Showing papers in "Zeitschrift Fur Kristallographie in 1935"

A Direct Method for the Determination of the Components of Interatomic Distances in Crystals

Abstract: In a recent publication) the author has given a brief account of a new method for the direct determination of the components of interatomic distances in crystals. The method is based on the application of the results of the theory of Four i e r series. I t is the object of the present paper to give a more detailed account of the underlying theory and to describe the application of the method to specific examples. In addition, two new results are presented which promise to increase the usefulness of the method for the investigation of complex structures. The paper is divided into two parts. In Part I, which contains the theory of the method, a Fourier Series whose coefficients are the observed values of F 2 (hkl) is derived. This series is shown to be a weighted average distribution of electrons about any point in the crystal. I t is then shown that the peaks of this distribution occur at points which correspond very closely in direction and magnitude with the interatomic distances in the crystal. A second series is then set up, with coefficients derived from those of the first, which corresponds closely to a weighted average distribution of atomic centres. This gives the interatomic distances with greater resolution than the first. Additional results which are of value in practical applications are also derived. In Part II, several known structures are discussed in terms of the new method.

The Crystal Structure of γ-Fe2O3 and γ-Al2O3

Abstract: \\. The oxides Fe203 and Al203 are known to crystallize in different modifications. The stable oxides are a-Fe203 (haematite) and a-Al2()3 (corundum); they form structures of the same type with hexagonal symmetry. Besides, cubic y-modifications of both oxides were obtained and studied by several authors1)2). γ-Α1203 is c. g. prepared by careful heating of ^-hydroxide precipitated with ammonia; it is transformed into the stable α-oxide at temperatures of 950° or higher. y-Fe203 (or ferromagnetic iron oxide) can be prepared by the oxydation of precipitated Fe304 at low temperatures or by the careful dehydration of goethite (lepidocrocite, y-Fe203 · H 2 0 ) ; it is not very stable and is transformed rapidly into «-oxide above 250°. The investigation by X-rays gave for these oxides spinellike diagrams. The structure of y-Fe203 has been discussed by W e l o and B a u d i s c h (1. c.) and more carefully by T h e w l i s (1. c.). The powder diagrams of Fe3Ot and y-Fe203 are practically identical. Fe3Oi has spinel structure. The unit cell contains 32 O-atoms and (8 + 16) i'e-atoms. I t is believed therefore that the structure of y-Fe203 is given by a spinel unit ccll with four additional O-atoms: Fe^O^. The positions assumed by T h e w l i s for these 4 atoms arc considered to be more probable than those of B a u disch and Welo with regard to ionic radii. For γ-Α1203 a similar structure has been proposed. However, B u r g e r s (1. c.) determined the length of the unit cell of γ-Α1203, a — 7.90 A. and pointed out that a structure with 12 molecules

Ergebnisse der Strukturbestimmung komplexer Sulfide

Abstract: I. Die Struktur von Zinnsulfür SnS und Ί eallit PbSnSa. Die Struktur von Teallit, eines seltenen bolivianischen Minerals, schien dem Verfasser von Wichtigkeit, da die Umgebung der Bleimit Schwefelatomen, die man bisher nur in Bleiglanz kennt (4), Schlüsse auf den Aufbau der kristallisierten Bleiarsen-, Bleiantimonsulfide und anderer komplexer Bleisulfide erwarten ließ. Die Kristallchemie der Sulfosalze wurde vom Verfasser seit einigen Jahren mit Unterstützung der Notgemeinschaft der Deutschen AVissenschaft bearbeitet. Die Ergebnisse dieser Arbeiten und der von anderen Autoren durchgeführten Strukturbestimmungen haben einige allgemeine Vorstellungen über den Aufbau der Sulfosalze gezeitigt, die zum Teil in vorläufiger Form (9) mitgeteilt wurden. Die Betrachtungen werden im zweiten Teil der Arbeit ausführlicher dargestellt werden, da Verfasser zur Zeit an der Weiterführung der Untersuchungen verhindert ist und diese zu einem vorläufigen Abschluß bringen möchte. Die unabhängig von den Arbeiten an Teallit begonnene Strukturbestimmung von Zinnsulfür führte zu dem überraschenden Ergebnis, daß beide Substanzen prinzipiell die gleiche Struktur besitzen. Sie entspricht einem neuen Strukturtyp für die Verbindungen AB und ist vermutlich in weiteren Verbindungen dieses Formeltyps, die noch nicht strukturell bearbeitet wurden, verwirklicht.

