Van der Waals gap

Figure 10. Schematic model showing the influence of the thickness of a graphite flake on the extent of co-intercalation of solvent molecules in the internal van der Waals gaps of graphite, (a) Thick graphite flakes (b) thin graphite flakes. Prepared with data from Ref. [169].

Thirdly, strong solvent co-intercalation, in particular into internal van der Waals gaps, can only be expected for kinetically stable ternary compounds Li solv) Cn. For example, comparison of DMC and DEC with dimethoxyethane (DME), shows that the kinetic stability of Li>.(DME)yCn can be considered much higher than that of LiJr(DMC)vC and LiJt(DEC)yC and of course Liy(EC)yCn [169]. With EC/DME, solvent co-intercalation proceeds on a macroscopic scale, i.e., the external van der Waals gaps and some internal ones can participate in the solvent co-... 

In the intercalation reactions, ions (anions X or cations M+) penetrate into the van der Waals gaps between the ordered carbon layers resulting in the enlargement of their inter-layer distance [23,24]. The corresponding charges are conducted by carbon and accepted into the carbon host lattice. 

Closs and Miller have also remarked that the intermolecular interaction across van der Waals gaps in frozen organic glasses does not give rise to large attenuations relative to intramolecular couplings (Science (1988) 240 446). 

Considerable attention has been devoted to the study of intercalation compounds of the dichalcogenides (Whittingham, 1978 Subba Rao Shafer, 1979). Intercalation compounds of dichalcogenides can be divided into three categories (a) compounds with Lewis base type molecules such as ammonia, n-alkylamines, pyridines etc. (b) compounds with metal cations or molecular cations, Li, Na, K, etc., or [(C5H5)2Co]" and (c) compounds containing both cations and neutral polar (solvated) molecules in the van der Waals gap. 

Layered InSe and GaSe crystals attract investigator interest because of heterostructures based on them possess good photosensitivity and find their application in solar cells [1-3], At the same time distinctive feature of layered crystals - sharp anisotropy of chemical bonds (strong ion-covalent inside crystal layers and weak van der Waals between them) paid great attention of researchers to intercalation, that is to insertion of atoms and molecules into interlayer space of a layered crystal - the so-called van-der-Waals gap . In particular for InSe and GaSe crystals volume of van der Waals gap makes under the attitude to all volume of a crystal about 40+45 %. 

We assume that at low concentrations atomic hydrogen enter into the van der Waals gap by the presented on the Fig. 2a schema and creates H2 molecules that occupy an ordered positions schematically shown in Fig. 2a. Appearance of hydrogen molecules in the gap, result in occurrence of interlayer pressure and in increasing of interlayer parameter C0. At higher concentrations atomic hydrogen incorporate into the crystal layers. 

Starting from this concept, let us estimate the concentration and pressure of H2 molecules in the van-der-Waals gap of HJnSe at x = 2. As the volume of y-InSc conventional hexagonal cell is equal V -351 A3, the concentration of elementary cells is N0 = l/ V = 2.85-1021 cm 3. At x = 2, one H2 molecule corresponds to one In2Se2 molecule. Hence, H2 molecular concentration is equal N = 37V0= 8.55 x 1021 cm"3. Using Clapeyron s equation for the ideal gas pressure P = NkBT, where kB is the Boltzmann constant and T is the absolute temperature, we can deduce that the pressure caused by H2 molecules in InSe van-der-Waals gap is equal to 9.4 MPa at... 

As seen from Fig. 2a, the van der Waals gap width is modulated periodically in positions of In atoms it is larger than in positions of Se atoms. This gap can be described as a layer of closely packed parallelepipeds, at the both ends of which pyramids are placed. The volume of this body (cavity) is equal to 50.5 A3, and for an ideal crystal, when defects of the dislocation type are absent the relative gap volume comprises 43% of the crystal bulk. It is obviously larger than the lower estimate 37% obtained using the ratio Q/(Q+Ci). 

Geometrical sizes of H2 molecule and van der Waals gap width of InSe crystal have such values that cause quantum-size effects. Using the well-known expression [12] for localization of a particle in the quantum well with infinite rectangular walls... 

As was shown in the previous part, the hydrogen concentration increase up to x = 2 gives rise to the pressure between layers up to P 9.4 MPa at T = XQK, which stimulates an increase of the van der Waals gap. Making extrapolation in accord with the analytical dependencies [9, 25], we obtained Eg growth by 0.2 meV at P =... 

At x = 2 and T < 80K this state of H2 in the gap can be treated as quazy-liquid monolayer . At temperatures of liquid helium molecular hydrogen in the van der Waals gap and layer crystal forms a supperlattice consisting from a layered crystal lattice and a lattice of molecular hydrogen cryocrystal built in its van-der-Waals gap. At x>2 atomic hydrogen begins to incorporate into interstices of the crystal lattice due to quantum-size effects arise in the gap and strong repulsion of between H2 molecules. 

Mo03 is known to crystallize in a layer lattice in which two-dimensional metal oxide sheets are separated by a van der Waals gap. Son et al.20) prepared linear Ni(II)-rubeanic acid coordination polymer (23) in the interlayer space of Mo03. 

In Clement s work, the van der Waals gap in the Mn2P2Se was expanded using an aqueous solution tetramethylammonium chloride at room (or near room) temperature in an intercalation reaction ... 

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