76 research outputs found
The effect of internal pressure on the tetragonal to monoclinic structural phase transition in ReOFeAs: the case of NdOFeAs
We report the temperature dependent x-ray powder diffraction of the
quaternary compound NdOFeAs (also called NdFeAsO) in the range between 300 K
and 95 K. We have detected the structural phase transition from the tetragonal
phase, with P4/nmm space group, to the orthorhombic or monoclinic phase, with
Cmma or P112/a1 (or P2/c) space group, over a broad temperature range from 150
K to 120 K, centered at T0 ~137 K. Therefore the temperature of this structural
phase transition is strongly reduced, by about ~30K, by increasing the internal
chemical pressure going from LaOFeAs to NdOFeAs. In contrast the
superconducting critical temperature increases from 27 K to 51 K going from
LaOFeAs to NdOFeAs doped samples. This result shows that the normal striped
orthorhombic Cmma phase competes with the superconducting tetragonal phase.
Therefore by controlling the internal chemical pressure in new materials it
should be possible to push toward zero the critical temperature T0 of the
structural phase transition, giving the striped phase, in order to get
superconductors with higher Tc.Comment: 9 pages, 3 figure
Molecular Chemistry to the Fore: New Insights into the Fascinating World of Photoactive Colloidal Semiconductor Nanocrystals
Colloidal semiconductor nanocrystals possess unique properties that are unmatched by other chromophores such as organic dyes or transition-metal complexes. These versatile building blocks have generated much scientific interest and found applications in bioimaging, tracking, lighting, lasing, photovoltaics, photocatalysis, thermoelectrics, and spintronics. Despite these advances, important challenges remain, notably how to produce semiconductor nanostructures with predetermined architecture, how to produce metastable semiconductor nanostructures that are hard to isolate by conventional syntheses, and how to control the degree of surface loading or valence per nanocrystal. Molecular chemists are very familiar with these issues and can use their expertise to help solve these challenges. In this Perspective, we present our group\u27s recent work on bottom-up molecular control of nanoscale composition and morphology, low-temperature photochemical routes to semiconductor heterostructures and metastable phases, solar-to-chemical energy conversion with semiconductor-based photocatalysts, and controlled surface modification of colloidal semiconductors that bypasses ligand exchange
Molecular Chemistry to the Fore: New Insights into the Fascinating World of Photoactive Colloidal Semiconductor Nanocrystals
Intrinsic Properties of Stoichiometric LaFePo
DC and ac magnetization, resistivity, specific-heat, and neutron-diffraction data reveal that stoichiometric LaFePO is metallic and non-superconducting above T=0.35K, with Îł=12.5 mJ mol K2. Neutron-diffraction data at room temperature and T=10K are well described by the stoichiometric, tetragonal ZrCuSiAs structure, and show no signs of structural distortions or long-range magnetic ordering to an estimated detectability limit of 0.07 ÎŒB /Fe. We propose a model based on the shape of the iron-pnictide tetrahedron that explains the differences between LaFePO and LaFeAsO, the parent compound of the recently discovered high-Tc oxyarsenides, which, in contrast, shows both structural and spin-density wave transitions
Self-Assembly and Enhanced Magnetic Properties of Three-Dimensional Superlattice Structures Composed of Cube-Shaped EuS Nanocrystals
First Synthesis of EuS Nanoparticle Thin Film with a Wide Energy Gap and Giant Magneto-Optical Efficiency on a Glass Electrode
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