The Structure of the electrolytical oxide Layer on Aluminium

Abstract: The structure of cubic ~/-Al203, formed by the electrolytical oxidation of Al, is described as an averaged structure of the cations. The unit cell contains 4 O-ions in face-centred positions; 2r! Al3~ ions are distributed statistically over all the interstices between the 02~ ions in such a way that 70% of the Ala+ have a coordination number 6 and 30% a coordination number 4 with regard to the oxygen. γ'-Α1203 and γ-Α1203 are intermediate cases between the amorphous and the completely arranged state. B u r g e r s , C l a a s s c n and Ze rn ike 1 ) have examined the crystal structure of the thin oxide layer that is formed by the electrolytic oxidation of an aluminium electrode. They found tha t the structure of this oxide varies according to the method of preparation of the oxide layer.) Sometimes it consists of amorphous Al203 (formation in oxalic acid solution). Under other conditions the X-ray diagrams showed tha t a cubic form of Al203 is obtained (formation in boric acid solution). The X-ray diagram of this cubic oxide proves tha t it is closely related to γ-Α1203. The spectograms are, however, not identical. Both the amorphous oxide and the cubic form are transformed into γ-Α1203 when the substances are heated during a few hours at 900°. We will denote the cubic oxide of B u r g e r s , C l a a s s e n and Z e r n i k e by γ'-Α1203. In a previous paper) we discussed the structure of y-Fe203 and γ-Α1203, and ascribed to these substances a spinel structure with a number of cation places vacant (averaged structure). γ-Α1203 contains 32 oxygen ions in the unit cell (a = 7.90 Ä). The structure of γ'-Α1203 will be the subject of this note. The study of the diagram of γ -Alfi3 revealed tha t γ'-Α1203 can be described as a face-centred lattice with 4 oxygen ions in the cell (a = 3.95 Ä)). This needs Al ions per unit cell. Jus t as for γ-Α1203

Untersuchungen uber dendritisches Wachstum von Kristallen

Abstract: Inhaltsübersicht. Einleitung. I . Bo l le der Diffusion. Dendritenbildung bei ebenen Kristallflächen. S. 90. — Desgl. bei runden Flächen. S. 93. II . Wirkungen der Oberflächenspannung. Destillationserscheinungen bei Ν H fil. S. 96. — Bestimmung der Oberflächenspannung bei NHfil. S. 99. III . Verschiedene Beobachtungen. An NHfil, NHiBr, Pb(N03)2, NaCl, NaBr, KCl, KBr, NHtN03, NaClO3. S. 102. IV. Vorgänge an der Spitze. Die Gleichungen des Wachstumsproblems. S. 108. — Wachstum von stabförmigen Kristallen. Beständige Formen. S. 110. — Feinheit der Spitze und Übersättigung. S. 112. — Bedingung für die Bildung von Stabformen. Erste kritische Übersättigung. S. 113. ·— Bedingung für die Bildung von Nebenspitzen. Zweite kritische Übersättigung. S. 114. V. Entwicklung der Nebenspitzen. Bildung von Nebenästen. S. 116. — Ein spezielles Diffusionsproblem: Einfluß des Hauptastes auf Nebenäste zweiter Ordnung. S. 118. — Vergleich mit der Erfahrung. S. 121. — Nebenäste erster Ordnung. S. 121. ·— Freie Äste. Formel für ihren mittleren Abstand. S. 123. — Vergleich mit der Erfahrung. S. 124. — Gesamtansicht eines wachsenden Dendriten. S. 126. VI. K r i s t a l l i s a t i o n aus der Schmelze. Nachweisbarkeit des dendritischen Wachstums bei der Kristallisation aus der Schmelze. S. 127. — Übertragung der früheren Ergebnisse auf den vorliegenden Fall. S. 127. ·— Vergleich mit der Erfahrung. S. 128.

