Research Projects

Running Research Projects

Cation and anion ordering and clustering in the system Lithian muscovite, Trilithionite and Polylithionite

The main interest of the project focuses on the spectroscopic investigation of the order / disorder of Si and Al in the tetrahedral sheets and of Al, Li, F, and OH in the octahedral sheets of polylithionite, trilithionite and Lithian muscovite. The emphasized analysis method will be Solid State MAS NMR spectroscopy, since it enables the characterization and quantification of the different local environments of 29Si, 27Al, 1H, 19F, 6Li and 7Li and since experiments and data evaluation can be done in a reasonable amount of time. The supposed preference of fluorine to Li cations as well as the preference of hydroxyl to Al represents a further interesting aspect with respect to the cation layers. NMR spectroscopic hetero and homonuclear correlation experiments can supply a vital contribution here. Significant questions arising are the transition from trioctahedral polylithionite-trilithionite to dioctahedral muscovite, alterations in the tetrahedral and octahedral sheets and the correlation between both, the change between the occupancy of the octahedral sites by cations Li and Al as well as the mutual replacement of anions OH and F and of the tetrahedral atoms Al and Si. Complementary X-ray diffraction studies refer to variations in lattice metrics and long-range order. Structure analyses based on X-ray powder diffraction data can show up the influence on the atomic environment along the substitutions in the polylithionite-trilithionite-(Lithian) muscovite binary line and the crystallization of the different polytypes based on the different cationic radii of Al3+ and Li+ and will be applied on selected samples. Moreover, X-ray diffraction will clarify any miscibility gaps along the binary lines. In addition, stacking faults and disorder of the whole crystal structure can be investigated.
The investigation of short-range order phenomena is another prominent goal of the project. It shall be clarified how fluorine / hydroxyl is included into the octahedral sheets; whether it occurs in the form of clusters or if a statistical distribution is present. The local distortion of the aluminium and lithium atomic environment can be easily correlated by analysing the electric field gradient parameters (EFG) from the electron shell extracted from the 27Al and 7Li MAS NMR spectra and evaluation of the lattice parameters from XRD. In addition, it is particularly interesting, whether there is any correlation of ordering between Si /Al in the tetrahedral sheets and Li / Al and OH / F in the octahedral sheets. The investigation of OH/F distribution between dioctahedral and trioctahedral sites and its influence on the c-lattice parameter is another question to solve. As an additional effect, the results derived from the different investigations will give an insight into phase and field stabilities. So far, very few studies exist on the stability of micas located at the join muscovite – trilithionite - polylithionite.

Financial Support by: DFG
Running since: February 2019

Finished Research Projects

Distribution of F/OH anions and Mg/Al cations in the octahedral sheets of phlogopite: A EuroMinScI project in the EUROCORES programme of the ESF

Cooperation with: Prof. Alberto Garcia Arribas, Barcelona, Prof. Vaclav Petricek, Prag, Prof. Tonci Balic-Zunic, Kopenhagen and
Prof. Volker Kahlenberg, Innsbruck

The EUROCORES Programme EuroMinScI draws together the experimental and computational activities, and the different experimental techniques, into integrated research projects. It addresses the need for young researchers with an academic background in earth sciences to be trained more in the physics-based techniques, where the methods are very different from traditional earth sciences. The current project aims at the elucidation of cationic or anionic order in minerals and the relation of the degree of ordering to the formation conditions and thermodynamic history. The main computational aid for the realisation of these studies will be ab-initio calculations within the framework of density functional theory.

The mica phlogopite is one of the preferred phases which enclathrate fluorine on partitioning of magmas. A deeper understanding of the fluorine local structure in the melt and the layer silicates can help to understand the mechanism of the element redistribution and the corresponding chemical and physical properties. Our main interest focuses on the order/disorder of the Al/Mg cations and F/OH anions in the octahedral sheets. Solid State NMR investigations already showed that hydroxyl-rich compositions indicate an increased Al-content in the tetrahedral sheets suggesting a stabilizing effect on the formation of Al-rich phlogopite.

Financial support by: DFG and ESF

Structural Bonding and Partitioning of Fluorine in Layer Silicates (micas) and Granitic Melts

Cooperation with: Prof. Dr. C.A. Fyfe and Prof. L.A. Groat, University of British Columbia,Vancouver, Canada
Prof. Dr F. Holtz and Dr. H. Behrens, Hannover

In general fluorine is a minority component in granitic rocks. Because of partitioning processes it can be strongly enriched in the melt as an incompatible moderate element. By means of fractional processes it can be enriched in silicate melts as a moderate incompatible element. Even small amounts of fluorine affect the chemical and physical properties of the melts as well as the phase relationships of partially molten granites. Besides the melt layer silicates like biotite, phlogopite and muscovite are the favourite phases, which intercalate fluorine on differentiation of magmas.
The partitioning of fluorine between melt and micas was studied less and selective up to now. A deeper understanding of the fluorine local structure in the melt and the layer silicates can help to get a better understanding of the mechanism of the element redistribution as well as the consequences of fluorine enclathration on the chemical and physical properties or to predict these.
An ideal possibility for local structure determination is presented by 19F MAS NMR: Strong signal intensities (high nat. Abundance, high gyro magnetic ratio) and a large chemical shift area could make obvious small changes in the local environment of the fluorine nucleus. On account of the spin oneself nucleus the signal areas enable an exact quantification of local structure elements. A distinction between glassy and crystalline parts makes no difficulties on account of the different line shapes. Thus reaction products from partitioning experiments could be analysed directly and the fluorine enclathration in crystalline and glassy phase can be correlated.

