One of the first important results of X-ray structure analysis after 1912 was the structural-crystallochemical mineral systematics for oxides, sulphides and silicates, which is still valid today. In particular, the discovery of the manifold and complex tetrahedral connections in the mineralogically basic group of silicates led to important insights into rock-forming and technical processes. Phase transformations as a function of temperature and pressure (polymorphism) and extensive atomic substitutions (solid solutions) are common in minerals; the Si/Al order disorder processes in feldspars are of particular petrological importance.
In the geosciences these crystallographic researches contribute to the understanding of the structure of the earth crust and the upper mantle, the moon and the meteorites, as well as the time-dependent reactions occurring there, often in geological periods. Technical Mineralogy uses crystallographic methods and ideas to improve technical materials and processes.
In the field of molecular chemistry, the rapid development of modern X-ray crystal structure analysis has made it the main method of constitution research today, from small molecules to polymers and proteins. It is routinely used in research institutes and industrial laboratories to characterize products and assess synthesis strategies. So far, about 70,000 structures of molecular crystals have been elucidated.
The results of structural analysis have given solid state chemistry a fundamentally new concept: the replacement of the pure molecular concept of the 19th century by the “collective structure” of the solid in which the atoms are connected in three dimensions. Furthermore, non-stochiometry was understood, especially the previously inexplicable “phase width” of intermetallic compounds. Only crystal structure analysis has made a deeper understanding of the “nature of chemical binding” in solids possible, e.g. through the direct imaging of valence electrons.
These developments make it possible to better understand important physical properties of solids and thus reduce gaps in solid-state research between chemistry and physics.
The relationship between physics and crystallography is traditionally particularly close. This is based on the one hand on the physical techniques predominantly used by crystallographers, which range from diffraction and topography to spectroscopy, and on the other hand on the great importance of crystallographic research results for various fields of physics.
In the field of solid state physics, these relationships are illustrated by two examples: Crystallography makes crystals with structures of increasing complexity available for physical experiments and their atomistic interpretation: from diamond and graphite to ferroelectrics and low-dimensional electrical conductors. Typical crystallographic concepts, such as distortion of coordination polyhedra in the crystal field, domain structures (twins) and structural disorder, are directly applied in physical research.
The theory of space groups and their representations has developed into an indispensable tool for the theoretical treatment of phase transformations, the chemical bond (band structure), the Brillouin zones and the thermal oscillations in crystals (phono dispersion, lattice dynamics).