Local Structural Units Research Articles

Nature of the disordered state of hydrogenated amorphous silicon as revealed by the study of anelastic relaxation behavior

The nature of the disordered state of hydrogenated amorphous silicon is examined for the first time by measurement of anelastic relaxation behavior. It is demonstrated that local structural units and their modifications control the relaxations in these films under different conditions of deposition, aging, and light exposure. Specifically, the light-induced state in this material is shown to be characterized by four distinct relaxations.

Dispersive X-ray absorption study of the kinetics of photo-induced structural changes in glassy As2S3

We studied the photo-induced XANES kinetics for the chalcogenide glass As2S3 using dispersive x-ray absorption spectroscopy. After numerically compensating for synchrotron instability, kinetic studies reveal subtle changes in XANES oscillations that suggest an anisotropy of of local structural units, in agreement with previous optical studies.

Electron microscopy of crystalloid structure in Ni-Cr small particles

Abstract The crystalloid structure, an aperiodic arrangement of small structural units, observed in Ni-Cr small particles was studied by selected-area electron diffraction, dark-field imaging, high-resolution electron microscopy and optical diffraction analysis using the high-resolution micrographs. The electron and optical diffraction patterns show an arrangement of sharp diffraction spots with twelvefold rotational symmetry and an intensity distribution which is (at least approximately) consistent with this symmetry. The high-resolution electron microscopy revealed that the crystalloid structure is non-periodic but has bond orientational order over a range of 60 nm. From these experimental results, as well as from those for crystalline phases coexisting in the same particles, a random tiling model of three local structural units (A15-type, Zr4Al3-type and 30° rhombus prism type) is proposed as a possible structural model of the crystalloid structure. An indexing scheme using five integers is also proposed for the crystalloid reflections. Using these indices, a precise analysis of the crystalloid diffraction pattern has been carried out.

Various contributions to magnetostriction in amorphous and polycrystalline ferromagnets

Magnetostriction of amorphous ferromagnets with random local anisotropy and of polycrystalline ferromagnets originates both from a modification of the local magnetic anisotropy strength and from a strain-induced reorientation of the easy axes directions of the local structural units. The magnitudes of these two contributions to the magnetoelastic coupling energy and to the effective magnetostriction constant are calculated for materials with hexagonal local magnetic anisotropy.

(Fe,Ni)-(B,Si) metallic glasses: local structural units using fluctuation hyperfine parameter correlations

Mossbauer studies are reported on several compositions of amorphous (Fe1-xNix)-(B1-ySiy) systems. Fluctuation correlation functions of the type mu n( Delta H Delta delta ) have been derived to infer the local structural units. It is shown that the structural variations in these amorphous ferromagnets manifest themselves through changes in the correlation functions. The structural variations have been discussed in relation to different predictions based on indirect measurements.

Theory of magnetostriction in amorphous and polycrystalline ferromagnets: I. Description of the formalism

Magnetostriction of amorphous and polycrystalline ferromagnets is discussed with special emphasis to the following questions: 1. 1. Is magnetostriction in these systems totally determined by the conventional mechanism via the strain-derivative of the local 2. anisotropy tensor as in crystalline materials, or is there also a contribution from the reorientation mechanism due to stiff 3. rotations of local structure units? 4. 2. What is the effect of the elastic coupling of the structural units on the effective magnetostruction tensor in disordered 5. ferromagnets? An iterative solution for the effective magnetostriction tensor is derived from two method, the balance-of-force 6. method and the incompatibility method.

Magnetoelastic evidence for a local structural transformation in Co-rich glasses

The magnetostriction of Fe-rich glasses is large and positive ( λ = +32×10 −6) whereas that of polycrystalline α-Fe is small and negative ( λ = -7×10 −6). This is understood to be a consequence of the sharp difference in local atomic order between these two materials: a 12-fold coordinated short-range order dominated by trigonal prismatic arrangements of Fe about B atoms in the glass, versus an eight-fold coordinated bcc structure in the crystal. Co-rich glases on the other hand show temperature and compositional dependences which closely parallel those of polycrystalline hcp (ϵ) cobalt and Co -Fe alloys. Specifically: (1) the addition of iron to ϵ-Co (polycrystalline λ = -16×10 −6) drives a martensitic transformation to the γ (fcc) phase in Co 1- x Fe x at approximately x = 0.07. Near this composition λ = 0 and for x 62 0.09, λ is positive. Similar behavior is observed with the addition of iron to amorphous Co 80B 20 ( λ = -4×10 −6) in the series (Co 1- x Fe x ) 80B 20 where λ passes through zero at x = 0.06. (2) The martensitic transformation from the hcp to the fcc phase can also be thermally driven and is observed in Co at 420°C, near which temperature λ again changes sign and is positive in the high-temperature phase. Similarly, amorphous Co 80B 20 shows by extrapolation to above crystallization [1] (and Co 78Fe 2B 20, Co 72Mn 8B 20 and Co 70Cr 10B 20 show explicitly [2]) a thermally induced transition to a positive magnetostriction phase at 400°C. To the extent that the magnetostriction of polycrystalline cobalt signals the occurrence of the long-range ε-γ transformation, the magnetostriction of amorphous cobalt-rich alloys can be taken to indicate a similar, but local, transformation in the glasses. These transformations are reversible in both the crystals and glasses. In the amorphous materials most other macroscopic manifestations of the cooperative, local atomic rearrangements involved in the transformation are masked by the random orientation of the local structural units. The electronic origin of the transformations in both crystalline and noncrystalline materials is suggested to be a Jahn-Teller mechanism involving two states close to E F which have orbital character similar to d x 2- y 2 (near E F- k B T/2) and d z 2- r 2 (near E F + k B T/2) levels. The ground state of the system is then characterized by an oblate spheroidal d-charge distribution and the ϵ phase (basal plane expansion, λ<0) is stable in the crystal and a similar local order is assumed to obtain in the glass. Increasing temperature (or shifting E F relative to these nearly degenerate levels by alloying) will tend to equalize their populations and drive the system to the higher-symmetry local order seen in the crystal as the γ phase.