Geometrical and Optical Properties, and Crystal Structure of Tenorite

Abstract: The goniometrical measurements of N. Story-l\\laskelyne ) on crystals of tenorite (also called melaconite) found with cuprite and chrysocolla in a chloritic veinstone from Lostwithiel, Cornwall, satisfy the symmetry requirements of the monoclinic system, and the crystal habit illustrated in the drawing of Story-l\\1askelyne is clearly that of the holohedral class of the monoclinic system. ~Ioreover, as A. Scacchi) has pointed out, the goniometrical measurements of J. J enzsch ) on artificial crystals of cupric oxide formed near a flue in a smelterhearth during roasting of copper matte with sodium chloride at Freiberg, yield calculated elements agreeing with those of Story-1\\Iaskelyne, and the crystals drawn by J enzsch exhibited the same habit and twinlaw as those of Story-·Maskelyne. J enzsch interpretE

Struttura Cristallografica del Gruppo Isomorfo (Si4+, Ti4+, Zr4+, Sn4+, Hf4+) P2O7

Abstract: I p iro fos fa t i di Si**, Ti1*, Zri+, Sni+, Hfi+, cos t i tu i scono , a q u a n t o risulta dall'esame microscopico dei loro cristalli e dalle loro formule chimiche, una serie isomorfa cubica, nella quale si devono probabilmente aggiungere quelli di Gei r c di Thi+, non ancora preparati, e quello di U i + che è già stato ottenuto sotto forma di piccoli cubottacdri (1). L'interesse della determinazione strutturale di questo gruppo è grande tanto dal punto di vista cristallografico, quanto da quello chimico, poche non è stata compiuta finora per nessun composto contenente il radicale acido P207 4 per il quale la struttura coi raggi X non ha quindi dato ancora una formula di struttura che convalidi quella proposta dalla chimica (vd. avanti).

The Orientation of the Oxalate Group in Oxalic Acid and some of its Salts

Abstract: The purpose of the work reported below was to determine the orientation of the oxalate group in a sufficiently large number of oxalates to permit correlation of the optical properties of the crystals with the polarizabilities of the constituent group. It is a matter of structural interest to ascertain possible variance in the geometry of the group, which involves a C—C single bond, as the surroundings are changed. The configuration of the oxalate radical in H2G2Oi · 2 H20 has been determined by W. H. Zachariasen 1 ) .

The Configuration of the Triiodide Group in Ammonium Triiodide Crystals

Abstract: Because of the usual methods of preparation, the triiodides of the alkali metals and of ammonium have sometimes been considered to be addition products of the simple iodides and iodine, into which components they readily revert by decomposition. Their properties, however, indicate that they are best represented as compounds of a metal ion or group and a complex iodine anion; that is, as the salts of hydrotriiodic acid. The crystal structure determination of a typical triiodide was undertaken in order to obtain accurate data concerning the interatomic distances and the bond directions in the triiodide group. According to Grace ) and to F o o t e , B r a d l e y and F l e i s che r 3 ) , the only true \"alkali\" polyiodides which exist as unsolvated solids a t twenty-five degrees and one atmosphere are those of caesium, rubidium and ammonium. The crystals) of the compounds all belong to the bipyramidal class of the orthorhombic system; they are isomorphous, the axial ratios differing only slightly; they are increasingly unstable in descending order of molecular weight. The ammonium compound is obviously the most suitable for an X-ray study of the triiodide group, since the scattering of ammonium is so small compared to that of iodine that the observed intensities may without serious error be attributed to the iodine contribution alone. Thus the location of the positions of the iodine atoms is greatly simplified, though on the other hand, it is impossible to draw any conclusions regarding the cation parameters. However, a study of the diffracted intensities in the isomorphous caesium salt may be used to deduce, at least approximately, the ammonium positions. Data. For both caesium and ammonium triiodide, observations were made by the single crystal oscillation method. Exposures of four

