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The accuracy of the determination of the structure factors from a powder diffraction measurement is studied in light of a representative example: nickel with Cu Kct radiation. Various sources of error in the absolute measurement and methods of sample characterization are discussed. The contributions of various extraneous scattering mechanisms are assessed. From these considerations it is possible to separate the Bragg scattering, providing measurements are made through virtually the entire angular range. Results are given in detail for a standard sample, a duplicate of which can be made available to interested workers. The relationship of these results to the structure factors of the sample is discussed in terms of a model calculation, in terms of line shape analysis, and in terms of some comparative measurements on various samples of Ni and MgO. All these analyses indicate the impossibility of preparing a single sample which is ideally mosaic. Thus it is incorrect to relate the measured integrated reflection to the structure factor with the customary simple formula when high accuracy is required.

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The thermal diffuse scattering (TDS) contribution to X-ray Bragg peaks is discussed in terms of its two experimental components, the included and the background parts. The discussion is primarily directed towald situations where detailed elastic constant information and a large computer are not available. Graphs are displayed which, in the spherical average approximation, allow an accurate assessment of the influence of all the experimental variables on the TDS contribution. It is shown that, even in the general case where spherical averaging is a poor approximation, the included TDS varies linearly with length of scan for small scans. A technique for determining the constant of proportionality is suggested. Thus the included TDS may be evaluated even though the elastic constants are not known.

A number of theories are examined for their predictions of extinction coefficients at large values of extinction, especially as applied to polarization ratios. Although several theories give the behavior expected on the basis of physical reasoning (a polarization ratio approaching unity), the popular theories of Zachariasen [Acta Cryst. (1967), 23, 558-564] and Becker & Coppens [Acta Cryst. (1964), A30, 129-147, 148-153] do not show the correct asymptotic behavior. Although this shortcoming may be of no consequence in ordinary crystallographic applications, it is misleading in predicting the correct polarization factor to be used in connection with a crystal-monochromated apparatus, where the monochromator is usually adjusted to maximize its extinction. The importance of measuring, rather than estimating, the polarization ratio of a crystal monochromator is therefore re-emphasized.

The heat capacity of CrBr 3 shows a lambda anomaly peaked at 32.55 °K, a temperature somewhat lower than the reported Curie temperature of 37°K. Assuming the validity of the spin-wave dispersion relation of Davis and Narath, the entire magnetic heat capacity was derived. As is typical of a predominantly twodimensional structure, this curve yields a large amount of entropy and energy of order (S^-S c )/R = 0.62 and {M"-E<)/(E"-E*)=Q.n.

The linewidth parameter IJ.p/p for nonresonant microwave absorption in CRaCI and in CHCbF has been determined at five temperatures between 10° and 150°C from cavity measurements of dielectric dispersion in the gases as a function of pressure at a frequency of 402 Mc. The linewidth is found to be proportional to T-l.5, as opposed to the T-1.0 dependence predicted by Anderson's theory of collision broadening.

of three J=\ holes coupled to a spin of / = |. Using (r n *) = RM m with i^0=1.2X10-13 cm, we obtain (2=+0.09 b. This is considerably smaller than the measured quadrupole moment, and conceivably is due to configuration mixing or small deformations of the nuclear core. The only other shell-model state likely to contain the 61 st proton is d 5/ 2, but this is not allowed because the configuration (d 5/ 2) 3 coupling to 7=f is forbidden by the Pauli principle.On the assumption that the nuclear core of Pm 151 is highly deformed, there are two possible state assignments for the 61 st proton that give the correct spin and parity. When the notation of Mottelson and Nilsson is used, 5 these are f+ [413] and f+ [402]. We have calculated the nuclear moments of these states for different values of the deformation parameter 8. The value obtained for the level f+ [402] is about 3.7 nm and is insensitive to the deformation. The level f+ [413] gives a moment of 0.91 nm with a deformation parameter of 5-0.4. This is in better agreement with the ATOMIC SCATTERING FACTOR OF Ne, Ar, K>, AND Xe 729