Arthur Bienenstock

Radial distribution studies of amorphous GexTe1−x alloys

Abstract Radial distribution studies of amorphous Ge χ Te 1−χ alloys have been calculated from X-ray diffraction data for x = 0.11 and 0.54. These show peaks at approximately 2.7 A and 4.2 A. The absence of a peak at the crystalline GeTe first neighbor separation of 3 A is shown to imply that the local coordinations in the amorphous materials are different from those in crystalline GeTe. The areas under the first neighbor peaks indicated that it is very unlikely that the coordination numbers of the Ge and Te atoms are the same. These areas are consistent with models in which the average Ge and Te coordinations are four and two, respectively. This implies that the similarity in the optical absorption edges of crystalline and amorphous GeTe cannot be explained by similarities in short range structures. The similarities in bonding over a wide range of compositions in these amorphous alloys appear to lend support to Motts picture for the failure of many impurities in many amorphous semiconductors to add significantly to the conductivity. The form of bonding postulated appears to be inconsistent with a microcrystalline picture for these alloys.

X-ray study of the structure of liquid water

The radial distribution function (RDF) of liquid water has been determined by x-ray scattering. Using a free-standing water jet and a diffracted beam analyzer to separate the Compton from the elastic scattering experimentally, several significant sources of error have been eliminated in the experiment. These have enabled the collection of quality data out to 19.7 A(-1). Data were collected both at 278 K and at 296 K. Our RDF presents fine structure, particularly, a distinct peak at 3.4 A and a peak split at 4.5 A. These features cannot be interpreted by a simple tetrahedral configuration suggested by continuum models. The uncertainties due to the truncation effects and the choice of scattering factors are found to have little influence on the RDF at intermolecular distances.

Radial Distribution Studies of Glassy GexS1−x Alloys

X‐ray diffraction radial distribution studies of glassy GexS1−x alloys, with x=0.33 and 0.42, have been performed. The interatomic distances in the x=0.33 sample, 2.2 and 3.55 A, are consistent with those in the covalently bonded crystalline GeS2. The distances in the x=0.42 sample, 2.3 and 3.65 A, are also consistent with covalent bonding, and show no relationship to those found in crystalline GeS. No evidence of a previously proposed octahedral coordination of Ge can be found. A tentative explanation for the existence of two separated glass‐forming regions in this system is put forth.

Replacement Partition Function for Small Crystals in Homogeneous Nucleation Theory

The replacement partition function was calculated for small fcc crystals by numerical normal mode analysis using exactly the same model, which considers nearest‐neighbor interactions only and neglects the surface relaxation and thermal expansion, to compute all the necessary quantities. A method of calculating the surface free energy of a small crystal from data on the macroscopic surface was introduced. The numerical value of the replacement partition function thus computed was, for example, 108 at T=2θ and for n=87. This is considerably larger than the value estimated for a crystal so far, which is 102‐−04 at T=2θ and for n ≅ 100. This result suggests the need of further work both in numerical calculation and in theoretical studies.

Neutron and X-ray diffraction radial distribution studies of amorphous Ge0.17Te0.83

Abstract Bulk amorphous samples of amorphous Ge 0.17 Te 0.83 have been prepared by quenching of small moltens droplets. Radial distribution functions have been constructed from both X-ray and neutron diffraction intensity data. These indicate first neighbor peaks at 2.75 A and 2.65 A, respectively, indicating the greater sensitivity of the neutron RDF to atoms pairs containing the smaller Ge atom. The first peak area in both RDFs is not consistent with twofold coordination of the Ge. It can be fit by two types of models. In the first, the Ge is fourfold and the Te twofold coordinated. In the second, the Ge is threefold coordinated, while some Te are threefold and others twofold coordinated. It is shown that both types of models are consistent with RDFs performed on other compositions.

Microstructural evolution of NiFe/Ag multilayers studied by X-ray diffraction and in situ high-resolution TEM

Abstract X-ray diffraction and in situ high-resolution electron microscopy experiments have been performed to study the effect of annealing on the microstructure of permalloy/silver multilayers. Both techniques show that the silver diffuses into the permalloy grain boundaries creating so-called ‘silver bridges’. This bridging is expected to be responsible for the giant magnetoresistance effect observed at low fields in these devices. Moreover, unexpected permalloy diffusion and recrystallization have also been observed in the in situ TEM experiments.

Liquid GeBr4. I. A test of the anomalous x‐ray scattering method as applied to disordered materials

This study tested the accuracy of the anomalous x‐ray scattering technique as part of an investigation of the molecular liquid GeBr4. The differential anomalous scattering technique proved to be quite accurate, giving nearest‐neighbor coordination distances and numbers correct to 0.02 A and 4%, respectively, as compared with known values. The high accuracy of the approach evidently depends on a cancellation of systematic errors when subtracting data sets at two nearby photon energies. The straightforward three‐wavelength approach to determining the partial‐distribution functions is quite ill conditioned and small experimental errors cause large errors in the resulting partial distribution functions. An alternate procedure, suggested by Munro, uses differences to determine the partial distribution functions (PDF’s) and hence takes advantage of the cancellation of systematic errors. This approach yields Br–Br, Ge–Br, and Ge–Ge PDF’s with error levels of approximately 5%–10%, 30%, and 100%, respectively. Sin...

Threefold coordinated model structure of amorphous GeS, GeSe and GeTe

Abstract The black P structure is presented as a model for the structures of amorphous GeS, GeSe and GeTe. It is shown that the short interatomic distances, low near neighbor coordinations and high covalencies of the amorphous materials, relative to the crystalline, can be rationalized with the model. When scaled to the near neighbor interatomic distances in the amorphous materials, the model yields satisfactory agreement with the observed position and area of the second neighbor X-ray radial distribution function peaks. The model predicts: (a) A first neighbor peak area for GeS which is significantly different from that predicted by the random covalent model and (b) phase separation in certain composition regions which, for the Ge-S system, should be observable by means of transmission electron microscopy. An explanation of why phase separation is not likely to be observable through transmission electron microscopy studies of amorphous GeTe and GeSe is also presented.

Anomalous scattering determinations of the pair distribution functions in amorphous GeSe

Abstract Techniques have been developed for utilizing synchroton radiation and the anomalous scattering of X-rays for the determination of partial distribution functions in amorphous alloys. Intensities of X-ray scattering from amorphous GeSe were measured at energies near the Ge and Se K absorption edges. These data were analyzed to obtain three partial structure factors and partial distribution functions. The results are consistent with models in which the Ge is fourfold coordinated but not with the model in which each Ge(Se) is surrounded by 3 Se(Ge) atoms.

Structure and bonding in amorphous GexTe1−x alloys☆☆☆

Abstract X-ray radial distribution studies of the system GexTe1−x have been interpreted in terms of a “random covalent” structural model. To further elucidate the structure and bonding type, X-ray absorption edge and X-ray photoemission studies were undertaken. Measured edge and photoemitted electron energies are consistent with covalent bonding. For x = 0.15 to 0.57, the Ge Kα edge and the Ge 3d 5 2 photo-emitted electron energies of the amorphous materials are close to those of crystalline Ge. For crystalline GeTe, GeSe and GeS, there are 2 to 3 eV shifts indicative of more ionic binding. These results imply that the amorphous materials are considerably more covalent than any of the crystalline Ge-chalcogenide phases and suggest that none of the crystalline structures can form the basis for a microcrystalline picture of amorphous GeTe.