Electronic Structure Stability Research Articles

Proton-Induced Disproportionation of Jahn-Teller-Active Transition-Metal Ions in Oxides Due to Electronically Driven Lattice Instability.

The interfacial chemical reactivity of Jahn-Teller-active transition-metal oxides remains an enigmatic area, often leading to uncontrollable phase transformations in the oxide-based technological applications. In particular, the higher tendency of unwanted transition-metal-ion dissolution and side-reactions in Jahn-Teller-active oxides is accompanied by performance degradation in many electrochemical systems, for example, lithium-ion batteries. We show here that the fundamental electronic structure instability that leads to Jahn-Teller (lattice) distortion in an octahedral ligand field is the active chemical driving force for the spontaneous disproportionation, phase transformation, and metal-ion dissolution in transition-metal oxides upon exposure to protons. On the basis of electronic structure analyses and 18O isotope labeling, we present a mechanism comprising a coupled acid-base/redox reaction that leads to a proton-induced disproportionation of Jahn-Teller-active transition-metal ions, as exemplified by the broad classes of respective oxides.

Electronic structure and phase transition engineering in NbS2: Crucial role of van der Waals interactions**Project supported by the National Natural Science Foundation of China (Grant No. 11864008) and Guangxi Natural Science Foundation, China (Grant Nos. 2018GXNSFAA138185 and 2018AD19200). High performance computational resources provided by LvLiang Cloud Computing Center of China and National Supercomputer Center on TianHe-2 are gratefully acknowledged.

Based on first-principles simulations, we revisit the crystal structures, electronic structures, and structural stability of the layered transition metal dichalcogenides (TMDCs) NbS2, and shed more light on the crucial roles of the van der Waals (vdW) interactions. Theoretically calculated results imply that the vdW corrections are important to reproduce the layered crystal structure, which is significant to correctly describe the electronic structure of NbS2. More interestingly, under hydrostatic pressure or tensile strain in ab plane, an isostructural phase transition from two-dimensional layered structure to three-dimensional bulk in the I4/mmm phase has been uncovered. The abnormal structural transition is closely related to the electronic structure instability and interlayer bonding effects. The interlayer Nb–S distances collapse and the interlayer vdW interactions disappear, concomitant with new covalent bond emerging and increasing coordination number. Present work highlights the significance of the vdW interactions, and provides new insights on the unconventional structural transitions in NbS2, which will attract wide audience working in the hectic field of TMDCs.

Premartensitic State of Ti-Pd-Fe Shape Memory Alloys Studied by Electrical Resistivity, Magnetic Susceptibility and Specific Heat Measurements

Transformation behavior in Ti-(50-x)Pd-xFe shape memory alloys has been investigated by electrical resistivity, magnetic susceptibility and specific heat measurements. The alloys with x ¼ 14, 16 and 18 exhibit an obvious first order martensitic transformation, but the alloys with x ¼ 19, 20 and 22 exhibit no first order transformation down to 4.2 K. However, the latter alloys show a negative temperature coefficient in electrical resistivity below a temperature Tmin, which decreases with increasing x. The increase in resistivity with decreasing temperature is associated with a decrease in magnetic susceptibility. Also, specific heat measurements at cryogenic temperatures have revealed that the electronic specific heat coefficient of the latter alloys (x ¼ 19, 20 and 22) is significantly larger than that of the former alloys (x ¼ 14, 16 and 18). These results suggest that the so-called precursor phenomena observed in Ti-Pd based shape memory alloys are caused by the instability of electronic structure of the B2-phase, as in the Ti-Ni based alloys previously studied by our group. [doi:10.2320/matertrans.M2009412]

ChemInform Abstract: Aspects of Electronic Structure Instability in Search for New Superconductors: Superconducting Boride at Liquid Nitrogen Temperature?

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Aspects of Electronic Structure Instability in Search for New Superconductors: Superconducting Boride at Liquid Nitrogen Temperature?

It has been shown that electron-phonon coupling in superconductors induces temperature-dependent electronic structure instability which is related to fluctuation of analytic critical point of some bands across the Fermi level. The band fluctuation results in a considerable reduction of chemical potential and to breakdown of the adiabatic Born-Oppenheimer approximation. At critical temperature Tc, superconducting system undergoes transition from the adiabatic electronic ground state into the antiadiabatic state at broken symmetry, which is stabilized due to the effect of nuclear dynamics. This effect is absent in non-superconducting compounds. In a good agreement with the experimental Tc of superconducting state transition, the critical temperature of the adiabatic ↔ antiadiabatic state transition has been calculated for three different superconductors. Two hypothetical compounds, LiB and ZnB2, are predicted to be potential superconductors with Tc about 17 and 77.5 K, respectively.

Uniaxial pressure effect on magnetic susceptibility of perovskite manganite La 0.66Ba 0.34MnO 3

Magnetization of La 0.66Ba 0.34MnO 3 and its temperature behavior under a uniaxial pressure of 0.1 kbar are measured between 5 and 270 K in magnetic fields 0< H<120 Oe. The magnetization represents nearly linear dependence on an external magnetic field. Temperature dependence of the magnetic susceptibility found represents a plateau, that is considered as an evidence of the formation of a long period magnetic structure (probably a sort of helix) below the Curie point. Pressure derivative of magnetization displays a sharp minimum at 200 K, pointing to an instability of electronic structure of the compound near this temperature.