Alkalimetallhydrogensulfide, Struktur und Dynamik
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Date
2001-06-01
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Universität Dortmund
Abstract
Die Hydrogensulfide der Alkalimetalle MHS mit M = Na, K, Rb, Cs haben die Eigenschaften eines Modellsystems zur Untersuchung von Reorientierungsprozessen ladungsasymmetrischer Ionen, was auf die einfachen Kristallstrukturen dieser ionogen aufgebauten Verbindungen (M+HS-) zurückzuführen ist. Die Strukturen leiten sich für MHS mit M = Na, K, Rb vom NaCl-Typ und für CsHS vom CsCl-Typ ab. Die Verbindungen bilden bei Normaldruck drei Modifikationen als Funktion der Temperatur aus. Diese sind für die verschiedenen Kationen der NaClartigen Substanzen isotyp, so daß eine Änderung der Reorientierungsdynamik der Anionen im wesentlichen durch die Stärke des Umgebungspotentials bedingt wird. Mit steigender Temperatur nimmt die dynamische Fehlordnung der Anionen zu. In der niedersymmetrischen Tieftemperaturmodifikation wird in erster Näherung eine antiferroelektrische Ordnung der Anionen beobachtet. Im Temperaturbereich der Mitteltemperaturmodifikation bilden zwei symmetrieäquivalente Wasserstoffpositionen mit der Position des Schwefels eine lineare Gruppierung und die Anionen führen 180°-Sprungreorientierungen aus. Die kubische Symmetrie der Hochtemperaturmodifikation geht mit einer sogenannten quasifreien Rotation der Anionen einher. Dennoch wird die Wahrscheinlichkeitsdichte der Wasserstoffatome durch das Umgebungspotential beeinflußt. Zudem ist in dieser Phase eine Translationsdiffusion der Anionen wahrscheinlich.
Zur Untersuchung des Zusammenhangs von Struktur und Dynamik der Anionen bzw. Kationen der genannten Verbindungen wurden elastische und quasielastische Neutronenstreuexperimente sowie NMRMessungen in einem weiten Temperaturbereich durchgeführt. Die NMRExperimente beinhalten eine Analyse der SpinGitterrelaxation sowie eine Analyse der zweiten Momente des Protonensignals und eine Linienformanalyse des Deuteronensignals. Zur Optimierung einzelner Experimente wurden vollprotonierte sowie volldeuterierte Präparate verwendet.
The hydrogensulfides of the alkali metals MHS with M = Na, K, Rb, Cs are a model system to investigate reorientational processes of charge asymmetric ions. This is due to the simple crystal structures of these ionic compounds (M+HS-). They are related to a NaCl-type for MHS with M = Na, K, Rb and for CsHS to a CsCl-type. Three modifications depending on temperature are known at ambient pressure. These are isotypic for the different cations of the NaCl like compounds. Therefore differences in the reorientational disorder of the anions of these compounds are mainly caused by the coordination potential. The degree of reorientational disorder increases with raising temperature. In a first approximation an antiferroelectric orientation of the anions was found in the low temperature modification. Two symmetry equivalent hydrogen positions build a linear group with the position of sulphur in the middle temperature modification. The anions reorientate by a two site 180°jump process. The cubic symmetry of the high temperature modification is achieved by nearly free reorientational disorder of the anions. But this is strongly linked to the coordination potential, which can be seen by the probability density function of the hydrogen atoms. Furthermore a diffusion of the cations seems to be likely for this modification. Elastic and quasielastic neutron scattering experiments and NMRmeasurements were done to gain insights into the relations of the crystal structures and the dynamics of anions or cations. The experiments were carried out in a wide temperature range. An analysis of NMR spinlattice relaxation and second moments of the proton signal was done. In addition a line shape analysis of the deuteron signal was executed. To optimise the experiments we used either fully deuterated or fully protonated samples.
The hydrogensulfides of the alkali metals MHS with M = Na, K, Rb, Cs are a model system to investigate reorientational processes of charge asymmetric ions. This is due to the simple crystal structures of these ionic compounds (M+HS-). They are related to a NaCl-type for MHS with M = Na, K, Rb and for CsHS to a CsCl-type. Three modifications depending on temperature are known at ambient pressure. These are isotypic for the different cations of the NaCl like compounds. Therefore differences in the reorientational disorder of the anions of these compounds are mainly caused by the coordination potential. The degree of reorientational disorder increases with raising temperature. In a first approximation an antiferroelectric orientation of the anions was found in the low temperature modification. Two symmetry equivalent hydrogen positions build a linear group with the position of sulphur in the middle temperature modification. The anions reorientate by a two site 180°jump process. The cubic symmetry of the high temperature modification is achieved by nearly free reorientational disorder of the anions. But this is strongly linked to the coordination potential, which can be seen by the probability density function of the hydrogen atoms. Furthermore a diffusion of the cations seems to be likely for this modification. Elastic and quasielastic neutron scattering experiments and NMRmeasurements were done to gain insights into the relations of the crystal structures and the dynamics of anions or cations. The experiments were carried out in a wide temperature range. An analysis of NMR spinlattice relaxation and second moments of the proton signal was done. In addition a line shape analysis of the deuteron signal was executed. To optimise the experiments we used either fully deuterated or fully protonated samples.
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Keywords
Reorientierungsfehlordnung, Anharmonizität, Pulver- und Einkristallneutronenbeugung, Quasielastische Neutronenstreuung, NMR, Alkalimetallhydrogensulfide