Advances in ab-initio theory of Multiferroics. Materials and mechanisms: modelling and understanding

Within the broad class of multiferroics (compounds showing a coexistence of magnetism and ferroelectricity), we focus on the subclass of "improper electronic ferroelectrics", i.e. correlated materials where electronic degrees of freedom (such as spin, charge or orbital) drive ferroelectricity. In particular, in spin-induced ferroelectrics, there is not only a {\em coexistence} of the two intriguing magnetic and dipolar orders; rather, there is such an intimate link that one drives the other, suggesting a giant magnetoelectric coupling. Via first-principles approaches based on density functional theory, we review the microscopic mechanisms at the basis of multiferroicity in several compounds, ranging from transition metal oxides to organic multiferroics (MFs) to organic-inorganic hybrids (i.e. metal-organic frameworks, MOFs)Comment: 22 pages, 9 figure

Hydrogen storage in open framework materials

Current methods for storing molecular hydrogen require extremely high pressures or very low temperatures, both of which are expensive and energy intensive. Thus, if hydrogen is to become a practical fuel for mobile applications a more efficient storage system is required. One strategy for on-board hydrogen storage involves utilizing nanoporous metal-organic or covalent organic frameworks as adsorbent materials. A combination of practical and theoretical studies has shown that pore size and enthalpy of adsorption (Qst) are critical to effective hydrogen storage. Here, we present the hydrogen uptake data for a series of open framework materials with high surface areas and highlight successful strategies for increasing Qst values for room temperature H2 storage based on simulations

Gas adsorption sites in a large-pore metal-organic framework

The primary adsorption sites for Ar and N-2 within metal-organic framework-5, a cubic structure composed of Zn4O(CO2)(6) units and phenylene links defining large pores 12 and 15 angstroms in diameter, have been identified by single-crystal x-ray diffraction. Refinement of data collected between 293 and 30 kelvin revealed a total of eight symmetry-independent adsorption sites. Five of these are sites on the zinc oxide unit and the organic link; the remaining three sites form a second layer in the pores. The structural integrity and high symmetry of the framework are retained throughout, with-negligible changes resulting from gas adsorption

Determination of the hydrogen absorption sites in Zn₄O(1,4-benzenedicarboxylate) by single crystal neutron diffraction

A variable temperature (5–300 K) single crystal Laue neutron diffraction study has been conducted, and the gas absorption sites within hydrogen-loaded Zn4O(1,4-benzenedicarboxylate) have been located

A Porous Covalent Organic Framework with Voided Square Grid Topology for Atmospheric Water Harvesting

Atmospheric moisture is a ubiquitous water resource available at any time and any place, making it attractive to develop materials for harvesting water from air to address the imminent water shortage crisis. In this context, we have been exploring the applicability of covalent organic frameworks (COFs) for water harvesting and report here a new porous, two-dimensional imine-linked COF with a voided square grid topology, termed COF-432. Unlike other reported COFs, COF-432 meets the requirements desired for water harvesting from air in that it exhibits an S-shaped water sorption isotherm with a steep pore-filling step at low relative humidity and without hysteretic behavior-properties essential for energy-efficient uptake and release of water. Further, it can be regenerated at ultra-low temperatures and displays exceptional hydrolytic stability, as demonstrated by the retention of its working capacity after 300 water adsorption-desorption cycles

What do we know about three-periodic nets?

An account is given of various classifications of three-periodic nets. It is convenient to classify nets according to the nature of their maximum-symmetry embeddings. Other classifications, particularly in terms of the tilings that carry the nets, are also discussed. Although there is an infinity of possible nets, for certain types the number of possibilities is limited-there are for example exactly five regular nets. An account is given of the enumerations of various types of special structures such as sphere packings, the nets of simple tilings and self-dual tilings. Some databases of relevant structures and computer programs are described. (c) 2005 Elsevier Inc. All rights reserved

Strong and reversible binding of carbon dioxide in a green metal-organic framework

The efficient capture and storage of gaseous CO(2) is a pressing environmental problem. Although porous metal organic frameworks (MOFs) have been shown to be very effective at adsorbing CO(2) selectively by dint of dipole-quadruple interactions and/or ligation to open metal sites, the gas is not usually trapped covalently. Furthermore, the vast majority of these MOFs are fabricated from nonrenewable materials, often in the presence of harmful solvents, most of which are derived from petrochemical sources. Herein we report the highly selective adsorption of CO(2) by CD-MOF-2, a recently described green MOF consisting of the renewable cyclic oligosaccharide gamma-cyclodextrin and RbOH, by what is believed to be reversible carbon fixation involving carbonate formation and decomposition at room temperature. The process was monitored by solid-state (13)C NMR spectroscopy as well as colorimetrically after a pH indicator was incorporated into CD-MOF-2 to signal the formation of carbonic acid functions within the nanoporous extended framework