Charge disorder effects in 3d transition metal oxide perovskites

The effects of the random electrostatic potential due to differences in formal charge between A site R³⁺ (lanthanide, Y), M²⁺ (Ca, Sr, Ba) and Th⁴⁺ cations have been investigated in ferromagnetic AMnO₃ and superconducting A₂CuO₄ perovskites. Series of samples in which the mean A site charge and the mean and variance of the A cation radius distribution are held constant, but the A site charge variance increases, show no significant changes of the electronic (Curie or superconducting) transition temperatures. The effect of the A cation random potentials on electronic transitions in the 3d metal oxide perovskites are insignificant in comparison to the lattice effects from the differing cation sizes.     

Structural, magnetic, and spectroscopic studies of YAgSn, TmAgSn, and LuAgSn

The rare earth-silver-stannides YAgSn, TmAgSn, and LuAgSn were synthesized from the elements by arc-melting and subsequent annealing. The three stannides were investigated by X-ray powder and single-crystal diffraction: NdPtSb type, P6₃mc, Z=2, a=468.3(1), pm, wR₂=0.0343, 353 F² values, 12 variables for YAgSn, and ZrNiAl type, P6¯2 m, a=726.4(2), , wR₂=0.0399, 659 F² values, 15 variables for TmAgSn, and a=723.8(2), , wR₂=0.0674, 364 F² values, 15 variables for LuAgSn. Besides conventional laboratory X-ray data with monochromatized Mo radiation, the structures were also refined on the basis of synchrotron data with , in order to clarify the silver-tin ordering more precisely. YAgSn has puckered, two-dimensional [AgSn] networks with Ag-Sn distances of 278 pm, while the [AgSn] networks of TmAgSn and LuAgSn are three-dimensional with Ag-Sn distances of 279 and 284 pm for LuAgSn. Susceptibility measurements indicate Pauli paramagnetism for YAgSn and LuAgSn. TmAgSn is a Curie-Weiss paramagnet with an experimental magnetic moment of 7.2 μ_B/Tm. No magnetic ordering is evident down to 2 K. The local environments of the tin sites in these compounds were characterized by ¹¹⁹Sn Mössbauer spectroscopy and solid-state NMR (in YAgSn and LuAgSn), confirming the tin site multiplicities proposed from the structure solutions and the absence of Sn/Ag site disordering. Mössbauer quadrupolar splittings were found in good agreement with calculated electric field gradients predicted quantum chemically by the WIEN2k code. Furthermore, an excellent correlation was found between experimental ¹¹⁹Sn nuclear magnetic shielding anisotropies (determined via MAS-NMR) and calculated electric field gradients. Electronic structure calculations predict metallic properties with strong Ag-Sn bonds and also significant Ag-Ag bonding in LuAgSn.     

High-temperature neutron diffraction study of the cation ordered perovskites TbBaMn2O5+x and TbBaMn2O5.5−y

The miscibility of TbBaMn₂O_5+x and TbBaMn₂O_5.5−y has been investigated at 100-600 °C using in situ powder neutron diffraction. No miscibility is observed, and the two phases remain oxygen stoichiometric (x,y=0) at 600 °C. Structure refinement results show that neither material undergoes a phase transition in this temperature range. TbBaMn₂O₅ is Mn²⁺/Mn³⁺ charge ordered and any charge melting transition is >600 °C. This symmetry-broken charge ordering is remarkably robust in comparison to that in other oxides.     

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Unconventional magnetic correlations in DyB2C and HoB2C

Layered borocarbides RB2C (R=Dy, Ho, and Er) have been studied by powder neutron diffraction at 2-30 K. ErB2C has two-sublattice antiferromagnetic order below T(N)=16.3 K, but DyB2C and HoB2C show a coexistence of a conventional canted k=(000) ferromagnetic structure and unconventional magnetic correlations. The k=(000) phase orders at T(c)=8.5 K (DyB2C) and 7.1 K (HoB2C), but low-Q diffraction peaks from the unconventional correlations appear above T(c) with different critical temperatures for different peaks: at 8, 10.5, and 15.7 K for HoB2C. This scattering is fitted as diffraction from a Warren-type random magnetic layer lattice and may result from quadrupolar interactions between R3+ spins.     

