High temperature powder diffraction and solid state DFT study of β‐cryolite (Na3AlF6)

Neutron and synchrotron powder diffraction data for β‐cryolite were collected within 550‐800(880) °C. Atomic coordinates of Al, F and Na atoms were obtained by both neutron Rietveld refinement and energy minimization in solid state at DFT level of theory. It was shown that although Rietveld refinements with fluorine atoms statically displaced from the 24e to the 96k special position of the Fm‐3m space group provide a reasonable fit to the experimental data, the refined structure differ from that obtained by the total energy minimization. The structure with the minimum of the total energy corresponds to the low temperature α‐phase with a zigzag arrangement of the polyhedra built around the Al and Na atoms. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Spin-lattice coefficients of Mn2+ in II–VI compounds

The spin-lattice coefficients of Mn²⁺ in CdTe and ZnTe have been measured. Our data allow us to evaluate the random stress distribution within the samples. Our results are compared with values obtained for Mn²⁺ in MgO and CaO.     

Spectral inequalities and G-functions

The results of Ostrowski on nonsingularity of matrices through diagonal dominance are generalized to the case of monotonic norms. From these results, combined with a basic theorem of Ky Fan, one obtains refinements and generalizations of the classical inequalities of Carleman and Schur.     

A convolution for the Hankel–Kontorovich–Lebedev transformation

The analysis of a kernel involving the product of three Bessel functions motivates the introduction of the translation operator and the convolution associated to the Hankel–Kontorovich–Lebedev tranformation, first in a classical framework, and then in certain spaces of generalized functions. The main properties of this convolution are investigated, the more important operational rules are obtained and some applications are shown. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A MOF@COF Composite with Enhanced Uptake through Interfacial Pore Generation

Herein, we describe a new class of porous composites comprising metal–organic framework (MOF) crystals confined in single spherical matrices made of packed covalent‐organic framework (COF) nanocrystals. These MOF@COF composites are synthesized through a two‐step method of spray‐drying and subsequent amorphous (imine‐based polymer)‐to‐crystalline (imine‐based COF) transformation. This transformation around the MOF crystals generates micro‐ and mesopores at the MOF/COF interface that provide far superior porosity compared to that of the constituent MOF and COF components added together. We report that water sorption in these new pores occurs within the same pressure window as in the COF pores. Our new MOF@COF composites, with their additional pores at the MOF/COF interface, should have implications for the development of new composites.     

The influence of oxygen vacancies on the magnetic state of La<Subscript>0.50</Subscript>D<Subscript>0.50</Subscript>MnO<Subscript>3−γ</Subscript> (D=Da,Sr) manganites

The crystal structure and magnetic and electric transport properties of polycrystalline La_0.50D_0.50MnO_3?γmanganites (D=Ca, Sr) were studied experimentally depending on the concentration of oxygen vacancies. The La_0.50Sr_0.50MnO_3?γ system of anion-deficient compositions was found to be stable and possess a perovskite structure only up to the γ=0.25 concentration of oxygen vacancies, whereas, for the La_0.50Ca_0.50MnO_3?γ system, we were able to obtain samples with the concentrations of oxygen vacancies up to γ=0.50. The stoichiometric La_0.50D_0.50MnO₃ (D=Ca, Sr) compositions had O-orthorhombic (Ca) and tetragonal (Sr) unit cells. The unit cell of the anion-deficient La_0.50Sr_0.50MnO_3?γ manganites also became O-orthorhombic when the concentration of oxygen vacancies increased γτ;0.16). Oxygen deficiency in La_0.50Sr_0.50MnO_3?γ first caused the transition from the antiferromagnetic to the ferromagnetic state γ～0.06) and then to the spin glass state γ～0.16). Supposedly, the oxygen vacancies in the reduced La_0.50Sr_0.50MnO_{3? γ} samples with γ≥0.16 were disordered. The special feature of the La_0.50Ca_0.50MnO_3?γ manganites was a nonuniform distribution of oxygen vacancies in the La_0.50Ca_0.50MnO_2.75 phase. In the La_0.50Ca_0.50MnO_2.50 phase, the type of oxygen vacancy ordering corresponded to that in Sr₂Fe₂O₅, which led to antiferromagnetic ordering. The specific electric resistance of the La_0.50D_0.50MnO_3?γ anion-deficient samples increased with increasing oxygen deficiency. The magnetoresistance of all samples gradually increased as a result of the transition to the magnetically ordered state. Supposedly, the La_0.50Ca_0.50MnO_3?γ manganites in the range of oxygen vacancy concentrations 0.09≤γ≤0.50 had a mixed state and contained microdomains with different types of magnetic ordering. The experimentally observed properties can be interpreted based on the model of phase layering and the model of superexchange magnetic ordering.