Pub Date : 1994-01-01DOI: 10.1107/s0108768194002284
E. N. Maslen, V. Streltsov, N. Streltsova, N. Ishizawa
{"title":"Synchrotron X-ray Study of the Electron Density in a-Fe2O3","authors":"E. N. Maslen, V. Streltsov, N. Streltsova, N. Ishizawa","doi":"10.1107/s0108768194002284","DOIUrl":"https://doi.org/10.1107/s0108768194002284","url":null,"abstract":"","PeriodicalId":7001,"journal":{"name":"Acta Crystallographica","volume":"46 1","pages":"435-441"},"PeriodicalIF":0.0,"publicationDate":"1994-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"84559063","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 1993-04-01DOI: 10.1107/S0108768192010000
A. Singh, A. King
In this paper we develop and apply techniques for computation of CSL, DSCL and step-vector data for grain boundaries in tetragonal materials for a range of axial ratios. This has application to L10 alloys including TiA1, which is a candidate for lightweight high-temperature structural applications. Our results are compared with others and found to be more accurate and complete. We provide data for a wider range of axial ratios than those considered by previous workers. We have also derived equivalent quaternions for tetragonal crystals in tetragonalcrystal coordinates and listed conditions for selecting a unique reduced rotation in tetragonal-crystal coordinates so that a disorientation description becomes available.
{"title":"Tables of Coincidence Orientations for Ordered Tetragonal L lo Alloys for a Range of Axial Ratios","authors":"A. Singh, A. King","doi":"10.1107/S0108768192010000","DOIUrl":"https://doi.org/10.1107/S0108768192010000","url":null,"abstract":"In this paper we develop and apply techniques for computation of CSL, DSCL and step-vector data for grain boundaries in tetragonal materials for a range of axial ratios. This has application to L10 alloys including TiA1, which is a candidate for lightweight high-temperature structural applications. Our results are compared with others and found to be more accurate and complete. We provide data for a wider range of axial ratios than those considered by previous workers. We have also derived equivalent quaternions for tetragonal crystals in tetragonalcrystal coordinates and listed conditions for selecting a unique reduced rotation in tetragonal-crystal coordinates so that a disorientation description becomes available.","PeriodicalId":7001,"journal":{"name":"Acta Crystallographica","volume":"34 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"1993-04-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"75272397","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 1993-03-01DOI: 10.1107/S0108767392008845
F. Wijnands, T. Janssen
Necessary and sufficient conditions are formulated for an n-dimensional arithmetic point group such that it may be the symmetry group of a d-dimensional quasiperiodic but not periodic, i.e. incommensurate, structure with Fourier modulus of rank n. Only point groups leaving invariant a d-dimensional subspace (the physical space) are considered. For an arithmetic point group describing an incommensurate structure, all equivalent choices for the internal space are related by the normalizer in Gl (n, bb Z) of the point group. Also, the conditions on arithmetic equivalence of two point groups allowing an incommensurate structure are discussed. These conditions yield a further partition of the arithmetic crystal classes.
{"title":"Arithmetic equivalence of point groups for quasiperdiodic structures","authors":"F. Wijnands, T. Janssen","doi":"10.1107/S0108767392008845","DOIUrl":"https://doi.org/10.1107/S0108767392008845","url":null,"abstract":"Necessary and sufficient conditions are formulated for an n-dimensional arithmetic point group such that it may be the symmetry group of a d-dimensional quasiperiodic but not periodic, i.e. incommensurate, structure with Fourier modulus of rank n. Only point groups leaving invariant a d-dimensional subspace (the physical space) are considered. For an arithmetic point group describing an incommensurate structure, all equivalent choices for the internal space are related by the normalizer in Gl (n, bb Z) of the point group. Also, the conditions on arithmetic equivalence of two point groups allowing an incommensurate structure are discussed. These conditions yield a further partition of the arithmetic crystal classes.","PeriodicalId":7001,"journal":{"name":"Acta Crystallographica","volume":"63 1","pages":"315-324"},"PeriodicalIF":0.0,"publicationDate":"1993-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"88521544","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 1992-07-15DOI: 10.1107/S010827019101452X
B. Doudin, G. Chapuis
Cristallisation dans Abma avec a = 7,538, b = 7,416 et c = 30,803 A, Z = 4; affinement jusqu'a R = 0,045. Cette phase lamellaire est semblable a la phase δ observee dans les composes parents, avec une inclinaison des octaedres CdCl 6 favorisant une liaison hydrogene specifique avec la fin ammonium des chaines aliphatiques. Aucun modele simple ne peut expliquer de facon convaincante le desordre des chaines, contrairement a ce qui avait ete le cas pour l'equivalent Mn. Observations de transitions de phase autour de 200 K.
