The Geometrical Optimization Of Alumina Clusters

In bulk solid of alumina, the ratio of metal to oxygen is2:3and alumina occurs in several different polymorphs on the basis of the variety of packing modes for Al and O atoms, such as a-alumina, β-alumina, and y-alumina. These isomers show great different properties because of differences in structure so that they are widely used in many fields of people’s daily life. But Alumina clusters are intermediates in size between molecules and bulk solids revealing its differences in property with them. It means that alumina clusters are unique in structure. Although a mass of investigations on alumina clusters have been performed, some problems have not been resolved, for instance, how does the structural transformation of alumina from molecules or atoms towards clusters occur? How may the cluster structures and properties change and what size do both alumina clusters and its bulk show same properties with further increasing in cluster size? So it is necessary to make a systematic research.In the paper, we respectively obtain the structures of alumina clusters via genetic algorithm plus density functional theory method and molecular mechanics doing some investigations about their properties.The work mainly is listed as follows:1. Structural determination of (Al2O3)n clusters based on density functional calculationWe obtained the geometrical structures of (Al2O3)n (n=1-7) clusters by using genetic algorithm plus density functional theory method. The structural properties are discussed. The global minimum structures are kite-shaped, cage and tea-cozy in structure for n=1,2and3respectively, disordered at n=4and5. At n=6and7, twelve and six lower-energy isomers are found respectively compared to the most stable structures obtained by Rahane et al. The structures show preference of disor-der and many new configurations are found with increasing cluster size, in which some isomers are nearly degenerate in energy. The different basis sets have great impact on the relative energies of various structures. 2. Structural determination of (A12O3)n clusters based on molecular mechanicsWe obtained the geometrical structures of (Al2O3)n (n=1-7) clusters via the evolutionary algorithms based on gradient optimization. The structural properties are discussed. The method can help to obtain a number of new structures in addition to some configurations that are similar to those calculated by the DFT method. Meanwhile, isomers with a higher symmetry are higher in energy suggesting that the structures show preference of disorder with further increase in cluster size.