Atomic Force Microscopy of Novel Zeolitic Materials Prepared by Top‐Down Synthesis and ADOR Mechanism

Top‐down synthesis of 2D materials from a parent 3D zeolite with subsequent post‐synthetic modification is an interesting method for synthesis of new materials. Assembly, disassembly, organisation, reassembly (ADOR) processes towards novel materials based on the zeolite UTL are now established. Herein, we present the first study of these materials by atomic force microscopy (AFM). AFM was used to monitor the ADOR process through observation of the changes in crystal surface and step height of the products. UTL surfaces were generally complex and contained grain boundaries and low‐angle intergrowths, in addition to regular terraces. Hydrolysis of UTL to IPC‐1P did not have adverse effects on the surfaces as compared to UTL. The layers remained intact after intercalation and calcination forming novel materials IPC‐2 and IPC‐4. Measured step heights gave good correlation with the X‐ray diffraction determined d₂₀₀‐spacing in these materials. However, swelling gave rise to significant changes to the surface topography, with significantly less regular terrace shapes. The pillared material yielded the roughest surface with ill‐defined surface features. The results support a mechanism for the majority of these materials in which the UTL layers remain intact during the ADOR process as opposed to dissolving and recrystallising during each step.     

Crystal Growth Mechanisms and Morphological Control of the Prototypical Metal–Organic Framework MOF‐5 Revealed by Atomic Force Microscopy

Crystal growth of the metal–organic framework MOF‐5 was studied by atomic force microscopy (AFM) for the first time. Growth under low supersaturation conditions was found to occur by a two‐dimensional or spiral crystal growth mechanism. Observation of developing nuclei during the former reveals growth occurs through a process of nucleation and spreading of metastable and stable sub‐layers revealing that MOFs may be considered as dense phase structures in terms of crystal growth, even though they contain sub‐layers consisting of ordered framework and disordered non‐framework components. These results also support the notion this may be a general mechanism of surface crystal growth at low supersaturation applicable to crystalline nanoporous materials. The crystal growth mechanism at the atomistic level was also seen to vary as a function of the growth solution Zn/H₂bdc ratio producing square terraces with steps parallel to the <100> direction or rhombus‐shaped terraces with steps parallel to the <110> direction when the Zn/H₂bdc ratio was >1 or about 1, respectively. The change in relative growth rates can be explained in terms of changes in the solution species concentrations and their influence on growth at different terrace growth sites. These results were successfully applied to the growth of as‐synthesized cube‐shaped crystals to increase expression of the {111} faces and to grow octahedral crystals of suitable quality to image using AFM. This modulator‐free route to control the crystal morphology of MOF‐5 crystals should be applicable to a wide variety of MOFs to achieve the desired morphological control for performance enhancement in applications.     

The development of a comprehensive toolbox based on multi-level,high-throughput screening of MOFs for CO/N2 separations

The separation of CO/N₂ mixtures is a challenging problem in the petrochemical sector due to the very similar physical properties of these two molecules, such as size, molecular weight and boiling point. To solve this and other challenging gas separations, one requires a holistic approach. The complexity of a screening exercise for adsorption-based separations arises from the multitude of existing porous materials, including metal–organic frameworks. Besides, the multivariate nature of the performance criteria that needs to be considered when designing an optimal adsorbent and a separation process – i.e. an optimal material requires fulfillment of several criteria simultaneously – makes the screening challenging. To address this, we have developed a multi-scale approach combining high-throughput molecular simulation screening, data mining and advanced visualization, as well as process system modelling, backed up by experimental validation. We have applied our recent advances in the engineering of porous materials'' morphology to develop advanced monolithic structures. These conformed, shaped monoliths can be used readily in industrial applications, bringing a valuable strategy for the development of advanced materials. This toolbox is flexible enough to be applied to multiple adsorption-based gas separation applications.

The separation of challenging mixtures through adsorption is a multidimensional problem that requires a holistic approach. Our toolbox combines experiments, molecular and process simulations with data visualization to find optimal, porous materials.