Designed Synthesis of Solid State Structural Isomers from Modulated Reactants

Abstract

A series of new, kinetically stable, crystalline superlattice compounds containing integral number of intergrown dichalcogenide layers were prepared through controlled crystallization of Ti/Se/Nb/Se superlattice reactants with designed compositional modulation. Discussed in this paper is a family of structural isomers with the overall stoichiometry of NbTiSe4. Specifically, the members of this series containing 1/1, 2/2, 3/3, 4/4, 6/6, 8/8, 9/9, 12/12, and 15/15 TiSe2/NbSe2 units as the unit cell of the superlattice were prepared as polycrystalline thin films oriented with the dichalcogenide layers perpendicular to the substrate surface. Low-angle diffraction data collected as a function of time and temperature reveal that the initial layered reactant contracts in the c-axis direction and that the interfaces become smoother during initial interdiffusion. Theta−theta scans show a gradual decrease of the (00l) diffraction line widths of the growing compound as a function of annealing time and temperature indicating an increase in the c-axis domain size. High-angle rocking curve diffraction data show increasing c-axis orientation, suggesting TiSe2/NbSe2 crystal growth perpendicular to the substrate surface upon low-temperature annealing. The end products of this synthetic approach are high quality c-axis-oriented TiSe2/NbSe2 crystalline superlattices with designed structure resulting from relatively low-temperature annealing at 500 °C. A working model for the formation of crystalline superlattices from the modulated reactants is presented which is consistent with the data.