Crystal chemistry
The crystal chemistry ( κρύσταλλος ( krystallos ) = ice; χημεία ( chemeia ) = chemistry) is a sub-discipline of crystallography and deals with the relationship between the chemical composition of crystalline materials and their structural composition, as well as the resulting physical properties. It is thus the link between the fields of crystallography and chemistry. A related area of expertise is structural chemistry, which is a branch of physical chemistry, and solid-state chemistry ( branch of chemistry ).
" [ The aim of the crystal chemistry is to ] determine ... lawful relationships between the chemical composition and physical properties of crystalline materials. In particular, it is to find an object of crystal chemistry in the narrow sense, the way in which the crystal structure of the chemical composition depends. "
History
The crystal chemistry has evolved from mineralogy ( around 300 BC Theophrastus: "On Stones" ): developed and later crystallography ( constant angles of mountain crystals Nicolaus Steno in 1669 ). In the 19th century the development of the invention of the Reflexionsgoniometers ( William Hyde Wollaston in 1809 ), the discovery of isomorphism and polymorphism by Eilhard Mitscherlich (1819 ) and the enantiomorphism by Louis Pasteur ( 1840) was continued.
Beginning of the 20th century was followed by the first X-ray diffraction experiments on crystals (Walter Friedrich, Paul Knipping and Max von Laue 1912) an important step towards systematic crystal structure analysis. 1923-1926 presented Goldschmidt, who is regarded as the founders of the crystal chemistry, its structural principles for simple compounds. The main rule stated this structure principles as in Geochemical distribution laws of the elements, VII, page 9, states: "The crystal structure of a substance due to size and polarization properties of its components; as components are atoms ( ions, respectively ) and atomic groups to call. "
Fundamentals of Crystal Chemistry
Goldschmidt and Fritz Laves introduced the space-filling postulates for the development of stable crystal structures with the smallest possible lattice energy ( the atoms / ions are purely geometrically considered as rigid spheres in these postulates ):
- Space principle: the atoms / ions are packed as tightly as possible
- Symmetry Principle: The crystal has the highest possible symmetry
- Interaction principle: Every atom / ion to surround himself with as many neighbors
In addition, the atomic or ionic radius ( depending on the type of bond may be different) plays a role. Thus, for example, some chemical compounds (such as the mineral olivine (Mg, Fe ) 2 [ SiO4 ] ) are explained in the crystal structure construction in that one type of atom forms a closest packing of spheres and the other (smaller) types of atoms occupy the remaining gaps.
The nature of the chemical bond in a crystal can ( a weave predominantly ) or heterodesmisch (stable, isolated groups of atoms or complexes which are in turn embedded in a larger unit ) homodesmisch. Pyrite FeS2 is an example of a compound heterodesmische (covalently between the sulfur atoms Ionar between sulfur and iron).
Contends For crystals with predominantly ionic bonding ( ionic crystals ) the Pauling linking rules.
Methods of investigation
The most important methods of investigation of the crystal chemistry, the structural analysis methods based on eg X-ray or neutron diffraction, as well as methods of analytical chemistry (especially the instrumental analysis, to include the spectroscopy part ) and physical chemistry (determination of phase diagrams and phase transformations ).
System
The crystal chemistry divides crystalline compounds in structure types, which are classified according to the nature of the stoichiometric compound and the order of their discovery. This classification goes back to the developed by Hermann and Ewald structure report. The order of the discovery is numbered ( 1,2, ... ), the stoichiometry of binding and is characterized by a letter:
- A: elements (eg Au)
- B: AB- compounds (e.g., NaCl)
- C: AB2 compounds (eg FeS2 )
- D: AnBm compounds (such as Al2O3)
- E: > 2 pronounced elements without complex formation (eg CaTiO3 )
- Q: with two - or three-atom complexes (eg NaNO2 )
- G: with four-atom complexes (eg Na2CO3)
- H: with five-atom complexes (eg Na2SO4)
- L: alloys (eg amalgams )
- M: Mixed crystals (for example, NaCl / AgCl)
- S: silicates (for example, Al2SiO5 )
"C4", for example, is the " rutile " (TiO2), "E2" is the " ilmenite - type" ( FeTiO 3 ), which is derived from the α -Al2O3 structure by alternately replacing the Al layers by Fe and Ti can be.