Determination of the crystal distortion caused by stereochemically active lone electron pairs.

点击翻译按钮获取中文摘要

Structural distortion away from ideal polyhedra and polyhedral arrangements can have multiple causes and always influences the crystal-chemical and crystal-physical properties of a solid. For the understanding of structure-property relationships, which are crucial for the development of any application, the origin of these distortions and their precise measurement is of pivotal importance.One such factors is a stereochemical active lone pair (LEP), whose strength can be theoretically described by the Liebau density vector. Experimentally however, up to now, we can only access the atomic distortion of a polyhedral coordination using the Wang-Liebau eccentricity (WLE) parameter. This measurement is intrinsically flawed in determining the influence of the LEP, because it does not allow to separate out this influence from all other possible influences on distortion (for example Jahn-Teller-distortion, Quadratic elongation, or Halasyamani distortion). However, nature may offer a perfect tool of exploration of this question because the lanthanide contraction results in the fact that elements of the 5th and 6th period have almost the same size, and hence should – geometrically at least – lead to isostructural materials. However, elements of period 5 display large LEPs, whereas those of period 6 are smaller due to the relativistic effect attraction. Thus, if one chooses a pair of materials, in which one contains a period 5 element and the other a period 6 element at the same position, one can make a direct assessment of the contribution of the LEP to the WLE. This would give only two distinct values, but if one views this pair of materials as the extremes of a solid solution in which these two elements can be mixed in any ratio, one may obtain a finely resolved tool for this assessment, which is of fundamental value for solid state chemistry and crystallography.We believe that mullite-type compound pairs of the composition PbMBO4 / SnMBO4 (M = Al, Ga, Cr, Mn, and Fe) and the solid solutions that connect them will provide the ideal platform to study and separate the influence of the lone pair.We therefore propose to synthesize these materials (some of which have been described by us and others), and to assess their crystal structures in terms of their physical properties (thermal expansion, thermal phase stability, bandgap behavior or magnetic properties) This will give the material and temperature dependent WLE in hand with the influence on the physical properties. In addition, DFT methods will be used to calculate the LDV, providing information on the electron density of the LEP and its direction. Having accumulated these data, we will synergize these findings and be able to generate a new conceptional model of the real influence of the LEP on distortion.