Ram Seshadri

Recently discovered alongside its sister compounds KV_{3}Sb_{5} and RbV_{3}Sb_{5}, CsV_{3}Sb_{5} crystallizes with an ideal kagome network of vanadium and antimonene layers separated by alkali metal ions. This work presents the electronic properties of CsV_{3}Sb_{5}, demonstrating bulk superconductivity in single crystals with a T_{c}=2.5 K. The normal state electronic structure is studied via angle-resolved photoemission spectroscopy and density-functional theory, which categorize CsV_{3}Sb_{5} as a Z_{2} topological metal. Multiple protected Dirac crossings are predicted in close proximity to the Fermi level (E_{F}), and signatures of normal state correlation effects are also suggested by a high-temperature charge density wavelike instability. The implications for the formation of unconventional superconductivity in this material are discussed.

Since the mid-1990s, phosphors have played a key role in emerging solid-state white-lighting technologies that are based on combining a III-nitride-based near-UV or blue solid-state light source with downconversion to longer wavelengths. Almost all widely used phosphors comprise a crystalline oxide, nitride, or oxynitride host that is appropriately doped with either Ce 3+ or Eu 2+ . These ions, with [Xe] 4f n 5d 0 configurations (n = 1 for Ce 3+ and 7 for Eu 2+ ) have proximal excited states that are [Xe] 4f n−1 5d 1 . Optical excitation into these states and concomitant reemission can be tuned into the appropriate regions of the visible spectrum by the crystal these ions are hosted in. In this article, we review the current needs and key aspects of the conversion process. We describe some currently used families of phosphors and consider why they are suitable for solid-state lighting. Finally, we describe some empirical rules for new and improved host materials.

Results of first-principles electronic structure calculations on the low-temperature monoclinic phase of the ferromagnetic perovskite BiMnO3 [Atou et al. J. Solid State Chem. 1999, 145, 639] are presented. In agreement with experiments, the calculations obtain an insulating ferromagnetic ground state for this material. The role of Bi 6s “lone pairs” in stabilizing the highly distorted perovskite structure is examined using real-space visualization of the electronic structure. Comparisons are drawn with the electronic structures of hypothetical cubic BiMnO3 and with the electronic structure of the prototypical perovskite manganite, LaMnO3. The exploitation of s electron lone pairs in the design of new ferroic materials is suggested.

Highly ordered mesoporous crystalline MoO(2) materials with bicontinuous Ia3d mesostructure were synthesized by using phosphomolybdic acid as a precursor and mesoporous silica KIT-6 as a hard template in a 10% H(2) atmosphere via nanocasting strategy. The prepared mesoporous MoO(2) material shows a typical metallic conductivity with a low resistivity ( approximately 0.01Omega cm at 300 K), which makes it different from all previously reported mesoporous metal oxides materials. Primary test found that mesoporous MoO(2) material exhibits a reversible electrochemical lithium storage capacity as high as 750 mA h g(-1) at C/20 after 30 cycles, rendering it as a promising anode material for lithium ion batteries.

We have investigated the paraelectric-to-ferroelectric phase transition of various sizes of nanocrystalline barium titanate (BaTiO3) by using temperature-dependent Raman spectroscopy and powder X-ray diffraction (XRD). Synchrotron X-ray scattering has been used to elucidate the room temperature structures of particles of different sizes by using both Rietveld refinement and pair distribution function (PDF) analysis. We observe the ferroelectric tetragonal phase even for the smallest particles at 26 nm. By using temperature-dependent Raman spectroscopy and XRD, we find that the phase transition is diffuse in temperature for the smaller particles, in contrast to the sharp transition that is found for the bulk sample. However, the actual transition temperature is almost unchanged. Rietveld and PDF analyses suggest increased distortions with decreasing particle size, albeit in conjunction with a tendency to a cubic average structure. These results suggest that although structural distortions are robust to changes in particle size, what is affected is the coherency of the distortions, which is decreased in the smaller particles.