Determination of the point and space groups for hydroxyapatite by computer simulation of CBED electron diffraction patterns

Experimentalmente se ha observado que la estructura de la hidroxiapatita natural nHAP (i.e. el componente inorgánico mayoritario de los dientes y los huesos) no ha sido completamente caracterizada hasta el momento. Esto nos lleva a estudiar las características estructurales y cristalográficas de la hidroxiapatita sintética sHAP (i.e. aquella que obedece la estequiometría de la fórmula Ca10 (PO4)6 (OH)2) por medio de diferentes técnicas, en particular la simulación computacional de los patrones de difracción electrónica, para contar con una base sólida que nos permita compararlas. En este trabajo se comentan los elementos de simetría que se observan en los patrones simulados computacionalmente de haz convergente (CBED) para sHAP, en sus versiones de zonas de Laue de orden cero (ZOLZ), de primer orden (FOLZ), de segundo orden (SOLZ) y de orden mayor (HOLZ), y se deduce su grupo espacial P63/m a partir de estos. Los resultados obtenidos se comparan con los reportados experimentalmente para nHAP

Palladium nanoparticles from solvated atoms - stability and HRTEM characterization

Palladium particles of nanometric dimensions were synthesized by the chemical liquid deposition method in which the Pd atoms were co-deposited at 77 K with 2-propanol, acetone, and tetrahydrofurane vapor to obtain colloidal dispersions. The colloidal dispersions were characterized by UV-visible spectrophotometry, transmission electron microscopy (TEM) and high-resolution TEM (HRTEM). The palladium colloids synthesized by use of these solvents are very stable. A strong absorption band in the UV region suggests that quantum confinement occurs in the nanoparticles obtained by this procedure. Studies of TEM micrographs reveal average size distributions between 1 and 5 nm for all Pd colloids. Whereas particle sizes in Pd-2-propanol colloids are not very sensitive to concentration change, the particle-size average in Pd-acetone and Pd-THF increases by 0.5 nm when the concentration increases from 10(-3) to 10(-2) mol l(-1). The HRTEM results show the high crystallinity of Pd nanoparticles and three low-energy structure shapes were found: cuboctahedron, tetrakaidecahedron, and icosahedron

Control of Texture During Vapor Deposition of Al on (111) Si

The growth of Al on (111) Si single crystal substrates by various techniques usually leads to films with (111) texture, sometimes with a small (100) component. Using X-ray diffraction and electron microscopy, the present study shows that the (100) texture component can be enhanced to the point of forming an oriented (100) continuous tricrystal structure. The formation of this texture is shown to be related the presence of Cu. It is concluded that an understanding of heteroepitaxy must take into account the effect of chemistry in addition to the crystallographic criteria of lattice matching