Large Interlayer Separation Research Articles

Magnetic Ordering in Three-Dimensional Metal–Organic Frameworks Based on Carboxylate Bridged Square-Grid Layers

Three isomorphous metal-organic frameworks of formula [M(ppdc)(H(2)O)(2)](n) [M = Mn(II), Fe(II), and Co(II)] were synthesized from sodium p-phenylenediacrylic (Na(2)ppdc). Crystallographic studies revealed that the compounds are layer-pillared 3D frameworks in which the square-grid M(II) layers with single carboxylate bridges are interlinked by long organic spacers with large interlayer separations of about 13 Å. Magnetic investigations indicated that they all display intralayer antiferromagnetic interactions through the carboxylate bridges in the unusual skew-skew coordination mode but the bulk behaviors are quite different. The Co(II) compound, like most compounds containing similar M-O-C-O-M layers, shows no 3D magnetic ordering down to 2 K, while the Mn(II) and Fe(II) compounds exhibit spin-canted ordering, behaving as a weak ferromagnet (T(C) = 3.8 K) and a metamagnet (T(N) = 3.8 K, H(c) = 650 Oe), respectively. Spin-canted ordering is still a rarity in this series of materials. Magnetostructural comparisons with analogous compounds indicate that the occurrence of spin-canted ordering can be related to the uncommon skew-skew and anti-anti coordination modes of carboxylate bridges, which induce stronger antiferromagnetic interactions than the common syn-anti mode.

Layering of ferrierite sheets by using large co-structure directing agents: Zeolite synthesis using 1-benzyl-1-methylpyrrolidinium and tetraethylammonium

In view of the success of our new synthesis methodology regarding the use of co-structure directing agents (co-SDA) in zeolite synthesis, and in a search for the discovery of new stable ferrierite-type layered materials, which are useful precursors for the productions of shape-selective catalysts, we have further explored the use of co-SDAs by using a new cation, tetraethylammonium (TEA), as co-SDA together with the larger 1-benzyl-1-methylpyrrolidinium cation (bmp). As a result, layered ferrierite-type materials with a large interlayer separation have been obtained. Interestingly, in contrast to the ferrierite layered materials obtained with quinuclidine and bmp using the same approach, these layered materials are stable in time, i.e., an increase of the crystallisation time does not lead in this case to an approach of the layers to finally yield a more condensed ferrierite (FER) structure, as was the case for the materials obtained with quinuclidine. A complementary computational study allowed us to gain insights into the mechanism of crystallisation of ferrierite-type materials in this system. Simulation results evidenced that the inability of TEA to direct the synthesis of the fully-condensed structure is due to the large size of the TEA cations that can be accommodated neither in the cavities nor in the 10 MR channels of the fully-condensed FER structure. Instead, the simulations show that the most stable situation for the accommodation of this co-SDA in the ferrierite cavities is reached when the FER layers are separated at an interlayer distance of ∼2 Å, in close agreement with the experimental findings.

Preparation and characterization of microporous SiO 2–ZrO 2 pillared montmorillonite

SiO 2–ZrO 2 pillared montmorillonite (SZM) was prepared by the reaction of Na-montmorillonite with colloidal silica–zirconia particles which were prepared by depositing zirconium hydroxy cations on silica particles. By pillaring with the colloidal particles, the basal spacing of montmorillonite was expanded to ca. 45 Å and the calcined SZM samples showed large specific surface areas up to 320 m 2/g at 400 °C. In spite of large interlayer separation, adsorption results indicated the presence of micropores generated between the colloidal particles. The microporous structure was maintained at least up to 600 °C and exhibited specific shape selectivity for the adsorption of large organic molecules, especially between toluene and mesitylene. According to the temperature-programmed-desorption (TPD) spectra of ammonia, the calcined SZM showed weakly acidic sites.

