The effect of minor addition of 3d transition metals on the formation enthalpy (△H) and electronic structure of MgH2 have been studied using first-principle calculations, and considering the phonon-calculated zero point energy. The results indicate that the partial substitution of Mg atoms by 3d transition metal atoms increases the formation enthalpy of MgH2. Both formation enthalpy and Mulliken population analysis showed that the ability to destabilize MgH2 generally increases with the atomic number, except Mn and Zn, which have half-filled and completely filled 3d orbital states. The destabilization of MgH2 by partially alloying 3d elements was due to relatively stronger covalent bonds between 3d elements and the H atom, and a weaker ionic bond between Mg and H in the alloyed material with respect to pure MgH2. Based on electronic structure analyses, MgH2 and MgH2 alloyed with Ti, Fe, and Zn show no spin magnetism, while MgH2 alloyed with Sc, V, Cr, Mn, Co, Ni, and Cu show spin magnetism. In the MgH2-3d metal system except Zn, the bonding peak near the Fermi energy is mainly contributed by 3d electrons of transition metals and weak H (s) states. The bonding nature of MgH2 is ionic, and the bonding nature of MgH2-3d metal systems is mainly ionic with covalent bonds between 3d metal atoms and their neighbor H atoms.

It is an interesting phenomenon for natural organisms to have control over the shapes and sizes of inorganic nanocrystals and arrange them into ordered superstructures. This phenomenon attracts many attempts to mimic the biomineralization process to synthesize novel materials. In the present work, a new superstructure of hexagonal vaterite mesocrystals consisting of nanocrystals has been synthesized via a mesoscale transformation process by controlled release of carbon dioxide through slow decomposition of dimethyl carbonate in the presence of cetyltrimethyl ammonium bromide at room temperature. The superstructures are composed of hundreds of well-stacked vaterite flakes, which build spheres with an axis and two poles. We can find that mesocrystal plates at the poles are arranged with their surfaces parallel to the axis, whereas the plates at the equator of the spheres are arranged with their surface vertical to their axes. The flakes present the unstable (001) planes and show hexagonal shapes with a thick core and thin edges. The subunits of vaterite flakes consist of oriented aggregation of nanocrystals which are transformed from the amorphous phase. New evidence of the mesoscale transformation process has been observed in detail by measuring the structures of the products at different reaction stages, revealing that the product evolves from amorphous oblate particles to doughnutlike particles, which crystallize and delaminate into superstructures. The mineralization process is discussed with the cooperative reorganization of coupled inorganic and organic components relevant for models of matrixmediated nucleation in biomineralization. This contribution will be helpful in understanding the aggregationdriven formation of complex and higher-order structured materials as well as the biomineralization mechanisms.

Highly ordered WO3/TiO2 composite nanotubes have been successfully prepared by the combination of the sol–gel chemical method and the anodic aluminium oxide (AAO) templating method. The diameter of the WO3/TiO2 composite nanotubes is about 100 nm, which is in good agreement with the pore diameter of the AAO template. The composite nanotubes are composed of mixed oxides of W6+ and Ti4+.

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