Experimental Crystallography from Atomic to Supramolecular Scale

Results of crystal structure analysis for various types of compounds are discussed in terms of a new concept of crystal formation. The effect of ordering “rigid” atomic groups ([Hg2]2+, [H2As · W18O60]7-, etc.) by families of crystallographic planes with interplanar distances comparable to the sizes of the groups is considered. The wide occurrence of symmetrical packings of atoms and/or centers of rigid atomic groups in structures is explained by a tendency toward maximum reduction in the number of degrees of freedom and, hence, a tendency toward minimum energy of the system.