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Register a new userMetal-Organic Framework Breathing in Electric Field: A Theoretical Study
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In this manuscript, we study the electrically induced breathing of Metal-Organic Framework (MOF) within a 2D lattice model. The Helmholtz free energy of the MOF in electric field consists of two parts: the electrostatic energy of the dielectric body in the external electric field and elastic energy of the framework. The first contribution is calculated from the first principles of statistical mechanics with an account of MOF symmetry. By minimizing the obtained free energy and solving the resulting system of equations, we obtain the local electric field and the parameter of the unit cell (angle $alpha$). The paper also studies the cross-section area of the unit cell and the polarization as functions of the external electric field. We obtain the hysteresis in the region of the structural transition of the framework. Our results are in qualitative agreement with the literature data of the molecular dynamics (MD) simulation of MIL-53(Cr).
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Within the so-called no-mans land between about 150 and 235 K, crystallization of bulk water is inevitable. The glasslike freezing and a liquid-to-liquid transition of water, predicted to occur in this region, can be investigated by confining water in nanometer-sized pores. Here we report the molecular dynamics of water within the pores of a metal-organic framework using dielectric spectroscopy. The detected temperature-dependent dynamics of supercooled water matches that of bulk water as reported outside the borders of the no-mans land. In confinement, a different type of water is formed, nevertheless still undergoing a glass transition with considerable molecular cooperativity. Two different length scales seem to exist in water: A smaller one, of the order of 2 nm, being the cooperativity length scale governing glassy freezing, and a larger one (> 2 nm), characterizing the minimum size of the hydrogen-bonded network needed to create real water with its unique dynamic properties
Hydrogen adsorption by the metal organic framework (MOF) structure Zn2(BDC)2(TED) is investigated using a combination of experimental and theoretical methods. By use of the nonempirical van der Waals density-functional (vdW-DF) approach, it is found that the locus of deepest H2 binding positions lies within two types of narrow channel. The energies of the most stable binding sites, as well as the number of such binding sites, are consistent with the values obtained from experimental adsorption isotherms and heat of adsorption data. Calculations of the shift of the H-H stretch frequency when adsorbed in the MOF give a value of approximately -30 cm-1 at the strongest binding point in each of the two channels. Ambient temperature infrared absorption spectroscopy measurements give a hydrogen peak centered at 4120 cm-1, implying a shift consistent with the theoretical calculations.
Although the magnetoelectric effects - the mutual control of electric polarization by magnetic fields and magnetism by electric fields, have been intensively studied in a large number of inorganic compounds and heterostructures, they have been rarely observed in organic materials. Here we demonstrate magnetoelectric coupling in a metal-organic framework [(CH3)2NH2]Mn(HCOO)3 which exhibits an order-disorder type of ferroelectricity below 185 K. The magnetic susceptibility starts to deviate from the Curie-Weiss law at the paraelectric-ferroelectric transition temperature, suggesting an enhancement of short-range magnetic correlation in the ferroelectric state. Electron spin resonance study further confirms that the magnetic state indeed changes following the ferroelectric phase transition. Inversely, the ferroelectric polarization can be improved by applying high magnetic fields. We interpret the magnetoelectric coupling in the paramagnetic state in the metal-organic framework as a consequence of the magnetoelastic effect that modifies both the superexchange interaction and the hydrogen bonding.
In the dense metal-organic framework Na[Mn(HCOO)$_3$], Mn$^{2+}$ ions ($S=frac{5}{2}$) occupy the nodes of a `trillium hyperkagome net. We show that this material exhibits a variety of behaviour characteristic of geometric frustration: the Neel transition is suppressed well below the characteristic magnetic interaction strength; short-range magnetic order persists far above the Neel temperature; and the magnetic susceptibility exhibits a pseudo-plateau at $frac{1}{3}$-saturation magnetisation. We demonstrate that a simple nearest-neighbour Heisenberg antiferromagnet model accounts quantitatively for each observation, and hence Na[Mn(HCOO)$_3$] is the first experimental realisation of this model on the trillium net. We develop a mapping between this trillium model and that on the two-dimensional Shastry-Sutherland lattice, and demonstrate how both link geometric frustration within the classical spin liquid regime to a strong magnetocaloric response at low fields.
The basic ingredients of osmotic pressure are a solvent fluid with a soluble molecular species which is restricted to a chamber by a boundary which is permeable to the solvent fluid but impermeable to the solute molecules. For macroscopic systems at equilibrium, the osmotic pressure is given by the classical vant Hoff Law, which states that the pressure is proportional to the product of the temperature and the difference of the solute concentrations inside and outside the chamber. For microscopic systems the diameter of the chamber may be comparable to the length-scale associated with the solute-wall interactions or solute molecular interactions. In each of these cases, the assumptions underlying the classical vant Hoff Law may no longer hold. In this paper we develop a general theoretical framework which captures corrections to the classical theory for the osmotic pressure under more general relationships between the size of the chamber and the interaction length scales. We also show that notions of osmotic pressure based on the hydrostatic pressure of the fluid and the mechanical pressure on the bounding walls of the chamber must be distinguished for microscopic systems. To demonstrate how the theoretical framework can be applied, numerical results are presented for the osmotic pressure associated with a polymer of N monomers confined in a spherical chamber as the bond strength is varied.
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