Ever since Wilhelm Conrad Rontgen discovered X-rays a century ago and von Laue, Friedrich, Knipping, Braggs and Braggs developed an analytical method almost nine decades ago, single crystal structure analysis has become the most powerful analytical tool used to elucidate unequivocally the three dimensional structure of solid matter. New X-ray detectors like image plates or CCD-detectors permit the collection of two datasets a day. However, this recent development should not only result in a higher ‘turn-over’ of crystal structure determinations, but should also be utilized in new research approaches. One can think of monitoring chemical reactions in terms of Echtzeit crystallography. The challenge is to synthesize, crystallize and analyze reaction intermediates by cryo crystal structure determination. Alkali organometallic compounds are omnipresent in any chemical laboratory where they are used as highly reactive starting materials in a vast number of reactions. It would be of great use to be able to freeze out intermediates along a reaction pathway in order to deduce new synthetic routes and concepts from the structural knowledge obtained by single crystal structure determinations.

Cryo crystal structure determination and application to intermediates

Theory of the formation of metallic glasses

The chemistry of metals in low valence states is marked by the frequent occurrence of metal clusters, which are easily recognizable when they occur as molecular units. Many metal-rich compounds of transition metals with p-elements (3rd to the 6th main groups) are closely related to the corresponding halides, since they are built up from metal clusters of the same type. The clusters are however, linked together (condensed) by metal-metal bonds. This principle of construction holds particularly well in the case of the novel reduced halides of the lanthanoids.

Condensed Metal Clusters

The ordered overlayer structures formed by Cs adsorbed on a Ru(0001) surface were analyzed by use of low-energy electron diffraction (LEED). The phase diagram reflects the dominance of dipole-dipole repulsions between the adparticles and comprises quasiliquid configurations characterized by diffraction rings up to a coverage \ensuremath{\Theta}=0.17, followed by a (2\ifmmode\times\else\texttimes\fi{}2) structure with maximum intensity of the diffraction spots at \ensuremath{\Theta}=0.23. Beyond \ensuremath{\Theta}=0.25, a series of structures with rotated unit cells is identified which are followed by a (\ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 )R30\ifmmode^\circ\else\textdegree\fi{} structure around \ensuremath{\Theta}=0.33 (\ensuremath{\approxeq}completion of the first monolayer). In the (2\ifmmode\times\else\texttimes\fi{}2) phase the Cs atoms are located in on-top sites with a Ru-Cs bond length of 3.25\ifmmode\pm\else\textpm\fi{}0.08 \AA{}, corresponding to a hard-sphere radius of 1.9 \AA{} for the Cs atom. In the (\ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 )R30\ifmmode^\circ\else\textdegree\fi{} structure, on the other hand, the adatoms occupy threefold hollow hcp sites with Ru-Cs bond lengths of 3.52\ifmmode\pm\else\textpm\fi{}0.02 \AA{}, corresponding to a Cs hard-sphere radius of about 2.2 \AA{}. The increase in bond length and effective radius of the adparticle is paralleled by the transition of the character of bonding from more ``ionic'' at \ensuremath{\Theta}=0.25 (large dipole moment) to more ``metallic'' at \ensuremath{\Theta}=0.33 (dipole moment reduced by about 30%). The associated change of the type of adsorption site (from on-top to hollow) is qualitatively rationalized by a model according to which inherently less favorable sites may become preferred due to improved effective screening of the dipole-dipole repulsion by the location of substrate atoms in the region between neighboring adatoms.

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Coverage dependence of adsorption-site geometry in the Cs/Ru(0001) system: A low-energy electron-diffraction analysis.

Die Chemie der Metalle in niederen Oxidationsstufen ist durch das vielfaltige Auftreten von Metall-Clustern gepragt, die in Molekulstrukturen leicht erkennbar sind. Sehr viele metallreiche Verbindungen der Ubergangsmetalle mit p-Elementen (3. bis 6. Hauptgruppe) sind ahnlich wie die niederen Halogenide dieser Metalle aus gleichartigen Metall-Clustern aufgebaut. Die Cluster sind jedoch miteinander durch Metall-Metall-Bindungen verknupft (kondensiert). In besonderem Mase wird dieses Aufbauprinzip bei den neuartigen niederen Halogeniden der Lanthanoide beobachtet.

Kondensierte Metall‐Cluster

Die bislang nicht bekannten, unterhalb etwa –90°C existierenden Verbindungen K2Cs und K7Cs6 werden beschrieben. Phasendiagramme und kinetische Zusammenhange bei der Entstehung der Verbindungen ergeben sich aus thermischen Analysen sowie der rontgenographischen Untersuchung poly-und einkristalliner Proben.



K2Cs ist isotyp mit der hexagonalen LAVES-Phase Na2K (MgZn2-Typ; a0 = 9,065, c0 = 14,755 A bei –95°C; modifizierte GUINIER-Technik). K7Cs6 kristallisiert gleichfalls hexagonal in einer bislang unbekannten Struktur vom FRANK-KASPER-Typ (Einkristallzuchtung bei –100°C; Raumgr. P63/mmc; ao = 9,078; co = 32,950 A bei –95°C). Zwischen K7Cs6 und den μ-Phasen bestehen vergleichbare Strukturbeziehungen wie zwischen den LAVES-Phasen MgZn2 und MgCu2. Die interatomaren Abstande in den LAVES-Phasen Na2K, K2Cs, Na2Cs folgen einheitlich dem Zusammenhang d = Z(2rA + rB).



Novel Compounds between Potassium and Caesium



The hitherto unknown compounds K2Cs und K7Cs6 exist at temperatures below −90°C. Phase relationships and kinetic aspects of phase formation are investigated by thermal analyses and x-ray work with poly- and single-crystalline samples.



K2Cs is isotypic with the hexagonal LAVES phase Na2K (MgZn2 type of structure; a0 = 9.065, c0 = 14.755 A at −95°C due to modified GUINIER-diagrams). K7Cs6 forms hexagonal crystals with a novel kind of FRANK-KASPER structure (single crystals grown at −100°C; space group P63/mmc; a0 = 9.078; c0 = 32.950 A at −95°C). Structure relationships of K7Cs6 and μ-phases are analogues to those of the LAVES phases MgZn2 and MgCu2.



The interatomic distances in all of the LAVES phases Na2K, K2Cs and Na2Cs are characterized by the identical expression d = Z(2rA + rB).

Neue Verbindungen zwischen Kalium und Cäsium

The specific heats of the metallic oxides Cs11O3, Cs7O ( = Cs10Cs11O3) and Rb7Cs11O3 have been measured in the temperature range of 0.50⩽T⩽100 K. The results are in agreement with a bond model of ionic clusters of the composition Cs11O3 which are weakly coupled to one another and to the matrix of additional alkali metal atoms via metallic bonding. The Debye temperatures are extremely low [θ0(Cs11O3) = 41 K, θ0 (Cs7O) = 45 K, and θ0 (Rb7Cs11O3) = 47 K]. Anomalies in the lattice specific heat at low temperatures (T⩽5 K) are interpreted as due to low frequency vibrations of the heavy Cs11O3 clusters (molecular weight m=1510 g mol−1), corresponding to a characteristic Debye temperature of θD≃4 K.

W. Brämer

Papers

Neue Verbindungen zwischen Kalium und Cäsium

Low temperature specific heat of metal‐rich alkali metal oxides