Polymeric Networks of Copper(II) Phenylmalonate with Heteroaromatic N-donor Ligands: Synthesis, Crystal Structure, and Magnetic Properties

Two new phenylmalonate-bridged copper(II) complexes with the formulas [Cu(4,4'-bpy)(Phmal)](n).2nH(2)O (1) and [Cu(2,4'-bpy)(Phmal)(H(2)O)](n)() (2) (Phmal = phenylmalonate dianion, 4,4'-bpy = 4,4'-bipyridine, 2,4'-bpy = 2,4'-bipyridine) have been synthesized and characterized by X-ray diffraction. Complex 1 crystallizes in monoclinic space group P2(1), Z = 4, with unit cell parameters of a = 9.0837(6) Angstroms, b = 9.3514(4) Angstroms, c = 11.0831(8) Angstroms, and beta = 107.807(6) degrees , whereas complex 2 crystallizes in orthorhombic space group C2cb, Z = 8, with unit cell parameters of a = 10.1579(7) Angstroms, b = 10.3640(8) Angstroms, and c = 33.313(4) Angstroms. The structures of 1 and 2 consist of layers of copper(II) ions with bridging bis-monodentate phenylmalonate (1 and 2) and 4,4'-bpy (1) ligands and terminal monodentate 2,4'-bpy (2) groups. Each layer in 1 contains rectangles with dimensions of 11.08 x 4.99 Angstroms(2), the edges being defined by the Phmal and 4,4'-bpy ligands. The intralayer copper-copper separations in 1 through the anti-syn equatorial-apical carboxylate-bridge and the 4,4'-bpy molecule are 4.9922(4) and 11.083(1) Angstroms, respectively. The anti-syn equatorial-equatorial carboxylate bridge links the copper(II) atoms in complex 2 within each layer with a mean copper-copper separation of 5.3709(8) Angstroms. The presence of 2,4'-bpy as a terminal ligand accounts for the large interlayer separation of 15.22 Angstroms. The copper(II) environment presents a static pseudo-Jahn-Teller disorder which has been studied by EPR and low-temperature X-ray diffraction. Magnetic susceptibility measurements of both compounds in the temperature range 2-290 K show the occurrence of weak antiferromagnetic [J = -0.59(1) cm(-1) (1)] and ferromagnetic [J = +0.77(1) cm(-1) (2)] interactions between the copper(II) ions. The conformation of the phenylmalonate-carboxylate bridge and other structural factors, such as the planarity of the exchange pathway in 1, account for the different nature of the magnetic interaction.

Frictional magnetodrag between spatially separated two-dimensional electron systems: Coulomb versus phonon mediated electron–electron interaction

We study the frictional drag due to Coulomb and phonon mediated electron–electron interaction in a double layer electron system exposed to a perpendicular magnetic field. We calculate the transresistivity as a function of magnetic field, temperature, and inter-layer spacing Λ. We find the strikingly different magnetic field and temperature dependence of the transresistivity for Λ=30 and 200 nm and ascribe this to the weak screening effect at large inter-layer separations.

Open-Framework Zinc Phosphates with Unusual Architectures

Open-framework zinc phosphates, I, [HMTA(H)3)]2[Zn3(PO4)2(HPO4)2][(H2PO4)2]·10H2O, and II [HMTA(H)3][Zn2(PO4)(HPO4)(H2PO4)][(H2PO4)], with unusual layered architectures, have been synthesized under ordinary conditions and characterized. The amine phosphate, HMTA-P, also has been synthesized and characterized. The structures of I and II contain strictly alternating ZnO4, PO4, H/H2PO4 tetrahedral units linked through their vertices forming macroanionic layers with a large interlayer separation of ∼20 Å. While I contains the charge compensating amine, the water, and the isolated free H2PO4 units in the interlamellar region, II possesses only the amine and the free H2PO4 units. In II, one H2PO4 units hangs from the layer in a direction perpendicular to the layers. All these structural features have been observed for the first time in open-framework zinc phosphates, in addition to the large interlamellar separation. The structure has extensive O−H...O, N−H...O, C−H...O hydrogen bonds that lend structural stability and support the extra-large separation between the inorganic sheets. The removal of the amine molecules in I and II causes the collapse of the framework forming condensed zinc phosphates. The structural details garnered from this study might be of some use in the design of more complex inorganic systems that involve larger amine molecules, especially for the synthesis of mesoporous solids.

Phonon-mediated transresistivity in a double-layer composite Fermion system

We consider the frictional drag in a double-layer system of two-dimensional electrons in the half-filled lowest Landau level. At sufficiently large interlayer separations the drag is dominated by the exchange of acoustic phonons and exhibits a variety of dependencies on temperature and distance between the layers. In all of the regimes, the phonon-mediated drag is found to be strongly enhanced compared to that in zero magnetic field.