在Abma中结晶，a = 7,538, b = 7,416, c = 30,803 a, Z = 4;细化至R = 0.045。这种层状相类似于在母体化合物中观察到的δ相，cdcl6八面体的倾斜度有利于与脂肪链的铵端形成特定的氢键。没有一个简单的模型可以令人信服地解释链的无序，不像等效Mn的情况。观测到200 K左右的相变。
{"title":"Structure of Bis(n-butylammonium) Tetrachlorocadmate(II)","authors":"B. Doudin, G. Chapuis","doi":"10.1107/S010827019101452X","DOIUrl":"https://doi.org/10.1107/S010827019101452X","url":null,"abstract":"Cristallisation dans Abma avec a = 7,538, b = 7,416 et c = 30,803 A, Z = 4; affinement jusqu'a R = 0,045. Cette phase lamellaire est semblable a la phase δ observee dans les composes parents, avec une inclinaison des octaedres CdCl 6 favorisant une liaison hydrogene specifique avec la fin ammonium des chaines aliphatiques. Aucun modele simple ne peut expliquer de facon convaincante le desordre des chaines, contrairement a ce qui avait ete le cas pour l'equivalent Mn. Observations de transitions de phase autour de 200 K.","PeriodicalId":7001,"journal":{"name":"Acta Crystallographica","volume":"78 1","pages":"1218-1220"},"PeriodicalIF":0.0,"publicationDate":"1992-07-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"77423586","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 1992-02-01DOI: 10.1107/S0108768191010509
E. N. Maslen, N. Spadaccini
Cristallisation dans Pmmm avec a = 3,847, b = 3,875 et c = 11,70 A (ensemble de donnees 1), a = 3,842, b = 3,886 et c = 11,690 A (ensemble de donnees 2), Z = 1; affinement jusqu'a R = 0,041 (O5 exclu) et R = 0,040 (O5 inclus). Discussion detaillee des cartes de distribution de la charge electronique et etablissement d'un modele de la supraconductivite.
{"title":"A position-space model for superconductivity in Y Ba2 Cu3O7-x","authors":"E. N. Maslen, N. Spadaccini","doi":"10.1107/S0108768191010509","DOIUrl":"https://doi.org/10.1107/S0108768191010509","url":null,"abstract":"Cristallisation dans Pmmm avec a = 3,847, b = 3,875 et c = 11,70 A (ensemble de donnees 1), a = 3,842, b = 3,886 et c = 11,690 A (ensemble de donnees 2), Z = 1; affinement jusqu'a R = 0,041 (O5 exclu) et R = 0,040 (O5 inclus). Discussion detaillee des cartes de distribution de la charge electronique et etablissement d'un modele de la supraconductivite.","PeriodicalId":7001,"journal":{"name":"Acta Crystallographica","volume":"12 1","pages":"21-30"},"PeriodicalIF":0.0,"publicationDate":"1992-02-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"91239254","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 1991-03-01DOI: 10.1107/S0108767390011308
D. Schwarzenbach, H. Flack
The raw data obtained with a single-crystal diffractometer are typically integrated profile and background intensities. All linear combinations of these data items, and in particular net intensities and total backgrounds, are equivalent observations. However, net intensities may be negative, in apparent contradiction with basic physics. Backgrounds are generally discarded from the list of observations once they have been subtracted from the profile intensities. In a crystal structure refinement on all observations, structural parameters including scale, extinction and possibly others, as well as individual background intensities, are refined on the observed profile intensities and on the observed background intensities. It is shown that, in such a refinement, the structural parameters are determined exclusively by the net intensities as in traditional least squares, whereas calculated backgrounds depend on the difference between observed and calculated net intensities. Negative net intensities are therefore not in contradiction with physics or with the structural model, and should be retained unaltered in the data set.
{"title":"On the Refinement on Profile, Background and Net Intensities","authors":"D. Schwarzenbach, H. Flack","doi":"10.1107/S0108767390011308","DOIUrl":"https://doi.org/10.1107/S0108767390011308","url":null,"abstract":"The raw data obtained with a single-crystal diffractometer are typically integrated profile and background intensities. All linear combinations of these data items, and in particular net intensities and total backgrounds, are equivalent observations. However, net intensities may be negative, in apparent contradiction with basic physics. Backgrounds are generally discarded from the list of observations once they have been subtracted from the profile intensities. In a crystal structure refinement on all observations, structural parameters including scale, extinction and possibly others, as well as individual background intensities, are refined on the observed profile intensities and on the observed background intensities. It is shown that, in such a refinement, the structural parameters are determined exclusively by the net intensities as in traditional least squares, whereas calculated backgrounds depend on the difference between observed and calculated net intensities. Negative net intensities are therefore not in contradiction with physics or with the structural model, and should be retained unaltered in the data set.","PeriodicalId":7001,"journal":{"name":"Acta Crystallographica","volume":"35 1","pages":"134-137"},"PeriodicalIF":0.0,"publicationDate":"1991-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"78938466","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 1989-01-01DOI: 10.1107/S0108767388013844
B. Doudin, G. Chapuis
The Patterson function can reveal the modulation of a basic structure if its summation terms include only satellite reflections. Examples of Patterson summations for various types of modulated structures illustrate the difficulties of interpretation and the bias occurring in the determination of the amplitudes and phases of the modulation. The opportunity to use the Patterson function to determine modulated structure is discussed.