ChemInform Abstract: THE LOCAL STRUCTURE OF OXIDE AND METALLIC GLASSES

Abstract On first acquaintance, oxide glasses, amorphous semiconductors and metallic alloy glasses appear to represent structurally distinct families. The former range from covalent “framework” materials with very open structures to mixed oxides with considerable ionic bonding. Amorphous alloys, on the other hand, appear to be essentially close-packed, with bonding dominated by delocalised “metallic” orbitals. There are, nonetheless, definite similarities in the structural questions appertaining to each class of amorphous solid. For example, the notion of local structural units — a concept generally accepted in covalent glasses but contentious in amorphous alloys. Where this description is appropriate, it then becomes necessary to define the method of interconnection and the detailed network topology. In this paper, an attempt is made to delineate current structural problems in each class of materials and to present a review of relevant experimental investigations. Structural information is dominated by the results of scattering studies (neutrons or X-rays), with a recent, growing emphasis being placed on extraction of partial structure factors and accurate comparison with atomic models. EXAFS allows a more detailed investigation of the surroundings of specific atoms, even in complex systems such as oxides and chalcogenides, Mossbauer and NMR spectroscopy have provided new information on the symmetry of first-neighbour coordination shells — particularly in metallic systems. The results of experiment and comouter-simulations appear to reinforce the view that structure in amorphous materials is marked less by its uniformity than by its variability. It does not seem possible to provide a simple, singular, definition of what constitutes a glass (in terms of random networks, for example) or why some form more easily than others. But a common set of principles and parameters is emerging which allow the various structures adopted by glasses to be investigated, measured and described.

The local structure of oxide and metallic glasses

On first acquaintance, oxide glasses, amorphous semiconductors and metallic alloy glasses appear to represent structurally distinct families. The former range from covalent “framework” materials with very open structures to mixed oxides with considerable ionic bonding. Amorphous alloys, on the other hand, appear to be essentially close-packed, with bonding dominated by delocalised “metallic” orbitals. There are, nonetheless, definite similarities in the structural questions appertaining to each class of amorphous solid. For example, the notion of local structural units — a concept generally accepted in covalent glasses but contentious in amorphous alloys. Where this description is appropriate, it then becomes necessary to define the method of interconnection and the detailed network topology. In this paper, an attempt is made to delineate current structural problems in each class of materials and to present a review of relevant experimental investigations. Structural information is dominated by the results of scattering studies (neutrons or X-rays), with a recent, growing emphasis being placed on extraction of partial structure factors and accurate comparison with atomic models. EXAFS allows a more detailed investigation of the surroundings of specific atoms, even in complex systems such as oxides and chalcogenides, Mössbauer and NMR spectroscopy have provided new information on the symmetry of first-neighbour coordination shells — particularly in metallic systems. The results of experiment and comouter-simulations appear to reinforce the view that structure in amorphous materials is marked less by its uniformity than by its variability. It does not seem possible to provide a simple, singular, definition of what constitutes a glass (in terms of random networks, for example) or why some form more easily than others. But a common set of principles and parameters is emerging which allow the various structures adopted by glasses to be investigated, measured and described.

Determination of Local Structure and Moving Unit Formed in Binary Solution of t-Butyl Alcohol and Water

Abstract The concentration dependences of the mutual diffusion coefficients at 24, 37, and 63 °C were observed for a binary solution of t-butyl alcohol (TBA) and water by the use of the light scattering method. The observed concentration dependences of the mutual diffusion coefficients were explained well by postulating the existence of a “moving unit”–a group of molecules which move together for a time much longer than the velocity correlation time. The structure of the moving units which are formed in the solution are (H2O)11TBA at 24 °C and (H2O)20TBA at 63 °C in the concentration range of 0&amp;lt;xTBA&amp;lt;0.1, where xTBA is the mole fraction of TBA. Taking into account the local structures, which had been determined from the concentration dependences of the mean-square concentration fluctuation values, we could conclude that water molecules form a polyhedron which encages a TBA molecule and that these cages then gather together to form an aggregate. It is essential that these polyhedra do not share their faces with each other in the aggregate. It was also concluded that the number of cages which form an aggregate increases with an increase in the temperature. This suggests the existence of a pseudo-critical temperature for this system. That is to say, the TBA–water solution can separate into two phases under high pressures, a TBA-rich phase and a water-rich phase, in the latter of which almost all the molecules form polyhedra.

Similarities in amorphous and crystalline transition metal–metalloid alloy structures

In many amorphous and glassy systems the local structure seems to be essentially the same in amorphous and crystalline phases. It thus becomes possible to define a ‘local structural unit’, such as the SiO4 group in silicates. Determination of the structure then reduces to the problem of describing how the local units are connected and packed in the medium-range structure. There is mounting evidence that alloys of transition metals with metalloids are more adequately described by arrangements of stereo-chemically-defined structural units than by alternative descriptions in terms of random packing of atoms. There are three phases, Fe3C, Ti3P and Fe3P, which may serve as models of local structural units. These lattices differ to a small degree in the symmetry of the metalloid nearest-neighbour shell but there are major differences in the arrangement of local structural units. Although the experimental evidence is limited, it seems that the distinctive features of the local and medium-range topologies of the crystalline phases are represented in the structures of each of the corresponding glasses.