Unconventional Metal-Framework Interaction in MgSi5
Cage compounds of silicon are promising materials in the fields of superconductivity and thermoelectricity. The extended networks serve as hosts, which hold cationic guest species in large cavities. A plethora of structural patterns has already been realized with a number of cations of the alkali or alkaline-earth group.
An outstanding exception is the light metal magnesium, which tends to avoid the guest position in such motifs of binary or ternary phases. Earlier experiments have shown that the application of high-pressure can play a beneficial role in the stabilization of new silicon framework patterns.
Scientists of the Max Planck Institute for Chemical Physics of Solids could now overcome the previous limits and facilitate the formation of the compounds MgSi5 by realizing chemical synthesis at extreme conditions.
The selected pressure-temperature combination corresponds to conditions, which are normally found in the earth mantle, i.e., reactions at pressures between 50,000 and 100,000 atmospheres combined with temperatures around 1000 °C.
Traces of the new compound were first detected by scanning electron microscopy in form of small domains. Samples for special electron diffraction experiments were manually isolated by focused-ion techniques so that the crystal structure could be identified. After this characterization of the target phase, synthesis conditions were optimized.
The improved synthesis finally yielded specimen, which included crystalline particles of sufficient size for elaborate single-crystal X-ray diffraction experiments. At last, crystals could be isolated by a leaching process. The results of the structure refinement evidence that MgSi5 realizes a new structure type.
Analysis of the chemical bonding in position space indicates magnesium species adopting almost spherical symmetry, a finding, which evidences a mostly cationic character of the magnesium atoms. Correspondingly, the distribution of a special bond indicator (ELI-D) shows the typical picture of four-bonded silicon atoms with maxima situated on or close to the bond lines between silicon atoms.
However, some features are different from those of typical cage compounds. Additional local maxima are observed between magnesium and silicon atoms revealing multi-center Mg-Si interactions. In addition, the results point towards polar - or lone-pair-like - interactions of silicon atoms surrounding smaller empty cavities. Thus, the findings clearly show that the new compound exhibits previously unidentified bonding properties which have the potential to substantially modify our understanding of chemical bonding in host-guest assemblies.