摘要:

The electron‐microscope technique of lattice imaging can in favourable circumstances be used to observe the structure of materials directly. The technique is described, and examples of cases in which a direct correlation between image contrast and structural features has been established are given. Procedures for computing contrast inn‐beam lattice images are outlined, and the important experimental parameters, thickness and orientation of the specimen, and spherical aberration and defocusing of the microscope objective, are discussed in some det

ISSN:1600-5767    

DOI:10.1107/S0021889873008459

出版商:International Union of Crystallography    

年代:1973

数据来源: WILEY

摘要:

A procedure is described for obtaining the standard deviations in cell dimensions derived front powder patterns by Cohen's method of least squares. The analysis applies to any crystal system and the error in the volume of the cell is evaluated. The method is particularly suited to the study of minerals where isomorphous replacements give rise to small changes in lattice parameters. As an example, the cell dimensions and their errors are obtained for an iron–rich actinolite asbestos and the powder pattern is indexe

ISSN:1600-5767    

DOI:10.1107/S0021889873008460

出版商:International Union of Crystallography    

年代:1973

数据来源: WILEY

摘要:

Reduction of random experimental errors and accounting for systematic errors require good experimental techniques and systematic handling of data. For the Straumanis–Ieviš, van Arkel, and Bradley–Jay methods, in the absence of error, the sum of the readings of the measuring instrument for corresponding lines on either side of the exit (or entrance) hole would be constant. Drift of this sum with θ reveals certain types of systematic error resulting from geometrical aberrations having low symmetry; random scatter about the drift line gives an immediate measure of the random error in setting on a line. Unfortunately no such internal assessment is possible with the usual Guinier or diffractometer arrangements. The standard deviation of a measurement of line position is found to be show little variation with θ, though there is some indication that it is dependent on line intensity, being greater for very weak and very strong lines. The standard deviations of Bragg angles and lattice spacings or parameters are easily determined from that of the position measur

ISSN:1600-5767    

DOI:10.1107/S0021889873008472

出版商:International Union of Crystallography    

年代:1973

数据来源: WILEY

摘要:

The effect of the counting losses on the values of the strain and particle size determined by the Warren–Averbach method is discussed. The analysis of the error transmission is performed by introducing a known error in simulated profiles, corresponding to a predetermined degree of deformation. The effect of the error on the Fourier transform of the profiles, and on the plots relating the squared strain and particle‐size coefficients to the column length is discus

ISSN:1600-5767    

DOI:10.1107/S0021889873008484

出版商:International Union of Crystallography    

年代:1973

数据来源: WILEY

摘要:

X‐ray and neutron‐diffraction techniques have been used to ascertain the structures of vanadium deuterides with compositions between V2D and V4D3over a temperature range 5–425 °K. The β phase, V2D, has a monoclinic crystal structure at room temperature witha≃c= 4.46;b= 3.00 Å, β = 95.5°. The deuterium atoms prefer octahedral interstitial sites. Near 425 °K this phase transforms to a b.c.c. phase, and the deuterium atoms prefer tetrahedral sites. The b.c.c. phase of V4D3has been designated α′ and transforms to α′′ at about 220 °K. This transition is an ordering of deuterium as shown by additional neutron‐diffraction peaks. This α′′ structure can be indexed witha≃c≃4.46;b≃3.00 Å; β = 90°. Below 160 °K an additional neutron peak appears at 2θ = 12° which indicates a doubling of theblattice parameter. This transformation to α′′′ phase involves further ordering. The metal sublattice in α′ and α′′′ is very near b.c.c. Therefore, the neutron patterns do not allow a determination of octahedral or tetrahedral site occupancy. The exact crystal structure cannot be determined. The V–D system is not complete

ISSN:1600-5767    

DOI:10.1107/S0021889873008496

出版商:International Union of Crystallography    

年代:1973

数据来源: WILEY

摘要:

A 7.5μ thick β‐Ta film with very high preferred orientation has been studied with both the Gandolfi and precession cameras. A hexagonal subcell witha= 2.831,c= 5.337 Å has been found. The smallest hexagonal cell that accounts for all the observed spacings contains 144 subcells witha= 34 Å. A structural model is proposed based on analysis of the subcell. It suggests that there are domains in the form of hexagonal prisms about 1000 Å or more in height and about 250 Å in diameter. The domains are divided into 12 coherently diffracting prismatic zones with impurity species, principally Ar and H2, incorporated into the zone boundaries. The structure within a zone is related to the simple h.c.p. structure. The domains are stacked together side by side, normal to the plane of the film, to form layers. Many layers in turn may be added together to form th

ISSN:1600-5767    

DOI:10.1107/S0021889873008502

出版商:International Union of Crystallography    

年代:1973

数据来源: WILEY

摘要:

A new X‐ray diffraction method for the determination of crystallinity of polymers is reported. A probability function is used to express the intensity distribution of an amorphous halo. The intensity of the halo buried under any crystalline peak can be calculated by this function. An amorphous‐standard‐addition method was used to determine crystallinity. A linear relationship between intensity and concentration is derived theoretically and applied to polyethylene terephthalate. No previous chemical or structural information about the polymer is necessary for this method. Very good agreement between X‐ray data and density measurements were obtained. This method is rapid, practical and suitable for routine a

ISSN:1600-5767    

DOI:10.1107/S0021889873008514

出版商:International Union of Crystallography    

年代:1973

数据来源: WILEY

摘要:

The geometrical and physical aberrations affecting the positions and breadths of diffraction maxima as determined by the energy distribution of photons diffracted through a fixed angle and analysed by a solid‐state detector differ in many points of detail from those determined by the angular distribution of photons of fixed energy analysed by a conventional diffractometer. The main differences are investigated and the accuracy of spacing measurements is discussed. There is no simple analogue of the extrapolation method of reducing errors in spacing measurement

ISSN:1600-5767    

DOI:10.1107/S0021889873008526

出版商:International Union of Crystallography    

年代:1973

数据来源: WILEY

摘要:

The method of variance has been applied to some cold‐worked hexagonal (β‐phase) silver–antimony alloys to determine the particle size and strain values, and the values thus obtained have been compared with those from Fourier analysis of line shapes. Assuming small anisotropy, the agreement in the particle size and strain values has been found to be quite satisfactory. The low values of the effective particle sizes obtained from variance analysis along the faulted planes 101, 102 and 103 clearly indicate the influence of stacking faults which has been found to decrease with increasing solute c

ISSN:1600-5767    

DOI:10.1107/S0021889873008538

出版商:International Union of Crystallography    

年代:1973

数据来源: WILEY

摘要:

Eleven alloys from the binary and ternary range (VMn)2Ga5have been investigated using neutron powder diffraction methods supported by measurements of magnetic susceptibility and superconducting transition temperatures. A magnetic structure is present in alloys in the composition range around (Mn0.2V0.8)2Ga5to Mn2Ga5. The magnetic moment found in the alloy Mn2Ga5increases on the addition of vanadium to a maximum around the composition (Mn0.2V0.2)2Ga5. It then decreases and disappears around the composition (Mn0.2V0.8)2Ga5. Possible changes in the magnetic structure are suggested in order to account for this. There is some evidence of weak superconductivity in the alloy V2Ga5. This disappears on the addition of manganese.

ISSN:1600-5767    

DOI:10.1107/S002188987300854X

出版商:International Union of Crystallography    

年代:1973

数据来源: WILEY