Abstract
Pressure is well known to dramatically alter physical properties and chemical behaviour of materials, much of which is due to the changes in chemical bonding that accompany compression. Though it is relatively easy to comprehend this correlation in the discontinuous compression regime, where phase transformations take place, understanding of the more subtle continuous compression effects is a far greater challenge, requiring insight into the finest details of electron density redistribution. In this study, a detailed examination of quantitative electron density redistribution in the mineral langbeinite was conducted at high pressure. Langbeinite is a potassium magnesium sulfate mineral with the chemical formula [K2Mg2(SO4)3], and crystallizes in the isometric tetartoidal (cubic) system. The mineral is an ore of potassium, occurs in marine evaporite deposits in association with carnallite, halite and sylvite, and gives its name to the langbeinites, a family of substances with the same cubic structure, a tetrahedral anion, and large and small cations. Single-crystal X-ray diffraction data for langbeinite have been collected at ambient pressure and at 1 GPa using a combination of in-house and synchrotron techniques. Experiments were complemented by theoretical calculations within the pressure range up to 40 GPa. On the basis of changes in structural and thermal parameters, all ions in the langbeinite structure can be grouped into 'soft' (potassium cations and oxygens) and 'hard' (sulfur and magnesium). This analysis emphasizes the importance of atomic basins as a convenient tool to analyse the redistribution of electron density under external stimuli such as pressure or temperature. Gradual reduction of completeness of experimental data accompanying compression did not significantly reduce the quality of structural, electronic and thermal parameters obtained in experimental quantitative charge density analysis.
Highlights
Establishing a detailed picture of electron density evolution as a function of depth in rock-forming mineral phases is absolutely crucial for the development of quantitative models, allowing us to predict chemical and physical transformations involved in major geological processes taking place in the deep interiors of Earth and other extraterrestrial planets, as well as in ore formation
In AIM theory, the many-electron system is separated into subsystems by zero-flux surfaces (ZFSs) that satisfy the following condition for every point on the surface: nr(r) = 0, where r(r) is the gradient vector field of the molecular electron density, r is a point on the zero-flux surface that separates two fragments and n is the vector normal to the surface at that point
We examined whether the wavelength of X-ray radiation used for data collection influences the final distributions of electron density, and compared the results of charge density studies obtained at two different wavelengths (Ag K and Mo K ) at ambient pressure
Summary
Establishing a detailed picture of electron density evolution as a function of depth in rock-forming mineral phases is absolutely crucial for the development of quantitative models, allowing us to predict chemical and physical transformations involved in major geological processes taking place in the deep interiors of Earth and other extraterrestrial planets, as well as in ore formation All these processes can be traced at subatomic levels of detail, achievable by combining experimental charge density studies and crystal structure investigations under extreme conditions. Some examples of experimental electron density determined for crystals under high pressure have already been published. The most common aspherical quantitative experimental charge density model is based on a finite spherical harmonic expansion of the electronic part of the charge distribution around each atomic centre. For more information on multipole refinement and topological analysis of electron density see the supporting information
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