摘要:

Low‐ and high‐angle X‐ray diffraction patterns have been obtained from one‐dimensional superlattice crystals prepared artificially by alternately depositing predetermined thicknesses of GaAs and AlAs, on the (001) face of a GaAs single‐crystal by molecular‐beam epitaxy. The positions and intensities of several superlattice reflections obtained along the 00l, 11land 02lreciprocal lattice rows have been recorded. The structure of the superlattice can be approximated by a model which incorporates elastic strains in the unit cell due to the lattice mismatch between GaAs and AlAs. The number of Ga and Al layers in the superlattice unit cell can be accurately determined from the low‐angle scattering data while the relative intensities of the high‐angle superlattice reflections are a sensitive measure of the elastic strain present in the lattice. It is shown that the elastic strain agrees with the value computed theoretically on the assumption that the strain is not relieved by dislocations at the GaAs

ISSN:1600-5767    

DOI:10.1107/S0021889877012679

出版商:International Union of Crystallography    

年代:1977

数据来源: WILEY

摘要:

The Rietveld profile‐analysis refinement procedure [Rietveld,J. Appl. Cryst.(1969),2, 65–71] is here applied to microdensitometer‐measured Guinier–Hägg X‐ray powder film data (strictly monochromated CuKα1radiation). Various functions for describing the profile form of an individual reflection are tested; the most satisfactory fit is obtained for a modified and asymmetrized Lorentz function. The crystal structure of α‐Bi2O3has been refined and no significant deviations are observed from the results of an earlier single‐crystal study [Malmros,Acta Chem. Scand. (1970),24, 384–396]. AnRFvalue [roughly comparable with the conventionalR(F) value of single‐crystal work] of 0.068 was obtained, with standard deviations on the refined positional parameters on average three times larger than those obtained from the single‐crystal study. Further examples of the refinement technique are also given which illustrate both its reproducibility an

ISSN:1600-5767    

DOI:10.1107/S0021889877012680

出版商:International Union of Crystallography    

年代:1977

数据来源: WILEY

摘要:

The theoretical expressions of Cooper&Sayer [J. Appl. Cryst. (1975),8, 615] for the peak shift from the true Bragg angle and for the peak broadening in powder neutron‐diffraction patterns are confirmed experimentally for detectors with finite aperture. It is shown that these factors can be important even at relatively large scattering angle

ISSN:1600-5767    

DOI:10.1107/S0021889877012692

出版商:International Union of Crystallography    

年代:1977

数据来源: WILEY

摘要:

The macroscopic yield stress of Ni3Al increases rapidly with temperature, reaching a maximum at about 700°C. Such anomalous behavior has been observed in other ordered alloys which undergo partial disordering with increasing temperature,e.g. Cu3Au. The long‐range order parameter,S, of stoichiometric Ni3Al powder was measured over the temperature range 25°C to 1000°C and great care was taken to ensure a fine particle size and thus avoid extinction effects. The results of this study showed that, in Ni3Al, S remains constant at about 0.93 over the entire temperature range investigated. This means that theories relating the strength of ordered alloys toSare not applicable to N

ISSN:1600-5767    

DOI:10.1107/S0021889877012709

出版商:International Union of Crystallography    

年代:1977

数据来源: WILEY

摘要:

The cooling of single‐crystal samples to helium temperatures within a four‐circle neutron diffractometer has been achieved by a novel continuous‐flow system. The system relies solely on helium for its cooling power with two flow circuits being used. The first delivers liquid to a reservoir cryostat of capacity 1–4 l located close to the Eulerian cradle of the diffractometer and the second draws the refrigerant from this cryostatviaa flexible transfer tube into the sample region of a compact cooling unit mounted off the cradle. This arrangement allows unhindered motion of the diffractometer circles for sample temperature operation in the range 3.9–300 K. The system requires a total helium flow of 1.0 l h−1to maintain a temperature of 20±0.1 K, decreasing to 0.5 l

ISSN:1600-5767    

DOI:10.1107/S0021889877012710

出版商:International Union of Crystallography    

年代:1977

数据来源: WILEY

摘要:

An APL program has been written to compute the locations and the radii of spherical voids between two packing layers of atoms, and to obtain plottings which correspond to the various types of standard sheets used for the representation of crystal structures according to the condensed‐model technique. Conversational facilities, together with the use of original APL routines, enable the crystallographer to get the results in a few minutes of real tim

ISSN:1600-5767    

DOI:10.1107/S0021889877012722

出版商:International Union of Crystallography    

年代:1977

数据来源: WILEY

摘要:

A differential formulation of the problem of the largest sphere inscribed betweenn+ 1 spheres in a space with n dimensions leads to a linear system ofn+ 1 equations with n unknowns which can be solved by the least‐squares method. This formulation is most efficient for APL computin

ISSN:1600-5767    

DOI:10.1107/S0021889877012734

出版商:International Union of Crystallography    

年代:1977

数据来源: WILEY

摘要:

Metastable precipitation as the formation of GP zones is governed by a metastable miscibility gap. It is shown that the limits of this gap in a ternary system can be determined from combined analysis of the small‐angle scattering (SAS) of X‐rays and of neutrons. The necessary condition is a significantly different ratio β = (bC−bA)/(bB−bA) of the atomic scattering amplitudes b of the componentsA,BandCfor both the X‐rays and the neutrons. This difference enables one to determine the slope of the tie‐line passing through an individual alloy by comparing the integrated SAS intensities of the two types of radiation. Thereafter, the determination of the gap limits follows the same procedure as already known for binary alloys. The change of the mean atomic volume with composition influences the result, but this can be taken into account provided the same Vegard's law is valid for both the depleted matrix and the

ISSN:1600-5767    

DOI:10.1107/S0021889877012746

出版商:International Union of Crystallography    

年代:1977

数据来源: WILEY

摘要:

The method described in the preceding paper [Gerold (1977),J. Appl. Cryst.10, 25–27] has been applied to the system Al–Zn–Mg where the small‐angle scattering has been measured from nine alloys having zinc and magnesium contents up to 5.4 and 2.5 at. %, respectively. The alloys were annealed at room temperature where the metastable equilibrium of GP zone formation is reached after about one month. From combined analysis of the X‐ray and neutron results the slopes of the tie‐lines have been evaluated in this region of the phase diagram. The further analysis leads to the approximate determination of the miscibility gap limits. The confidence limits of the determination were restricted by it not being possible to make measurements far enough out in the tail of the neutron scattering profile from the small GP zones (diameter less than 30 Å) to allow the background intensity to be separated completely satisfactorily. The ultimate limiting factor for this type of analysis may be the accuracy to which the mean atomic v

ISSN:1600-5767    

DOI:10.1107/S0021889877012758

出版商:International Union of Crystallography    

年代:1977

数据来源: WILEY

摘要:

The dielectric constants (ɛr) of several cadmium iodide polytypes have been measured and found to be higher than that of the parent 4Hstructure. The dielectric constants for several long‐period polytypes have been found to lie between 60 and 190 whereas the 4Hparent structure has a value in the range of about 15 to 30. In particular, both increasing polytype periodicity and increasing disorder increase the dielectric constant,e.g.from 16 for an ordered 4Hstructure to 114 for a 32Hstructure and 67 for a highly disordered 4Hstructure. In the absence of disorder a particular polytype has its own characteristic dielectric constant value,e.g.the two 14Hpolytypes which have a common ɛrvalue of

ISSN:1600-5767    

DOI:10.1107/S002188987701276X

出版商:International Union of Crystallography    

年代:1977

数据来源: WILEY