In nature the formation of minerals generally takes place at non-ambient conditions, e.g at elevated temperatures T and pressures P. Many materials, however, exhibit a series of phase transitions as a function of T and P in as much that the structure, determined at normal conditions is different to the one of materials formation and – for technical products – materials operation; As a function of T, P or chemical composition in solid solutions, the atomic structures may change sometimes drastically, depending on the nature of the transition. The knowledge of these phase transitions is of fundamental importance both for the Earth Sciences as well as for Materials Science and technical application.
One focus of the crystallographic work in our research group is to precisely characterize the thermal behavior of minerals at T between 5 and approximately 1200 K by means of X-ray and neutron diffraction including the study of the nature of possible phase transitions. Here we make use of laboratory as well as large scale facility diffraction. The materials under investigation widely vary, but most of them have one thing in common: they are derived from natural mineral structures. One main focus of our crystal chemical work is to understand the connection between crystal structure, variation with temperature and chemical composition and their influence on physical properties, such as thermal expansion tensor, magnetic properties, ionic mobility and (electric) conductivity.
The issue behind these investigations is: “From minerals to materials”.
In general our research topics include:
- (low dimensional) magnetism in minerals and synthetic materials, derived from mineral structures, mainly in chain silicates and germanate compounds,
- Investigation of temperature induced structural phase transitions,
- Crystal chemistry of pyroxene-type compounds as a function of X-P-T,
- Structure and temperature variation of Li- and Na-bearing materials, e.g. of NASICON-type structures,
- Structural investigations in Li-garnets
- Crystal chemistry of Sheet silicates
- General interest in the crystal chemistry of germanate compounds
- Crystal chemistry and amgetism in transition metal bearing phosphates
- Micro and mesoporous structure with alkali-metal cations
- Neutron diffraction.
- Structure determination of metal-organic compounds.
One of the main focuses in recent research activities is the investigation of structure – property relations in the mineral group of pyroxenes (more than 40 papers in peer review journals). However we do not investigate natural mineral species but characterize synthetic material, obtained in our laboratories with controlled chemical substitutions, both as polycrystalline powders and single crystals.
As a function of temperature and composition, these materials pass through up to three structural phase transitions, which are detected and investigated by means of in situ X-ray and neutron diffraction. By means of controlled variations in chemical composition changes in the atomic structure can be induced in the pyroxene also at ambient conditions. Besides silicates our research on crystal chemistry and phase transformations especially focuses on germanate – compounds, but also include phosphates, borates, sulphates and arsenates. In recent days we focus intensively on the structure – property relations in fast Li-conduction compounds with garnet structure and NASICON-type structres.
The pyroxenes, and many other transition metal bearing minerals show interesting magnetic properties at very low temperatures. Another – steadily growing – aspect in our research is concerned with the magnetic phase transitions in minerals / materials. The magnetci structures thereby can be determined by means neutron diffraction. Again, the focus of the research is currently at the material group of pyroxene, where we could – for example – prove a complex incommensurate modulated magnetic chiral structure in NaFeGe2O6.
- Synthetic crystals of NaFeGe2O6, the Ge4+ – analogue to the mineral specimen aegirine, and the complex chiral magnetic structure of the Ge-compound at low temperatures (Redhammer et al. 2011)