A new thermally stable SiO 2–Cr 2O 3 sol pillared montmorillonite with high surface area

A new sol pillared clay is prepared by ion-exchange of Na + ions in montmorillonite with SiO 2–Cr 2O 3 mixed oxide sol particles, which are prepared by titrating a Cr 3+ aqueous solution with NaOH in the presence of silica sol particles. Upon pillaring of oxide sols, the basal spacing of montmorillonite is expanded up to 47 Å and the porous textures of pillared compounds are retained at least up to 700°C. In spite of large interlayer separation, the pillared clay exhibits characteristic adsorption behavior of microporous solid with a large BET surface area of 420 m 2/g. The temperature-programmed-desorption spectra of ammonia revealed that the sample calcined at 400°C has both strong Brönsted and weak Lewis acid sites, whereas the sample heated at 600°C shows significant Lewis acidity due to the formation of a new Si–O–Cr linkage.

Impurity states of two-dimensional magnetic trion and biexciton in a spatially separated electron-hole system

An interacting system of two-dimensional spatially separated electrons and holes containing charged impurities is considered in the strong magnetic field limit. Particles are assumed to occupy the zeroth Landau levels. The energies of the lowest impurity states of three-particle complexes and biexcitons are calculated for different projections of the total angular momentum M. It is shown that with increasing the interlayer separation the impurity states with higher momentum become more favorable and some of them become stable and bound if the interlayer separation is larger than a certain critical value. For impurity states of magnetic biexciton the lowest level corresponds to the state with an even value of M at any interlayer separation. At a very large interlayer separation the states with negative binding energy corresponding to the certain value of M have disappeared both for biexcitons and for three-particle complexes. The nature of the ground state in the presence of impurities as well as the possible implications of the results in experiments are discussed for such a system.

The crystal structure of nanpingite-2M2, the Cs end-member of muscovite

Other| February 01, 1996 The crystal structure of nanpingite-2M2, the Cs end-member of muscovite Yunxiang Ni; Yunxiang Ni Miami University, Department of Geology, Oxford, OH, United States Search for other works by this author on: GSW Google Scholar John M. Hughes John M. Hughes Search for other works by this author on: GSW Google Scholar American Mineralogist (1996) 81 (1-2): 105–110. https://doi.org/10.2138/am-1996-1-213 Article history first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Yunxiang Ni, John M. Hughes; The crystal structure of nanpingite-2M2, the Cs end-member of muscovite. American Mineralogist 1996;; 81 (1-2): 105–110. doi: https://doi.org/10.2138/am-1996-1-213 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyAmerican Mineralogist Search Advanced Search Abstract Nanpingite, ideally CsAl2(AlSi3)O10(OH)2, is a Cs end-member dioctahedral mica, crystallizing in C2/c, with a = 9.076(3), b = 5.226(2), c = 21.41(5) Å, and β = 99.48(6)°. The crystal structure of type nanpingite was refined to R = 0.058 using single-crystal X-ray diffraction methods. Nanpingite is a 2M2 polytype dioctahedral mica rather than a 2M2, polytype as reported originally.In comparison with the interlayer cations in common muscovite and paragonite, Cs+ in nanpingite increases the [001] interlayer separation between adjacent 2:1 layers but has little effect on the lateral (a, b) cell dimensions. The existence of the rare 2M2 polytype is attributed to this large interlayer separation, which minimizes the repulsion of the superimposed (along [001]) O anions in the basal planes of neighboring tetrahedral layers. The 2M2 polytype involves ditrigonal prismatic coordination of the interlayer Cs+, in contrast to the octahedral coordination of interlayer K+ and Na+ in muscovite-2M1 and paragonite-2M1. The tetrahedral rotation in nanpingite is the smallest among dioctahedral micas and is also caused by the incorporation of the large interlayer cation Cs+. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Phase transitions in a multiple quantum well in strong magnetic fields.

We study the phases of a multiple-quantum-well system of layered two-dimensional electrons in a strong perpendicular magnetic field. Solid phases are found at small fillings of the lowest Landau level which are the hexagonal-close-packed lattice at large interlayer separations and the body-centered-tetragonal lattice at small separations. We find that a sequence of incompressible-Laughlin-liquid-like states occurs at larger fillings. The energies and correlation functions of these states have been calculated by using Monte Carlo simulations. We also discuss the quasiparticle and collective excitations of the liquid states. We find that the electric charge carried by a quasiparticle can have a contribution in one layer which is an irrational fraction of the electron charge. It should be possible to observe the transitions between different Laughlin-like phases experimentally. Transport and microwave experiments are particularly promising in this regard.