Financial support by: Feodor-Lynen Research Fellowship of the Alexander von Humboldt-Stiftung

An NMR-Spectroscopic study of the phase transition of letovicite, (NH4)3H(SO4)2: The behaviour of the order parameter on an atomic length scale

Cooperation with Prof. Dr. U. Bismayer, Hamburg

Letovicite (NH4)3H(SO4)2 is a compound of theM3H(XO4)2 type, with M = NH4, Rb, Cs und X = S, Se which forms different ferroic compounds. Crystals of this species own technical relevant superconducting properties due to their high protonic mobility.

Letovicite shows a formation of non-reorientable ferroelastic domains just 30 K below the critic temperature and therefore is named as a hard ferroelastic. The thermal instability can be described by terms of higher order in the Landau potential of the Gibbs-free enthalpy and can be characterized as Slater-instability. The behaviour of the order parameter in this system is defined by order/disorder effects of the free protons as well as by the displacive behaviour of the ammonium groups.

A distinction of the different influence of free protons and ammonium groups on the local order parameter should be possible by variable temperature 1H T1-Spin-Lattice-relaxation and static 1H NMR-spectroscopic studies. The estimated correlation times and activation energies can be used for the simulation of static 2H NMR spectra. Subsequently, the data can be compared with experimental 2H NMR spectra of full deuterated (ND4)3D(SO4)2. The 2H NMR lineshapes enable conclusions of the manner of motion forms in the intermediate dynamic range. Two-dimensional 1H-1H homonuclear correlation experiments (NOESY) should give a contribution to a better understanding of free proton motion below and above the critic temperature.

Financial support by: DFG

Synthesis and Characterization of template free Cancrinites: A nano-structured Host-Guest Matrix with one-dimensional Channel System

On account of their structure channels nanoporous materials of the cancrinite type present an ideal Host-Guest matrix of aligned enclathration of molecules in case of template free voids. Since in conventional hydrothermal synthesis the channels are blocked by inorganic anions, the latter could not be reached. Subsequent removal by thermal treatment failed due to the decomposition of the framework
It is the aim of our work to find new ways of cancrinite synthesis by application of organic solvents (alcohols, diols) and to optimize the followed calcination process to receive a complete template free framework with one-dimensional porous system Besides synthesis of polycrystalline material the preparation of single crystals is of immediate importance.
Cooperation with: Prof. Dr. J.-C. Buhl, Hannover

Application of analytic methods (X-ray powder and single crystal structure analysis, IR/Raman-spectroscopy, Solid State NMR Spectroscopy, Scanning- and Transmissions electron microscopy, simultaneous thermo analysis) reveals an adequate characterization of the products.

Financial support by: DFG

High Resolution Multinuclear Solid State NMR studies of carbonate and nitrate cancrinites

Supervisor: Prof. Dr. J-C. Buhl, Hannover

In behalf on inclusion possibilities of different guest compounds in channel and cages of the aluminosilicate framework, cancrinites are of particular interest for the zeolite chemistry. To understand the fundamental inclusion and dynamic processes of cations and guest-anions inside the cancrinite structure we carry out structural investigations to get more information about the hydrothermal inclusion and dynamic of various guest compounds.

In the centre of the research project stands the application of MAS NMR spectroscopy as a local probe for characterization of cancrinite compounds with guest species CO32- and NO3-. Besides the model character of the planned investigations for other representatives of the zeolite group, new results of the formation kinectics and specific Host-Guest interactions are expected to form a base of specific alumosilicat framework compounds with modified crystal properties.

Financial support by: DFG

Substitution and Local Symmetry of Cations in Lead Phosphate

Cooperation with Prof. Dr. U. Bismayer, Hamburg

Compounds of the lead-phosphate type are a ferroic model compound, which potential applications lie in the area of optic and acoustic compounds. Lead phosphate as well as isotpyic arsenates and vanadates show a ferroelastic phase transition from a monoclinic to a trigonal phase in each case. Pb can be replaced by cations without lone pair and oxidation grade +2. This leads to normalization of the ferroelastic transformation.

It is shown, from 207Pb static and MAS NMR spectra, that the chemical shift patterns provide valuable information about the local symmetry of Pb atoms in Pb3(PO4)2 and Ba diluted samples. The application of both, MAS and static NMR experiments is an ideal combination to obtain the maximum amount of information. MAS experiments yield the number and kind of fit functions and good estimates of the spectral parameters. The static experiment provides the correct chemical shift parameters and signal areas. The results for pure lead phosphate are correlated with the structural data obtained earlier by X-ray diffraction.

The spectra of the doped material provide accurate information on the mechanism by which Ba substitutes in the structure and the consequences for the structure. For the doped lead phosphate with 8% Ba2+ an inhomogenous Ba cation distribution is found. The larger effect of doping on the Pb(1) site, together with „spin-counting" experiments indicate clearly and unambiguously that the Pb(2) sites are preferentially occupied by Ba.

Financial support by: DFG

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