclosed in P. The incommensurate character of the host-guest and P IV structures is shown to be imposed by the energetically most favorable path relating the A7 and bcc structures.

Displacive mechanisms and order-parameter symmetries for the A7-incommensurate-bcc sequences of high-pressure reconstructive phase transitions in Group Va elements

Hannelore Katzke

Papers

Theory of morphotropic transformations in vanadium oxides

Phase transitions between polytypes and intralayer superstructures in transition metal dichalcogenides

Theory of the high-pressure structural phase transitions in Si, Ge, Sn, and Pb

Displacive mechanisms and order-parameter symmetries for the A7-incommensurate-bcc sequences of high-pressure reconstructive phase transitions in Group Va elements

One-dimensional chains in uranyl tungstates : Syntheses and structures of A8[(UO2)4(WO4)4(WO5)2] (A = Rb, Cs) and Rb6[(UO2)2O(WO4)4]