The Crystal Structure of Gallium

Abstract: The crystal structure of the metal gallium was first investigated by J a e g e r , T e r p s t r a and Westenbrink 1 ) , but the tetragonal structure suggested by them was incorrect. Later Laves 2 ) showed that this element was orthorhombic, and not tetragonal, although like the earlier workers he considered that two of the axes were equal. He also determined atomic coordinates, owing however to the lack of quantitative intensity data with less accuracy than desirable. In view of these facts it seemed advisable to attempt a reinvestigation of the structure from entirely different experimental data. The present work shows conclusively that all three axes are unequal, so that the anomaly of an orthorhombic crystal with two equal axes has now been removed. Moreover the atomic coordinates have been fixed within narrower limits by using a new method for comparing observed and calculated intensities.

The Structure of a-Quartz

Abstract: Introduction. The structure of α-quartz has been studied by various investigators1, 2· 3> 4). It belongs to the space group D\\ or D\\ according to whether it is left-handed or right-handed. The unit cell contains three silicon-dioxide molecules arranged spirally about the screw axis of symmetry. Its axial units are a = b = 4.903 Á and c = 5.39s Â. Its structure involves four unknown parameters designated analytically as u, x, y, and ζ by Wyckoff. On account of various difficulties the exact values of the unknown parameters have never been determined. Gibbs attempted to narrow down the uncertainty in the parameter values in his paper of 19263), but did not indicate its range. As pointed out by Ewald and Hermann5), his structure is not unique. It has thus been found impossible to apply his results in the study of various properties of α-quartz. Considerable interest in the structure of α-quartz has been felt following the discovery by Bozorth and Haworth9) of its crystalline perfection. They measured the spectral width of MoKu lines reflected from an etched Curie cut quartz plate and found it to agree very closely with that predicted by Dar win's theory7) of reflection from perfect crystals. Their measurements were later confirmed by Parratt 8 ) . Quartz thus promises to replace calcite as reflector in the double crystal spectrometer in the study of X-ray spectra, and also to give a final test of the Darwin perfect crystal intensity formula, when its exact structure is known. The present paper is an attempt at a more accurate determination of the said structure. Data were obtained by the usual method of oscillation photographs taken with a Bernal apparatus and indexed according to Bernal's method9). A set of absolute intensities including reflections

Crystal Structure of Sodium Uranyl Acetate

Abstract: There are several points in this paper which are open to criticism. The structure proposed by De J o n g is based on the estimate of intensities of a comparatively small number of lines in a powder photograph. Zacha r iasen 3 ) in his paper on NaClOz, which is also T, points out that the powder method does not afford adequate data to attack a structure of this type as the hkl and Jc Τι I reflections fall on the same spot while they have completely different structure amplitudes. A reasonable model of the acetate group was assumed by De J o n g with one exception. The radius of the acetate oxygen was taken as .60 A irrespective as to whether one is considering its relationship with atoms in its own acetate group or atoms outside it. With regard to the latter relationship, the radius used should be about 1.35 A. The placing given of the acetate group would make the structure a molecular one, which seems unlikely. In computing intensities, the TJOi and (Cyy302) groups were considered as scattering centers containing respectively 109 and 31 electrons. Especially in the case of the acetate group, this approximation is very poor. I t was therefore thought desirable to attempt a more complete solution making use of single crystal data.

