Chiral II-VI semiconductor nanostructure superlattices based on an amino acid ligand

Reaction of L-cysteine with M(NO3)2 x xH2O (M = Cd, Zn) generates M(L-cysteinate), which feature one-dimensional substructures that can be viewed as fragments of bulk structures of CdS (rock salt high pressure phase) and ZnS (wurtzite) because of the bridging modes accessible to the sulfur atom of L-cysteine. The MS substructures are arranged in a regular and periodic fashion within the crystal via the carboxylate function of L-cysteine. Considering the structural similarities with bulk materials, the optical properties of M(L-cysteinate) were studied and indicate blue shifts of the band gap of 2.59 eV (M = Cd, compared to CdS rock salt) and 1.37 eV (M = Zn, compared to ZnS wurtzite) with respect to the bulk MS structures, due to the low dimensionality of the metal-sulfur arrangement. The chelating nature of the cysteine ligand imposes an unusual mer arrangement of three binding S moieties at Cd with a correspondingly high Cd coordination number in a chalcogenide-based material. Density of states calculations show strong electronic structure similarities with the bulk phases and rationalize the band gap changes.     

Modular construction of oxide structures--compositional control of transition metal coordination environments

The effects of reaction temperature and pO2 were investigated on a series of (Ba,Ca,Nd)FeO3-delta perovskite systems in order to isolate phases containing ordered arrangements of the distinct vacancy and cation ordering patterns identified in less compositionally complex iron oxide systems. Initial synthesis in air at high temperature yields cubic perovskite phases (I) with average iron oxidation states higher than 3; selected area electron diffraction together with diffuse features observed in the synchrotron X-ray diffraction (SXRD) patterns of these materials show evidence of small domains of short-range cation and vacancy order. Annealing these materials in nitrogen or in a sealed tube in the presence of an NiO/Ni buffer yielded the Fe(3+) phase Ca2Ba2Nd2Fe6O16 (II), closely related to Sr2LaFe3O8 but with partial cation order as well as anion order present the larger Ba cations are largely present in the 12-coordinate site between the octahedral iron layers, and Ca is largely present in 10-coordinate sites between octahedral and tetrahedral sites. Further reduction of Ca2Ba2Nd2Fe6O16 using a Zr getter yields the mixed-valence phase Ca2Ba2Nd2Fe6O15.6 (III). The structure of III was solved by maximum entropy analysis of XRD data coupled with analysis of high-temperature neutron diffraction data and refined against combined SXRD and high-Q ambient-temperature neutron data. This material crystallizes in a 20-fold perovskite super cell (Imma, a approximately square root(2 x a(p), b approximately 10 x a(p), c approximately square root(x 2a(p)) and can be visualized as an intergrowth between brownmillerite (Ca2Fe2O5) and the YBa2Fe3O8 structure. There are three distinct iron coordination environments, octahedral (O), square-pyramidal (Sp), and trigonal planar (Tp, formed by distorting the tetrahedral site in brownmillerite), which form a Sp-O-Tp-O-Sp repeat. Bond valence calculations indicate that Tp is an Fe(2+) site, while the O and Sp sites are Fe(3+). The A-site cations are also partially ordered over three distinct sites: 8-coordinate between the Sp layers, 10-coordinate between Tp and O layers, and 12-coordinate between Sp and O layers. Mossbauer spectroscopy, magnetometry, and variable-temperature neutron diffraction show that the material undergoes two magnetic transitions at approximately 700 and 255 K.     

Chemical and Structural Stability of Zirconium‐based Metal–Organic Frameworks with Large Three‐Dimensional Pores by Linker Engineering

The synthesis of metal–organic frameworks with large three‐dimensional channels that are permanently porous and chemically stable offers new opportunities in areas such as catalysis and separation. Two linkers (L1=4,4′,4′′,4′′′‐([1,1′‐biphenyl]‐3,3′,5,5′‐tetrayltetrakis(ethyne‐2,1‐diyl)) tetrabenzoic acid, L2=4,4′,4′′,4′′′‐(pyrene‐1,3,6,8‐tetrayltetrakis(ethyne‐2,1‐diyl))tetrabenzoic acid) were used that have equivalent connectivity and dimensions but quite distinct torsional flexibility. With these, a solid solution material, [Zr₆O₄(OH)₄(L1)_2.6(L2)_0.4]?(solvent)_x, was formed that has three‐dimensional crystalline permanent porosity with a surface area of over 4000 m² g^?1 that persists after immersion in water. These properties are not accessible for the isostructural phases made from the separate single linkers.     

