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Structural Phase Transitions In Layered Transition Metal Compounds (Physics And Chemistry Of Materia

Transition metal phosphorous trichalcogenides, MPX3 (M and X being transition metal and chalcogen elements, respectively), have been the focus of substantial interest recently because they are unusual candidates undergoing Mott transition in the two-dimensional limit. Here we investigate material properties of the compounds with M=Mn and Ni employing ab initio density functional and dynamical mean-field calculations, especially their electronic behavior under external pressure in the paramagnetic phase. Mott metal-insulator transitions (MIT) are found to be a common feature for both compounds, but their lattice structures show drastically different behaviors depending on the relevant orbital degrees of freedom, i.e., t2g or eg. Under pressure, MnPS3 can undergo an isosymmetric structural transition within monoclinic space group by forming Mn-Mn dimers due to the strong direct overlap between the neighboring t2g orbitals, accompanied by a significant volume collapse and a spin-state transition. In contrast, NiPS3 and NiPSe3, with their active eg orbital degrees of freedom, do not show a structural change at the MIT pressure or deep in the metallic phase within the monoclinic symmetry. Hence NiPS3 and NiPSe3 become rare examples of materials hosting electronic bandwidth-controlled Mott MITs, thus showing promise for ultrafast resistivity switching behavior.

Structural Phase Transitions in Layered Transition Metal Compounds (Physics and Chemistry of Materia


The investigation of materials at extreme conditions of high pressure and temperature (high-PT), has been one of the greatest scientific endeavors in condensed mater physics, chemistry, astronomy, planetary, and material sciences. Being subjected to high-PT conditions, materials exhibit dramatic changes in both atomic and electronic structure resulting in an emergence of exceptionally interesting phenomena including structural and electronic phase transitions, chemical reactions, and formation of novel compounds with never-previously observed physical and chemical properties. Although new exciting experimental developments in static and dynamic compression combined with new diagnostics/characterization methods allow to uncover new processes and phenomena at high P-T conditions, there are some fundamental limitations on what can be achieved experimentally. Therefore, theory/simulations play an important role in uncovering interesting physics and chemistry of materials at much smaller cost and at much higher accuracy.

energetic molecular crystals, layered transition metal chalcogenides, binary Ni-Xe system, ternary H-S-O compounds, and single-element carbon, all of them being in the focus of funded research projects of my PhD advisor, Dr. Ivan Oleynik. The important results obtained in this PhD project include: (1) very accurate equation of state, properties of individual phases and phase transitions of several materials such as energetic materials and carbon, which allowed to address outstanding challenges and long-term controversies of previous experimental and simulation studies; (2) prediction of novel phases and compounds with accompanying phase transitions, such as VSe2, Sn-Se, Ni-Xe and H-S-O compounds, which will drive future experiments as well as follow up theoretical studies of these novel compounds. This work sets up new standards for high-quality theoretical prediction of physical properties of materials at extreme conditions, but most importantly, encourages experimentalists to perform more precise measurements of such properties as equation of state, phase transitions, and melting curves, as well as to attempt to synthesize newly predicted compounds with new emergent properties such as ferromagnetism or superconductivity.

I use crystal growth, solid state chemistry, and metallurgical synthesis techniques, crystallographic studies, and physical property measurements to explore structure-property relationships in complex materials, with particular emphasis on magnetism. My research is currently focused on layered transition metal compounds, anisotropic magnets, and topological materials. I am also interested in materials discovery, crystallography, crystallographic phase transitions and their effects on physical properties, and birds.

Progress in the research on phase transitions during Li + extraction/insertion processes in typical battery materials is summarized as examples to illustrate the significance of understanding phase transition phenomena in Li-ion batteries. Physical phenomena such as phase transitions (and resultant phase diagrams) are often observed in Li-ion battery research and already play an important role in promoting Li-ion battery technology. For example, the phase transitions during Li + insertion/extraction are highly relevant to the thermodynamics and kinetics of Li-ion batteries, and even physical characteristics such as specific energy, power density, volume variation, and safety-related properties.