Die Kristallstruktur des Tief-Cristobalits SiO2

Abstract: Eine röiitgenographisehc Untersuchung der von Professor Dr. P. R a m d o h r gesammelten Cristo balite1) im Göttinger mineralogischen Institut hat außer der Auffindung der zweidimensionalen Strukturgebilde in den nach Tridymit pseudomorphen Kristallen (Cristobalite dritter Tracht R a m d o h r s), worüber früher berichtet wurde2), auch zu einer Strukturbestimmung der regulär ausgebildeten Kristalle erster Tracht geführt. Die oktaederförmigen Kristalle sind bekanntlich sehr innig verzwillingt. Die tetragonalen (oder rhombischen) Individuen sind stets mit ihren Hauptachsen parallel zu den pscudokubischen Hauptachsen angeordnet, wie aus optischen Befunden zu schließen ist3). Es ist befürchtet worden, daß diese Verzwillingung die Anwendbarkeit des Drehkristallverfahrens in Frage stellen würde; die erhaltenen Drehaufnahmen erwiesen sich aber als durchaus interpretierbar4).

A Back-Reflection Laue Method for determining Crystal Orientation

Abstract: The back-reflection L a u e method for determining crystal orientation is both simple and rapid, and does not involve the use of calculations or projections in solving the patterns obtained. One photograph, which can be taken in less than an hour and interpreted in a few minutes, will completely establish the orientation of the crystal being studied. The attainable accuracy is limited only by the perfection of the crystal. No special camera is needed, and crystals of any size or thickness may be studied.

Die Steinsalzstruktur der Verbindung Li2TiO3 und ihre Mischkristallbildung mit MgO und Li2Fe2O4

Abstract: Die Untersuchungen zahlreicher Forscher über die Isomorphiebedingungen besonders während der letzten 15 Jahre haben ergeben, daß bei der isomorphen Substitution einzelner Gitterkomponenten der g l e i c h e n W e r t i g k e i t nicht die große Bedeutung zukommt, wie früher angenommen wurde. Neben anderen Voraussetzungen, wie z. B. der gleichen Koordinationszahl und ähnlichen Polarisationseigenschaften, muß in erster Linie die Ähnlichkeit der R a u m b e a n s p r u c h u n g der einander ersetzenden Gitterbausteine erfüllt sein. Besonders bei k o m p l i z i e r t e n Verbindungen, wie z. B. bei den Feldspaten, Pyroxenen und Amphibolen hat man sich schon längst daran gewöhnt, daß einzelne Ionen bzw. Atome durch andere u n g l e i c h e r Valenz ersetzt werden können, sofern durch g e k o p p e l t e Substitution weiterer Ionen die Elektroneutralität des Gitters gewahrt bleibt. Das Ziel der hier mitgeteilten Untersuchungen war, zu zeigen, daß unter günstigen Bedingungen selbst bei a l l e r e i n f a c h s t e m Gitterbau, wie z. B. vom S t e i n s a l z t y p , ganz normale Mischkristalle erhalten werden können, in denen Ionen v e r s c h i e d e n e r W e r t i g k e i t einander in gleichen Positionen ersetzen. Verfasser wählte für seine Untersuchungen die Ionen Li, Mg, Fe und Ti, deren Ionenradien recht ähnlich sind, und die allesamt gegenüber Sauerstoff die Koordinationszahl 6 betätigen können. Zugleich war bekannt, daß diese Ionen sich in komplizierten Verbindungen, z. B. in einigen natürlichen Alumosilikaten, einander z. T. isomorph zu vertreten vermögen.

The Structure of Strontium and Barium Peroxides, Sr O~2~ and Ba O~2~

Abstract: Strontium and Barium Peroxides are shown to have the Calcium Carbide structure and the size of the unit cells is given by: Strontium Peroxide α = 5.02 Á, c = 6.55 Á. Barium Peroxide a = 5.34 A, c = 6.77 A. The structures are confirmed by density measurements and by the comparison between the calculated and observed intensities, the results in both cases being very satisfactory. The distance between the centres of the oxygen atoms in the Ο2 ion are shown to be : Strontium Peroxide d0_0 = 1.35 Á Barium Peroxide d ^ = 1.29 Á. These distances are the length of the oxygen-oxygen single bond, the difference in the two cases being ascribed to contrapolarisation. Subject to allowances for this the length of the bond is taken to be : d^, = 1.31 ± 0.03 Á.

