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Elastic neutron diffraction has been used to study the structure of N2 films adsorbed on the (0001) surfaces of an exfoliated graphite substrate at coverages 0 between 1.0 and 1.67 layers and at temperatures below 11 K. For 0=1.0, the diffraction patterns can be fit by the in-plane rectangular V 3 X 3 herringbone structure denoted C (commensurate), previously inferred from low-energy electron diffraction (LEED) experiments. Analysis of the relative Bragg-peak intensities in the neutron diffraction pattern of the C phase extends the LEED results by yielding a value of /=45'+5 for the angle between the N -N bond and the short axis of the unit cell. Also, we find a substantially smaller Debye-Wailer factor for this phase than previously inferred from x-ray experiments. At coverages 8=1.13 and 1.27, the diffraction patterns are consistent with the uniaxial incommensurate (UI) phase seen by LEED. The patterns can be fit with the same molecular orientational parameters as for the C phase. We find the compression of the monolayer to be complete at 6=1.67 where the film density is -10% greater than for the C phase. Fits to the diffraction pattern at this coverage indicate a slight oblique distortion of the unit cell from hexagonal symmetry. For this nearly triangular incommensurate (TI) phase, we obtain orientational parameters in the ranges 30' & P &45' and 10 & P & 20 where l3 is the tilt angle of the N -N bond with respect to the surface.At coverages between 6=1.27 and 1.40, there is evidence of coexistence of the UI and TI phases.

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A series of Mössbauer, x-ray, and magnetic measurements has been made on wustite (Fe1−xO). The main new results are: (a) The dependence of the Mössbauer quadrupole splitting on heat treatment is very similar to that of the atomic constant, confirming Hoffman's conclusions on the variation of x. (b) The Zeeman Mössbauer lines of untreated wustite below the Curie temperature are completely smeared out; for a sample treated so as to give minimum quadrupole splitting (minimum x) these lines appear. (c) In a heat-treated sample a Zeeman splitting in the Mössbauer spectrum is observed well above the bulk wustite Curie temperature. This is interpreted as the imposition by the magnetic decomposition products of a magnetic ordering on the otherwise paramagnetic residual wustite with which they are in intimate contact.

Sn impurities in Pb and Ag hosts have been investigated by Mossbauer effect and in Pb by x-rayabsorption fine-structure (XAFS) studies. The Sn atoms are dissolved up to at least 2 at. %%u o inP ban dup to at least 8 at. % in Ag for the temperature ranges investigated. The concentration limit for Sn-Sn interactions is 1 at. % for Pb and 2 at. % for Ag as determined experimentally by lowering the Sn concentration until no appreciable change occurs in the Mossbauer effect. XAFS measurements verify that the Sn impurities in Pb are dissolved and predominantly at substitutional sites. For both hosts the temperature dependence of the spectral intensities of isolated Sn impurities below a temperature To is as expected for vibrating about a lattice site. Above To the Mossbauer spectral intensity exhibits a greatly increased rate of drop-off with temperature without appreciable broadening. This drop-off is too steep to be explained by ordinary anharmonic effects and can be explained by a liquidlike rapid hopping of the Sn, localized about a lattice site. Higher-entropy-density regions of radii somewhat more than an atomic spacing surround such impurities, and can act as nucleation sites for three-dimensional melting.

A Mossbauer resonance study of FeC12 monolayers deposited on oriented basal planes of graphite (Grafoil) is reported. Samples with fractional monolayer coverages between 0.2 and 0.9 were studied between 300 and 80'K. The spectra show two distinct quadrupole doublets; one with a room-temperature splitting of -2 mm/sec {designated L) and the other -0.8 mm/sec (8), which is identical to the splitting of bulk FeC1, and has the same shift. There is a marked difference between the intensities of the two lines of each doublet: For each sample, the ratio I+/I depends on the orientation of the planes of the Grafoil sheets relative to k". The intensity ratio for k"normal to the plane of the foilsprogresses from I+/I p 1 to I+!I & 1 with reduction of coverage. The total intensity of the 3 doublet relative to L is reduced with decreasing coverage. The velocity shift of L and the temperature dependence of the intensity of both L and B doublets differ markedly from those of bulk FeC1~and other known Fe++ compounds. The results suggest the existence of at least two distinct phases of FeC12 in the film samples, and that the FeC12-graphite samples involve monolayer surface states rather than bulk FeCl, aggregates. A calculation of the adsorption binding energy from the temperature dependence of the Debye-%'aller factor yields about 80 kcal/mole for 8 and about 60 kcal/mole for the L phase; both values are larger than the heats of fusion (10 kcal/mole) and evaporation (30 kcal/mole) of bulk FeCl"and are comparable with the heat of formation {80kcal/mole). Possible models for surface arrangements of ferrous chloride molecules are discussed. 1876