Face-centered tetragonal (fct) η-phase manganese nitride films have been grown on magnesium oxide (001) substrates by molecular-beam epitaxy. For growth conditions described here, reflection high energy electron diffraction and neutron scattering show primarily two types of domains rotated by 90° to each other with their c axes in the surface plane. Scanning tunneling microscopy images reveal surface domains consisting of row structures which correspond directly to the bulk domains. Neutron diffraction data confirm that the Mn moments are aligned in a layered antiferromagnetic structure. The data are consistent with the fct model of G. Kreiner and H. Jacobs for bulk Mn3N2 [J. Alloys Compd. 183, 345 (1992)].
Coherent quasielastic neutron scattering has been used to investigate the character of the rotational dynamics in the high-temperature solid phase of C6o. The observed scattering can be described by a model in which each molecule undergoes rotational diffusion which is uncorrelated with the motions of adjacent molecules. The rotational diffusion constant DR is (I.4±0.4)x I0'^ s~' at 260 K and (2.8 ±0.8)X 10'^ s~' at 520 K. The temperature dependence of DR is consistent with a thermally activated process having an activation energy of 35 ± 15 meV. 36.90.+f, 82.80.Jp C60 (buckminsterfullerene) forms a mostly orientationally ordered [1] simple cubic phase belonging to the space group Fa2> [2] below -255 K. Near this temperature it undergoes a transition [3] to a face-centered-cubic phase which is an excellent example of a broad class of molecular solids, sometimes termed "plastic crystals," which exhibit rapid molecular reorientations [4,5]. Quasielastic neutron scattering is capable of determining both the time scale of reorientations and, more importantly, the detailed nature of the reorientations [6]. Here we present such a study of the rotational dynamics of C6o in the orientationally disordered, high-temperature phase and show that the observed scattering is well described by a model in which there are no intermolecular correlations, and each molecule reorients completely randomly, as opposed to jump rotations between a specific set of sites.The sample consisted of -2.5 g of powdered Ceo of which 1.5 g were prepared at the University of Pennsylvania and 1.0 g at Exxon, using standard methods [7,8]. After the chromatographic separation of Ceo from the Ceo-Cyo extract, great care was taken to remove the solvent, since neutrons are -15 times more sensitive to hydrogen than to carbon. Prompt gamma activation analysis [9] showed a hydrogen content of 0.008 ± 0.002 atom per C atom for the Exxon sample and 0.0057 ±0.0011 atom per C atom for the Penn sample. Scattering from the H in the residual solvent was observed in our experiments, but does not affect our conclusions.The neutron scattering results were obtained using triple-axis spectrometers at the Neutron Beam Split-Core Reactor (NBSR) at NIST. For scattering vectors, g, below about 4.0 A~\ the neutrons were monochromated and analyzed using the (002) reflection of pyrolytic graphite. Collimators having angular spreads of 10'-20'-20'-20' were placed before and after both crystals, resulting in a measured energy resolution of 0.50 meV full width at half maximum (FWHM) at the elastic position. To reach larger values of g a shorter wavelength was necessary. In order to maintain a relatively good energy resolution, the Cu (220) reflection was chosen to monochromate the beam and the graphite (004) reflection was used to analyze the energy of the scattered neutrons. Combined with collimations of 40'-40'-40'-40' , this resulted in a measured resolution of 1.2 meV FWHM. For measurements in the fixed final energy configuration, the data were corrected for the separately m...
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