Crystal Chemistry

Abstract

Ionic Radii. Our previous discussion has shown the symmetries which are possible in point arrays and how they are related to crystal form. We are now familiar with the geometric framework into which the collective crystal world can be arranged. We now want to inquire further as to what relationships exist between the kinds of particles — atoms or molecules — and their crystal structural arrangement. Why, for example, do crystals of the three compounds BN, ZnS, and NaCl each possess a unique structure and crystallize in different classes ? Chemists and mineralogists have concerned themselves with these questions for more than 100 years. In the beginning, attempts were made to establish some universal generalities, by collecting data from as many kinds of crystals as possible. This approach culminated in 1919 with the publication of P. von Groth’s “Chemische Kristallographie”. His first volume was published in 1906. This is a book which even today is an important reference work, especially in the field of organic crystallography. In it is compiled all that was known at that time about the crystal forms of the elements and their compounds. It was not possible in this way, however, to organize the enormous volume of data according to some unifying concept. An important principle was revealed around 1920, when the first crystal structures were experimentally determined. Different investigators, especially V. M. Goldschmidt and H. G. Grimm, attempted to explain the spatial arrangement of particles in terms of their volume requirements.