Recent developments of studies on proton transfer dynamics in the hydrogen bonds of carboxylic acid dimers are reviewed. The phonon assisted tunneling in an asymmetric double well potential in the crystalline lattice is discussed together with a coupling effect between proton and heavy atom motions. The tunneling rates in the low temperature limit have been determined by proton and deuteron NMR to be 107 s-1-109 s-1 for most of investigated carboxylic acids. In the high temperature region, a new dynamic phenomenon was found in the H/D mixed crystals of benzoic acid. The transfer rate of an HD pair in hydrogen-bonded dimeric unit of carboxyl groups is quenched as a concentration of D in the surrounding hydrogen bounds is increased. This suggests a new mechanism of the thermally activated proton transfer mediated by short-lived quantum correlations between protons.
The crystal structures of polymorphisms of free amino acids have been reviewed. The polymorphisms were classified into three types: The first, including DL-methionine, DL-norleucine and DL-a-amino-butyric acid, has polymorphs of different packing arrangement of molecular double layers which is rigidly bound by hydrogen bonds. In this type, the polymorphs can interconvert in solid state by temperature. The second type, glycine, L-cystine and L-histidine, has polymorphs in which hydrogen bonding schemes are different although molecular conformations are similar. The third type, L-cystein, L-aspartic acid and L-glutamic acid, has polymorphs with different hydrogen bonding schemes and different molecular conformations. In this type, metastable form converts irreversibly to stable form by solvent-mediated transformation mechanism.
A combined method of the channeling enhanced x-ray microanalysis (ALCHEMI) and the intersecting Kikuchi-line (IKL) technique, both of which are based on dynamical electron diffraction, is used for determining long range order parameters in a multi-component system. An application of the method to Ll0 ordering in CuAuPd ternary alloys is described, and the kinetic paths in time-evolution of long range order are discussed. This method expands a field of research on atomic configuration in multi-component alloys.
Microstructures of MBE grown Si12/Ge4 and Si18/Ge6 strained-layer superlattice were studied using a transmission electron microscope. The Ge/Si heterointerfaces were observed to be flat and abrupt. Those of Si/Ge were neither flat nor distinct. Dislocations mostly identified as 60 degree type, were observed in the Si substrate near the interface with the strained-layer superlattice. Strain introduced in the preparation of thin-films for electron microscopy was observed. Optical properties of the Si/Ge strained-layer superlattice were discussed on the basis of results of the microstructure study.
The defect structure models of LiNbO3 (lithium niobate) were reviewed, with emphasis on the “Li-site vacancy model” we recently proposed on the basis of the results of single crystal X-ray and powder neutron diffraction experiments. A detailed scheme of Mg-incorporation in the LiNbO3 structure was also presented using our defect model.
Recently, it is found that structures formed upon alkali-metal adsorption are strongly dependent on substrate temperature. At low temperature, alkali-metal atoms form overlayers occupying the highest coordination sites of substrate surfaces. At room temperature, some adsorption systems do no exhibit overlayer-formation. Instead, alkali-metal atoms replace substrate surface atoms initially following ordered surface-alloy formation with increasing coverage.