摘要:

The transition temperature of ferro‐ and antiferroelectric crystals can be simply determined by continuous pyroelectric temperature analysis (PTA) of unsintered pressed powder. In order to avoid excessive stored charge the samples were not previously treated by a polarizing electric field. Experiments performed with (NH4)2SO4, KH2PO4and NH4H2PO4showed the transition points with a precision of 1

ISSN:1600-5767    

DOI:10.1107/S0021889879011766

出版商:International Union of Crystallography    

年代:1979

数据来源: WILEY

摘要:

Five new PbI2polytypes, 8H(1232), 10H(11112112), 10H(112222), 22H[11(22)5] and 30R[111313]3, were solved by the X‐ray diffraction method. It was found on polytypic change that (a) the 2Hpolytype is changed into various polytypes on heating at 130°C and most of these further change to 12R[13]3on heating at 260°C; (b) the polytypes 4H, faulted 4H, 14H, 22H, 26Hand 30Rwith layer structures similar to the 12R[13]3are fairly stable at high temperatures; (c) the 2H‐30R[111313]3transformation occurs rever

ISSN:1600-5767    

DOI:10.1107/S0021889879011778

出版商:International Union of Crystallography    

年代:1979

数据来源: WILEY

摘要:

At present, over 30 000 powder diffraction patterns are available as references. It is proposed that the patterns on file as well as new patterns submitted for publication be assigned quantitative quality factors. A simple‐to‐use figure of merit,FN, covering both accuracy in the measurement of the positions of the diffraction lines and completeness of the pattern, has been derived:FN= (1/||) (N/Nposs), whereNpossis the number of independent diffraction lines possible up to theNth observed line and || is the average absolute discrepancy between observed and calculated 2θvalues. This figure of merit provides a rapid evaluation of powder patterns, in much the same way as theRfactor provides a rating for single‐crystal structure determinations. This figure of merit also provides a means to assess the reliability of a unit derived solely from powder data. At presentFNranking scheme is shown to be superior to de Wolff'sM20for ranking patterns. It is recommended that use of the latter be discontinued for that purpose. Guidelines are given on the use and implementation of theFNrating of powder diffraction pa

ISSN:1600-5767    

DOI:10.1107/S002188987901178X

出版商:International Union of Crystallography    

年代:1979

数据来源: WILEY

摘要:

The reactivity of hematite depends among other factors on the morphology of the crystal. The choice of the orientation of the reference axes has been made with the help of stereographic projection combined with the morphology of the etch figures on the different faces of the crystal. That the most stable face of a crystal corresponds to the composition plane is shown for crystals of hematite from Elba.

ISSN:1600-5767    

DOI:10.1107/S0021889879011791

出版商:International Union of Crystallography    

年代:1979

数据来源: WILEY

摘要:

The lattice defects of beryl single crystals from Brazil and Madagascar have been studied by X‐ray topography. Two types of linear defects are observed: channels and grown‐in dislocations. Slip systems and Burgers‐vector moduli are determined. Some calculations of directions of minimum dislocation energy are given and they are in quite good agreement with the observa

ISSN:1600-5767    

DOI:10.1107/S0021889879011808

出版商:International Union of Crystallography    

年代:1979

数据来源: WILEY

摘要:

Growth defects in natural single crystals of beryl have been examined by X‐ray topography. The distribution and the configuration of the defects: dislocations, channels, growth bands and growth sector boundaries, are related to the development of the crystals in nature. For some Brazilian samples, characteristic stops and starts during their development and dissolution phenomena have revealed similar conditions of growt

ISSN:1600-5767    

DOI:10.1107/S002188987901181X

出版商:International Union of Crystallography    

年代:1979

数据来源: WILEY

摘要:

A simple and universal technique is described for the preparation of dry powder specimens for transmission electron diffraction and microscopy. An elaborate study of the obtainment and the interpretation of Debye–Scherrer electron diffraction patterns is presented, with particular attention to matters which determine the possibility of identifying substances with low crystal symmetry or large parameters: the observation of rings near the center of the pattern, the accuracy of the measurements of interplanar spacings, the resolving power, the contrast. It appears that voltages of some hundreds of kilovolts are generally the most appropriate for rapid electron diffraction. A method is given for computing and drawing with sufficient accuracy theoretical Debye–Scherrer patterns of substances with complex crystalline struct

ISSN:1600-5767    

DOI:10.1107/S0021889879011821

出版商:International Union of Crystallography    

年代:1979

数据来源: WILEY

摘要:

The equilibrium phase CuAl2contains a small excess of Al atoms at all temperatures. From accurate X‐ray and density measurements, it was found that the simplest defect structure model consistent with the experimental results is substitutional, Cu1 −xAl2 +x(x= 0.005–0.022 at room temperature). Such a defect type appears to be very unfavourable energetically because of the short Cu‐Cu‐site bond length of CuAI2compared to otherC16 compounds, and would be in contrast to that of the other knownC16

ISSN:1600-5767    

DOI:10.1107/S0021889879011833

出版商:International Union of Crystallography    

年代:1979

数据来源: WILEY

摘要:

Real‐time back‐reflection Laue patterns have been made possible through the construction of a specially designed multiwire proportional chamber. This ability to view Laue patterns in live time on anx−ydisplay scope has greatly reduced the magnitude of effort involved in orienting single crystals and of characterizing the nature of less than perfect cry

ISSN:1600-5767    

DOI:10.1107/S0021889879011845

出版商:International Union of Crystallography    

年代:1979

数据来源: WILEY

摘要:

In neutron‐diffraction measurements of the orientation of apatite crystals, for studying the stress distribution in bone, it is necessary to remove substantially the collagen in order to reduce the incoherent scattering. Data from X‐ray and neutron diffraction indicate the effect of chemical treatment with hydrazine and ethylenediamine and of heat treatment up to 650°C. Crystal growth commences above 500°C, with a slight sharpening of the curve of preferred orientation but without change of the preferred direction. Typically the half‐width at half‐intensity is reduced from 28° to 24° at 650 °C. Fortuitously, because of the differences of scattering amplitudes for the constituent atoms in apatite, the neutron pattern is much more informative concerning crystallinity than is the X

ISSN:1600-5767    

DOI:10.1107/S0021889879011857

出版商:International Union of Crystallography    

年代:1979

数据来源: WILEY