Short-range Order Regions Research Articles

On the TiAl phase diagram

The recent results of Swartzendruber et al. on the α−( α + α 2) phase boundary in the TiAl phase diagram are compared with the earlier results of Namboodhiri et al. Both of these modifications suggest a shifting of the α−( α + α 2) transition point to lower aluminium concentrations and to higher temperatures than those proposed by Blackburn. The region of short-range order determined by electrical resistivity measurements is also delineated.

High energy electronic Raman scattering in the one-dimensional ferromagnet CsNiF3

High energy (6000 to 14000 cm−1) electronic Raman scattering in the one-dimensional ferromagnet CsNiF3 has been studied in the short-range order region above the three dimensional magnetic ordering temperature (2.6 K). Our measurements involve transitions from the 3A2g ground term to the 3T2g and 3T1ga excited terms of the Ni2+ ion 3d8 configuration. Besides broad phonon assisted bands, the spectrum consists of purely electronic sharp lines. The successive moments of the sharp lines depend upon the spin correlation functions, which were experimentally monitored through a temperature or a magnetic field variation. Our experimental results qualitatively agree with the predicted ones.

Chain conformation in the crystalline state by means of neutron scattering methods

Neutron scattering experiments have been performed with n-alkane and polyethylene (PE) samples crystallized from melt and solution. Differences between different conformation models are most pronounced in the short-range order region covered by intermediate angle neutron scattering (IANS) experiments. The angular range has been extended up to k≲ 1.2 A–1 and highly concentrated mixtures of PE(H)+ PE(D) have been used. The problem of phase segregation of PE(H) and PE(D) was investigated by wide angle neutron scattering (WANS). In connection with extensive analytical and Monte Carlo calculations conformation models are proposed which explain the marked differences of the scattering curves in the IANS range between melt and solution crystallized samples. During rapid crystallization from the melt sequences of the randomly oriented molecules are apparently incorporated into the growing crystal with a minimum of chain reorganization (solidification model), whereas during crystallization from dilute solution sheets along (110) planes are formed (stacked-sheets model). IANS experiments with n-hexatriacontane reveal no short-range order tendencies between tagged alkane molecules.

Electron microscope study of phase transformations in cubanite

Direct observations of the phase transformations in cubanite both on heating and cooling have been made in a transmission electron microscope. Low temperature orthorhombic cubanite undergoes a cation disordering process at about 200° C resulting in a hexagonal wurtzite type structure. The reordering process takes place by the growth of independently nucleated regions of short range order. Long range order is difficult to attain in short term experiments due to the lack of order correlation between regions. At a slightly higher temperature a transformation from hexagonal close-packing to cubic close-packing takes place within the sulphur structure, by the propagation of partial dislocations along alternate close-packing planes. This transformation is irreversible. On cooling the disordered cubic structure, cation ordering takes place as an alternative to the transformation back to hexagonal close-packing, but in the absence of an ordering scheme at this composition based on the cubic subcell, the ordering process results in the exsolution of chalcopyrite. This transformation is interpreted in terms of metastable, kinetically controlled behaviour.

Short-range order and clusters in mictomagnetic MnCu

Experimental evidence for the existence of atomic and magnetic short-range order in 16.7 at.% MnCu is examined in relation to the magnetic susceptibility at low temperatures and its dependence on heat treatment. A neutron scattering experiment indicates the existence of magnetic clusters of size greater than 35 AA at 4.2K. A model for clusters is proposed, based on a lattice of second- and third-nearest-neighbour Mn spins, and which is centred on regions of atomic short-range order about 18 AA in size.

Anisotropic Behavior of the Magnetic Susceptibility of KCuF3

The anisotropic behavior of the magnetic susceptibility of the one-dimensional antiferromagnet KCuF 3 is discussed by taking the anisotropic exchange interaction mechanism into account. The results of analysis suggest the existence of the Ising-like anisotropy in the short range order region.

Magnetic Ordering in Gold-Iron Alloys

In an attempt to characterize the magnetic ordering in $\mathrm{Au}\mathrm{Fe}$ alloys, systematic studies were made of the temperature dependences of the thermoelectric power $S$ and the low-field magnetic susceptibility $\ensuremath{\chi}$ for Fe concentrations $C$ from 1 to 22 at.%. The concentration dependences of the magnitude and temperature of the maximum in $S(T)$ showed transitions clearly related to the magnetic ordering. Data analyses based on molecular field theories indicate the existence of small regions of short-range ferromagnetic order which undergo longrange interactions as the temperature is lowered. For $C\ensuremath{\gtrsim}12$ at.%, long-range ferromagnetism is dominant. Lower-concentration alloys ($C\ensuremath{\lesssim}12$ at.%) exhibit an antiferromagnetism with some properties similar to those of a magnetic spin-glass, but with well-defined ordering temperatures characterized by sharp cusps in $\ensuremath{\chi}(T)$, and with a negative Curie $\ensuremath{\theta}$ for $C=1 \mathrm{and} 2$ at.%. These properties indicate a more perfect antiferromagnetic order than that expected for a random alloy or a spin-glass, and this may be related to preferred local lattice arrangements in these alloys.

Short-range order, thermal vibration and expansion, and other properties of pseudosymmetric and mixed crystals of small organic molecules - Calculation of dimensions of ordered regions in triclinic and monoclinic pseudosymmetic crystals from the intensity of diffuse scattering

Mathematical relations are obtained giving the X-ray scattering for a crystal the unit cell of which contains one or two asymmetric molecules, each of which can adopt either of two centrosymmetrically-related orientations, giving pseudocentrosymmetry. It is shown that measurements of the integral breadth of maxima of the diffuse ‘disorder’ scattering can give the dimensions of the short-range order regions. The temperature variation of intensity of such diffuse scattering is different from that of thermal diffuse scattering.

The lifetimes and intensities of positron states in several solid polymers as an indication of structure

AbstractPositrons which penetrate and become trapped in polymeric solids form states which display complex annihilation time spectra. The average lifetimes and intensities of the posiltron states can be obtained from the exponentially decaying components of the time spectra. Usually three components can be resolved for solid polymers; with long, Intermediate and short average lifetimes. Experimental results of lifetimes and intensities for positrons annihilating in trans 1–4 polybutadiene, polyethylene, polyethyleneterephthalate are used to conclude that the intensity can be associated with regions of short range order and the intermediate lifetime and intensity with regions of long range order. It is also concluded that for both regions of long range and short range order there is a tightening of the microstructure in the polymer as the amount of long range order increases and/or molecular weight decreases.

Resistivity Minimum in an Amorphous Ferromagnetic Alloy

The electrical resistance-vs-temperature curve of an amorphous Fe80P13C7 alloy, obtained by rapid quenching from the liquid state, has a minimum at low temperature. Because the quenching conditions cannot be accurately controlled, the temperature at which the minimum occurs varies from specimen to specimen and covers a range from 10° to 50°K. A study of 14 different specimens shows a correlation between this temperature and the ratio of the resistances at helium temperature to that at room temperature. This correlation is interpreted in terms of the existence of regions of short range order, or small amounts of crystalline phases in imperfectly quenched specimens.