Sur la Syntaxie et la Polytypie

Abstract: Dans un memoire precedent1), j 'ai introduit le terme de s y n t a x i e pour designer un cas d'epitaxie d'espece tres particuliere: il s'agit des groupements a axes paralleles de deux modifications polymorphiques d'une meme substance chimique, avec rapport rigoureusement rationnel entre les parametres des deux mailles elementaires et entre leurs parametres cristallograpliiques, tous les caracteres cristallins etant nettement distincts d'unc »espece« ä l'autre. J ' a i decrit tout au long le cas extremement curieux de la coquimbite et de la paracoquimbite, mais je crois bon de rappeler ici de fafon succincte de quoi il s'agit. Le sulfate ferrique ä neuf molecules d'eau de cristallisation: Fe^Os · 3S03 · 91I20 se rencontre ä Tierra Amarilla sous deux formes totalement differentes, l'une senaire en apparence, la coquimbite proprement dite avec cja = 1,5643, une liste de formes, des clivages et des figures de corrosion entierement conformes avec l'holoedrie senaire ou au moins l'antihemiedrie senaire; l'autre ternaire, la paracoquimbite, avec cja = 4,6928, dont tous les caracteres exterieurs concordent entierement avec les postulate de l'holoedrie rhomboedrique, tels que repartition des faces, clivage, figures de corrosion sur les rhomboedres. La radiographic met en evidence une profonde difference de structure entre les deux modes de coquimbite, tout en revelant dans les deux cas une symetrie intime plus basse: parahemiedrie senaire ä axe ternaire pour la coquimbite, parahemiedrie rhomboedrique pour la paracoquimbite. Ces deux »especes« mineralcs paramorphes peuvent se trouver chacune pour soi dans des echantillons differents; mais plus souvent ils se trouvent associes dans une meme geode, avec des caracteres qui prouvent evidcmment que leur cristallisation a ete simultanee et soumise aux memos influences exterieures. II se produit alors une syncristallisation d'unc espece toute particuliere, une epitaxic qui peut se repeter plusieurs fois, par empilement a axes paralleles de rondelles des deux »especes«, et creer des groupements extremement varies, l'une ou l'autre des »especes« predominant ou terminant soit les deux extremites, soit une seule extremite de l'edifice complexe. C'est cette epitaxie indefiniment repetee de deux

The Crystal Structure of Au2Bi

Abstract: In a previous communication1) a new phase was shown to exist in the system gold-bismuth, at 66.6 at. proc. gold; though the crystal structure was mentioned, its systematic deduction from the experimental results was not given. This phase has always the same composition, whatever the ratio of the components is in the liquid from which it crystallizes, the lines of the Debye-Scherrer diagram remaining in the same place. This points to the fact, that Au2Bi is an intermetallic compound in which none of the components dissolve.

On the Symmetry of the Rhombic Pyroxenes

Abstract: The writer has recently tried to find the structural relationship between epididymite and eudidymite, both of which have the same chemical composition HNaBeSi3Os with different symmetry properties, one being rhombic and the other monoclinic1). The conclusion thereby arrived a t reveals that one of these dimorphous minerals may be looked upon as the twinned form of the other. Twinning in this case, however, is used in a rather more extended sense than is usual in crystallographic work. The operation for the formation of the \" twins \" (epididymite) is already effected within the unit cell. I t may probably be regarded as a limiting case of the twinning of crystals in general and offers a most interesting case in which polymorphism of crystals is due to twinning rather than to different structures. I t is then natural to enquire whether there are other examples in nature corresponding exactly to the above case or approximating closely to it. In this paper the writer will show what is the effect on X-ray photographs if the twinning takes place in a crystal a t intervals comparable with dimensions of the unit cell but not within the unit cell as in epididymite. As material for this study we have taken the rhombic pyroxenes, since the experimental examination of this group of minerals carried out by Β . E . W a r r e n and D. I. M o d e l l 2 ) does not exclude the much discussed possibility that they are really the twinned forms of the monoclinic pyroxenes. W a r r e n and M o d e l l based their determination on the space group V a n d went so far as to say that the orthorhombic unit cell of enstatite is essentially two monoclinic cells joined on the \" a \" face through a glide plane of reflexion. The present writer will reverse the process and start from the monoclinic space group G%h.