{"title":"Limits of the Patterson Method for the Determination of one-dimensionally Modulated Structures","authors":"B. Doudin, G. Chapuis","doi":"10.1107/S0108767388013844","DOIUrl":"https://doi.org/10.1107/S0108767388013844","url":null,"abstract":"The Patterson function can reveal the modulation of a basic structure if its summation terms include only satellite reflections. Examples of Patterson summations for various types of modulated structures illustrate the difficulties of interpretation and the bias occurring in the determination of the amplitudes and phases of the modulation. The opportunity to use the Patterson function to determine modulated structure is discussed.","PeriodicalId":7001,"journal":{"name":"Acta Crystallographica","volume":"40 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"1989-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"86320351","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 1988-10-15DOI: 10.1107/S0108270188006651
F. Nicolò, J. Bünzli, G. Chapuis
Le compose du titre cristallise dans C2 avec affinement jusqu'a 0,049. Le complexe est constitue d'anion [Gd(NO 3 ) 6 ] 3− de symetrie 2 et de deux types de cations [Gd(NO 3 ) 2 (18-crown-6)] + , l'un en position speciale de symetrie 2, l'autre en position generale. Ces trois ions etant dans le rapport 1:1:2
{"title":"Structure of the 4:3 Complex Between Gadolinium Nitrate and 18-Crown-6 Ether: [Gd(NO3)2(C12H24O6)]3[Gd(NO3)6]","authors":"F. Nicolò, J. Bünzli, G. Chapuis","doi":"10.1107/S0108270188006651","DOIUrl":"https://doi.org/10.1107/S0108270188006651","url":null,"abstract":"Le compose du titre cristallise dans C2 avec affinement jusqu'a 0,049. Le complexe est constitue d'anion [Gd(NO 3 ) 6 ] 3− de symetrie 2 et de deux types de cations [Gd(NO 3 ) 2 (18-crown-6)] + , l'un en position speciale de symetrie 2, l'autre en position generale. Ces trois ions etant dans le rapport 1:1:2","PeriodicalId":7001,"journal":{"name":"Acta Crystallographica","volume":"68 1","pages":"1733-1738"},"PeriodicalIF":0.0,"publicationDate":"1988-10-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"89656937","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 1988-07-01DOI: 10.1107/S0108767388002697
H. Flack, D. Schwarzenbach
The theory of fixing the origin of the coordinate system in a polar space group by use of restraints (soft constraints or pseudo-observations) is developed for any space group in any setting. The coefficients of the optimal restraint equation are on the average proportional to the square of the atomic numbers. They are determined directly from the unrestrained singular normal-equations matrix. Application of the restraint results in a positive-definite matrix which is as nearly diagonal as possible for the atomic positional coordinates along the origin-free axes. Correlations between these coordinates are therefore minimized. A very compact completely general and easily implemented algorithm results which functions without user intervention.
{"title":"On the Use of Least-Squares Restraints for Origin Fixing in Polar Space Groups","authors":"H. Flack, D. Schwarzenbach","doi":"10.1107/S0108767388002697","DOIUrl":"https://doi.org/10.1107/S0108767388002697","url":null,"abstract":"The theory of fixing the origin of the coordinate system in a polar space group by use of restraints (soft constraints or pseudo-observations) is developed for any space group in any setting. The coefficients of the optimal restraint equation are on the average proportional to the square of the atomic numbers. They are determined directly from the unrestrained singular normal-equations matrix. Application of the restraint results in a positive-definite matrix which is as nearly diagonal as possible for the atomic positional coordinates along the origin-free axes. Correlations between these coordinates are therefore minimized. A very compact completely general and easily implemented algorithm results which functions without user intervention.","PeriodicalId":7001,"journal":{"name":"Acta Crystallographica","volume":"25 1","pages":"499-506"},"PeriodicalIF":0.0,"publicationDate":"1988-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"81730359","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 1988-06-15DOI: 10.1107/S0108270188000885
F. Nicolò, H. E. Ghaziri, G. Chapuis
[CdBr 2 (SCN 3 H 5 )]•H 2 O cristallise dans le systeme monoclinique avec le groupe d'espace Ia. Affinement de la structure jusqu'a 0,031. La geometrie de coordination autour de Cd est un octaedre forme par 2 atomes Br, 3 atomes S et 1 atome N
{"title":"Structure of Dibromo(thiosemicarbazide)cadmium(II) Monohydrate","authors":"F. Nicolò, H. E. Ghaziri, G. Chapuis","doi":"10.1107/S0108270188000885","DOIUrl":"https://doi.org/10.1107/S0108270188000885","url":null,"abstract":"[CdBr 2 (SCN 3 H 5 )]•H 2 O cristallise dans le systeme monoclinique avec le groupe d'espace Ia. Affinement de la structure jusqu'a 0,031. La geometrie de coordination autour de Cd est un octaedre forme par 2 atomes Br, 3 atomes S et 1 atome N","PeriodicalId":7001,"journal":{"name":"Acta Crystallographica","volume":"16 1","pages":"975-977"},"PeriodicalIF":0.0,"publicationDate":"1988-06-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"79694590","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
京公网安备 11010802042870号
京ICP备2023020795号-1
扫码关注我们
求助内容：
应助结果提醒方式：