Oxygen engineering techniques performed under adequate controlled atmosphere show that the CaMnO(3)-CaMnO(2) topotactic reduction-oxidation process proceeds via oxygen diffusion while the cationic sublattice remains almost unaltered. Extra superlattice reflections in selected area electron diffraction patterns indicate doubling of the CaMnO(2) rock-salt cell along the cubic directions of a distorted rhombohedral cell originated by ordering of Ca(2+) and Mn(2+) ions distributed in nanoclusters into a NaCl-type matrix, as evidenced by dark field electron microscope images. The local nature of the information provided by the transmission electron microscopy techniques used to characterize the rock-salt type Ca(1-x)Mn(x)O(2) solid solution clearly hints at the existence of subtle extra ordering in other upper oxides of the Ca-Mn-O system. The combination of local characterization techniques like electron microscopy with more average ones like powder X-ray and neutron diffraction allows a very complete characterization of the system.
A careful and wide comparison between Al and Ga as substitutional dopants in the ZnO wurtzite structure is presented. Both cations behave as n-type dopants and their inclusion improves the optical and electrical properties of the ZnO matrix, making it more transparent in the visible range and rising up its electrical conductivity. However, the same dopant/Zn ratio leads to a very different doping efficiency when comparing Al and Ga, being the Ga cation a more effective dopant of the ZnO film. The measured differences between Al-and Ga-doped films are explained with the hypothesis that different quantities of these dopant cations are able to enter substitutionally in the ZnO matrix. Ga cations seem to behave as perfect substitutional dopants, while Al cation might occupy either substitutional or interstitial sites. Moreover, the subsequent charge balance after doping appear to be related with the formation of different intrinsic defects that depends on the dopant cation. The knowledge of the doped-ZnO films microstructure is a crucial step to optimize the deposition of transparent conducting electrodes for solar cells, displays, and other photoelectronic devices. V
The stability of hydroxyapatite (OHAP) and beta-tricalcium phosphate (beta-TCP) ceramics depends on the temperature of preparation and the partial pressure of water in the reaction atmosphere. In bioceramics used in the study of biomaterials, these two phosphates often coexist in distinct proportions; therefore, structural characterization of the individual phases is hindered. The structure of these two individual calcium phosphates have been studied using X-ray diffraction, and mainly by high-resolution electron microscopy. Whereas the X-ray diffraction profile of OHAP has been refined in the space group P6(3)/m, the HREM images oriented along the [001] direction at the edges of the crystal show a threefold symmetry. The X-ray diffraction pattern of beta-TCP has been refined in the space group R3c and the HREM image oriented along [0001] zone axis shows regular contrast. Both these calcium phosphate ceramics are unstable to the electron beam for a prolonged exposure time, and show areas of decomposition in HREM images of thin regions of the crystal. The regions of decomposition in the beta-TCP crystal show the presence of CaO as one of the major phases.
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