Kristallchemische Untersuchungen über Aluminiumverbindungen mit spinellartigem Gitterbau und über γ-Fe2O3.

Abstract: 1) Zusatz nach Eingang obiger Arbeit bei der Redaktion: Durch freundliche Vermittlung von Herrn Professor Dr. P. Niggli hatte Verfasser nach Absendung des Manuskripts Gelegenheit, den Inhalt der mittlerweile in dieser Zeitschrift erschienenen Arbeit von E. J. W. Verwev \"The crystal structure of y-Fe20:i and γ-Α120Λ\" aus einem Fahnenabzug zu erfahren. Verwey und der Verfasser deuten beide in ihren vo l lkommen u n a b h ä n g i g e n Untersuchungen die Struktur dieser zwei Oxyde im Prinzip in gleicher Weise. Ebenso wie der Verfasser erteilt auch Verwey den O-Ionen dieselbe Anordnung wie in den normalen Spinellen, so daß in einer Elementarzelle mit 32 Ü-Ionen 21J Albzw. •Fe^'-Ionen enthalten sein müßten. Verwey diskutiert ebenfalls, wenn auch nur ganz kurz, die Möglichkeit einer größeren Elementarzelle oder einer niedrigeren Symmetrie, doch konnte er in den Pulverdiagrammen von */-Fe203 hierfür keine Anhaltspunkte finden. Bei der y-Tonerde benutzte er die von Η. B. Barle t t veröffentlichten Daten. Verfasser gelangte auf Grund seiner expe r imen te l l en Untersuchungen an γ-Αl203 zu neuartigen Ergebnissen, die über diejenigen von Verwey hinausgehen. Vor allem wurde die pro Elementarzelle mit 32 O-Ionen übrigbleibende eine Sauerstoffvalenz berücksichtigt und in diesem Zusammenhang die stabilisierende Bolle kleiner Beimengungen von Oxyden oder Wasser gittermäßig zu erklären versucht. Außerdem wurden vom Verfasser in den sehr lange belichteten Aufnahmen von 3/j-Spinell, Spinell und vor allem von ;'-Tonerde bisher bei Spinellen noch nicht beobachtete neue Interferenzlinien erhalten, die Veranlassung zu einer genaueren Nachprüfung der bisherigen Strukturdeutung der Spinclle geben. 2) Der Inhalt dieser Arbeit ist am 9. Mai 1935 in der Leipziger »Chemischen Gesellschaft« vorgetragen worden; über die Untersuchungen am ϋ -Λ /-Spinell ist vom Verfasser in derselben Gesellschaft schon im Mai 1934 berichtet worden.

Die Kristallstruktur von Calciumchlorid, Ca Cl2

Abstract: Sie enthält 2 Moleküle CaCl2, entsprechend dem spezifischen Gewichte -Dröntg. = 2,17, Ait. = 2,22. Die rhombische Symmetrie ist in Übereinstimmung mit den optischen Befunden von C. B . S l a w s o n 1 ) . Die beobachteten gesetzmäßigen Auslöschungen hOl und 0kl für h + l, bzw. k + l ungerade, führen zu den Raumgruppen Pnn oder Pnnm (C^, oder V),2). Die Drehaufnahme mit Mo-Strahlung zeigte weiter völlige Parallelität der Intensitäten der 0. und 2. und 4. Schichtlinie, und der 1. und 3. Es ist also I(h kl) = I(h kl + 2). Dies besagt, daß die Atome in Ebenen senkrecht zur c-Achse angeordnet sind, welche einander im Abstand | c folgen. Für die Cl gibt es eine derartige vierzählige Lage nur in Pnnm, nämlich