Control of magnetic ordering by Jahn--Teller distortions in Nd(2)GaMnO(6) and La(2)GaMnO(6).

The substitution of Ga(3+) into the Jahn--Teller distorted, antiferromagnetic perovskites LaMnO(3) and NdMnO(3) strongly affects both the crystal structures and resulting magnetic ordering. In both compounds the Ga(3+) and Mn(3+) cations are disordered over the six coordinate sites. La(2)GaMnO(6) is a ferromagnetic insulator (T(c) = 70 K); a moment per Mn cation of 2.08(5) mu(B) has been determined by neutron powder diffraction at 5 K. Bond length and displacement parameter data suggest Jahn--Teller distortions which are both coherent and incoherent with the Pnma space group symmetry of the perovskite structure (a = 5.51122(4) A, b = 7.80515(6) A, c = 5.52947(4) A) at room temperature. The coherent distortion is strongly suppressed in comparison with the parent LaMnO(3) phase, but the displacement ellipsoids suggest that incoherent distortions are significant and arise from local Jahn--Teller distortions. The preparation of the new phase Nd(2)GaMnO(6) has been found to depend on sample cooling rates, with detailed characterization necessary to ensure phase separation has been avoided. This compound also adopts the GdFeO(3)-type orthorhombically distorted perovskite structure (space group Pnma, a = 5.64876(1) A, b = 7.65212(2) A, c = 5.41943(1) A at room temperature). However, the B site substitution has a totally different effect on the Jahn--Teller distortion at the Mn(3+) centers. This phase exhibits a Q(2) mode Jahn--Teller distortion similar to that observed in LaMnO(3), although reduced in magnitude as a result of the introduction of Ga(3+) onto the B site. There is no evidence of a dynamic Jahn-Teller distortion. At 5 K a ferromagnetically ordered Nd(3+) moment of 1.06(6) mu(B) is aligned along the y-axis and a moment of 2.8(1) mu(B) per Mn(3+) is ordered in the xy plane making an angle of 29(2) degrees with the y-axis. The Mn(3+) moments couple ferromagnetically in the xz plane. However, along the y-axis the moments couple ferromagnetically while the x components are coupled antiferromagnetically. This results in a canted antiferromagnetic arrangement in which the dominant exchange is ferromagnetic. Nd(2)GaMnO(6) is paramagnetic above 40(5) K, with a paramagnetic moment and Weiss constant of 6.70(2) mu(B) and 45.9(4) K, respectively. An ordered moment of 6.08(3) mu(B) per Nd(2)GaMnO(6) formula unit was measured by magnetometry at 5 K in an applied magnetic field of 5 T.     

Desolvation of a Novel Microporous Hydrogen-Bonded Framework: Characterization by In Situ Single-Crystal and Powder X-ray Diffraction

Despite significant structural rearrangement upon desolvation of a three-dimensional molecular framework of hexaaquacobalt cations and redox-active functionalized tetrathiafulvalene anions (see the picture; the area filled with water molecules is shown in gray), monocrystallinity and microporosity are retained. X-ray analyses show that a unique combination of hydrogen bonds and π⋅⋅⋅π interactions within the framework gives this material a structural flexibility not seen in zeolites or their analogues.     

Encapsulation of Crabtree's Catalyst in Sulfonated MIL‐101(Cr): Enhancement of Stability and Selectivity between Competing Reaction Pathways by the MOF Chemical Microenvironment

Crabtree's catalyst was encapsulated inside the pores of the sulfonated MIL‐101(Cr) metal–organic framework (MOF) by cation exchange. This hybrid catalyst is active for the heterogeneous hydrogenation of non‐functionalized alkenes either in solution or in the gas phase. Moreover, encapsulation inside a well‐defined hydrophilic microenvironment enhances catalyst stability and selectivity to hydrogenation over isomerization for substrates bearing ligating functionalities. Accordingly, the encapsulated catalyst significantly outperforms its homogeneous counterpart in the hydrogenation of olefinic alcohols in terms of overall conversion and selectivity, with the chemical microenvironment of the MOF host favouring one out of two competing reaction pathways.