The phenomenon of phase transitions and the resultant phase diagrams in Li-ion batteries (LIBs) are often observed in the synthesis of materials, electrochemical reaction processes, temperature changes of batteries, and so on. Understanding those phenomena is crucial to design more desirable materials and facilitate the overall development of LIBs. For instance, if the conditions of phase transition such as sintering temperature or composition are well known, more excellent materials for LIBs can be designed and prepared. [ 1 ] Such knowledge is also beneficial for choosing an electrolyte that is more stable during the electrochemical reaction process. [ 2 ] Thermodynamics and kinetics of phase transition have to be discussed first, as far as the study of phase transition is concerned. It should also be noted that the investigations and understanding of those phase transitions in LIBs now from macro to micro and from thermodynamics to kinetics are gradually being realized on the basis of newly developed advanced characterization techniques. Thus, advanced study tools play a significant role in understanding phase transition clearly and in depth.

Almost all the research methods used to study the material structure in other fields can also be applied to investigating the phase transitions of LIBs. Among them, x-ray diffraction (XRD) technology is the most direct method to distinguish the phase structure of LIBs materials. Thus, it has been widely used in researching phase analysis and studying phase transitions. [ 6 ] However, some special phase structures cannot be distinguished well by the diffraction method, so other local characterization methods such as Raman spectra have to be adopted. [ 7 ] Transmission electron microscopy (TEM) is also a powerful tool to get the microstructure information of materials, especially to obtain high spatial resolution. [ 8 ] Only recently could lithium ions be seen directly via electron microscope imagery, by using the spherical aberration correction electronic microscopy. [ 9 ] Some in situ experimental techniques are also being developed to understand in depth the structural changes of electrode materials during Li + extraction/ insertion. The characterization methods now available for LIBs, with different spatial resolution levels, have been summarized, [ 10 ] as shown in Fig. 1 . The nature of phase transition processes will be better described by combining those methods in the future.

Below, three typical electrode materials, LiCoO 2 , LiFePO 4 cathode and Si anode, are taken as examples to illuminate the study of phase transitions during the process of Li + extraction/intercalation in LIBs from the viewpoints of thermodynamics, kinetics, and characterization methods.

The first commercialized cathode material of LIBs is the layered LiCoO 2 , which still dominates the portable electric device market due to high volumetric density. [ 15 , 16 ] LiCoO 2 prepared by a conventional high-temperature solid state reaction method has an O3 structure in which the oxygen anion has a cubic close-packed arrangement in the form of ABCABC. During the electrochemical process, many complex phase transitions, especially in the lithium extraction stage are involved. In the past 30 years, plenty of investigations have focused on these phase transitions to understand the reactions and structural evolution mechanisms. The main investigations with respect to thermodynamics and dynamics are summarized here.

We have reviewed some phase transition mechanisms in LiFePO 4 . However, the occurrence of the phase-transition mechanisms also strongly depends on the limitation of the diffusion of Li + between the two-phase interfaces. This is related to electrochemical kinetics. Thus, the crystal structure of the compound often plays an essential role in determining the shape of the voltage profile as a function of Li concentration, which is related to the kinetic behavior of the material. Firstprinciples statistical mechanical approaches can be applied to study the chemistry and crystal structures that give rise to kinetic properties. [ 57 ] A flat voltage plateau (LiFePO 4 , at 3.45 V versus Li/Li + ) caused by two-phase electrochemical reaction can be verified from the Gibbs free energy according to the suggestions of Van der Ven et al. [ 58 ]

We discover that certain two-dimensional transition metal dichalcogenides undergo structural metal-to-insulator phase transitions under tension. Our calculations reveal that MoTe2 transforms at the smallest tensile strains: between 0.3 and 3% under uniaxial conditions.

Triangular magnetic structures have gained considerable interest due to their rich magnetic behavior and structural simplicity. These structures contain the motif of a triangle as the main structural feature, leading to geometric frustration and implicitly to degenerate magnetic ground states. Most of the previous work on triangular lattice structures was performed on simple transition metal halides or oxides. Therefore, it presents an interesting challenge for materials scientists to synthesize new class of materials that preserve the quasi-two dimensionality of the structures. This talk will feature two class of materials (1) triangular materials synthesized using high-pressure hydrothermal method (2) AREQ2 (A = Alkali metal, RE= rare earth, Q = O, S, Se) triangular magnetic materials. 350c69d7ab


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