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.     

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.     

Determination of the Preassembled Nucleating Units That Are Critical for the Crystal Growth of the Metal–Organic Framework CdIF‐4

Identifying the form and role of the chemical species that traverse the stages of crystallization is critical to understanding the formation process of coordination polymers. Herein, we report the combined use of in situ atomic force microscopy and mass spectrometry to identify preformed, complex, cadmium 2‐ethylimidazole containing solution species in the growth solution of the cadmium 2‐ethylimidazolate metal–organic framework CdIF‐4, and show that they are critical in the surface nucleation for the crystal growth of this material. Surface nucleation appears to be instigated by these [Cd_x(CH₃CO₂)_y(C₅H₇N₂/C₅H₈N₂)_z]‐containing solution species and not by sole addition of the ligand molecules. The CH₃CO₂^? or Cd(CH₃CO₂)₂ groups of the former are substituted subsequently as the framework growth proceeds. Our greater understanding of such solution species and their role in crystallization will guide future syntheses of designed functional coordination polymers.     

Large coupled magnetoresponses in EuNbO2N

We have explored a new strategy to discover materials with large resistive or capacitive responses to magnetic fields by synthesizing EuMO2N (M = Nb, Ta) perovskites that combine ferromagnetic order of S = 7/2 Eu2+ spins with possible off-center distortions of the d0 M5+ cations enhanced by covalent bonding to N. EuNbO2N shows colossal magnetoresistances at low temperatures and a giant magnetocapacitance. However, the latter response originates from a microstructural effect rather than an intrinsic multiferroism.     

Combined powder neutron and X-ray diffraction study of charge and orbital order in Bi0.75Sr0.25MnO3

The room temperature structure of Bi_0.75Sr_0.25MnO₃ has been fitted to high-resolution synchrotron X-ray and time-of-flight neutron powder diffraction data. Constrained structural models were refined using a Pn11 supercell (, , , and α=89.894(1)°) of the underlying Pnma perovskite structure. The best-fit model evidences a 3:1 Mn³⁺/Mn⁴⁺charge ordering with only 30% of the ideal separation of bond valence sums. An ordered intergrowth of antiferro-orbitally ordered (LaMnO₃ type) and charge and ferro-orbitally ordered (YBaMn₂O₆ type) blocks is observed. Off-centre Bi/Sr displacements are ferroelectrically ordered in this model.     

Revelation of the molecular assembly of the nanoporous metal organic framework ZIF-8

Crystalline nanoporous materials are one of the most important families of complex functional material. Many questions pertaining to the molecular assembly mechanism of the framework of these materials remain unanswered. Only recently has it become possible to answer definitively some of these questions by observation of growing nanoscopic surface features on metal organic frameworks (MOFs) through use of in situ atomic force microscopy (AFM). Here we reveal that a growth process of a MOF, zeolitic imidazolate framework ZIF-8, occurs through the nucleation and spreading of successive metastable unenclosed substeps to eventually form stable surface steps of the enclosed framework structure and that this process is reliant on the presence of nonframework species to bridge the developing pores during growth. The experiments also enable identification of some of the fundamental units in the growth process and the stable crystal surface plane. The former findings will be applicable to numerous nanoporous materials and support efforts to synthesize and design new frameworks and to control the crystal properties of these materials.     

Sub-Tc electron transfer at the HTSC/polymer interface

The first kinetic measurements for electron transfer (ferrocene/ferricinium reaction) at the interface between an HTSC (Tl,Pb1223) and a redox polymer (ferrocene-tagged poly-pyrrole) show that superconductivity affects electron transfer rate, which thus offers a novel probe of the superconducting state.