Crystallochemistry

crystalline chemistry (from the Greek κρύσταλλος, 'ice' and χημεία, 'chemistry') is the study of the relationships between the chemical composition of crystalline materials and their structures, as well as their effects on physical properties. It includes the study of chemical bonds, morphology and the formation of crystalline structures, according to the characteristics of the atoms, ions or molecules, as well as their type of bond. In fact it is a discipline that is the link between crystallography, solid state chemistry and condensed matter physics. A neighboring branch of crystallochemistry is the chemistry of complexes.

History

Crystallochemistry developed from mineralogy and later from crystallography. In the 19th century, its development was continued by the invention of the "reflection goniometer" (William Hyde Wollaston, 1809), for the discovery of isomorphism and polymorphism (Eilhard Mitscherlich, 1819), as well as chirality (Louis Pasteur, 1848).

At the beginning of the XX century, the first X-ray diffraction experiments (Walter Friedrich, Paul Knipping and Max von Laue, 1912) were an important step in the systematic analysis of crystal structures. From 1923 to 1926, Victor Goldschmidt, considered one of the founders of crystallochemistry and geochemistry, established his structural principles for simple compounds. The main rule of these principles is the following: “The crystalline structure of a material is caused by the size and polarization properties of its components; these components are atoms (or ions) and groups of atoms.”

Basics of crystallochemistry

Victor Goldschmidt and Fritz Laves laid down the replacement space-filling postulates for a stable crystal structure of minimum lattice energy (atoms or ions are considered geometrically for these postulates as hard balls):

• spatial principle: atoms or ions are stacked as compact as possible;
• symmetry principle: the crystal has the highest possible symmetry;
• interaction principle: each atom or ion is surrounded by as many neighbors as possible.

In addition, the atomic or ionic radius (which can vary depending on the type of bond) influences the structure. For example, in some compounds such as olivine (Mg,Fe)
2[SiO
4 ], the crystal structure can be described as a hexagonal close-packing of oxygen atoms, the other smaller atoms occupying the octahedral sites (magnesium and iron atoms) and tetrahedral sites (silicon atoms).

The nature of chemical bonds in a crystal can be homodesmatic (a predominant type of bond) or heterodesmatic (stable and isolated groups of atoms or complexes, included in a larger structure). Pyrite FeS
2 is an example of a heterodesmatic compound (covalent bond between sulfur atoms, bond ionic between sulfur and iron).

Pauling's rules apply to ionic crystals, in which ionic bonds predominate.

Experimental methods

The most important experimental methods in crystallochemistry are radiocrystallography (X-ray diffraction, neutron diffraction, etc.), as well as analytical chemistry (such as spectroscopy) and physical chemistry (pattern determination). and [[phase transition[phase transitions]]).

Classification of crystal structures

Crystallinochemistry groups mineralogical compounds into structural types, which are divided according to their stoichiometry and the order of their discovery. This classification is based on the Strukturbericht classification developed by Carl Hermann and Paul Peter Ewald. The order of the discoveries is numbered (1, 2,...) and the stoichiometry is denoted by a letter:

• A: elements (e.g., Au);
• B: AB formula (NaCl; CsCl; ZnS, wurtzite and blenda);
• C: Formula compounds AB
  2 (FeS2);
• D: Formula compounds A
  nB
  m (Al2O3);
• E: more than two elements without complex formation (CaTiO3);
• F: with a complex containing 2 or 3 atoms (Nano
  2);
• G: with a complex containing 4 atoms (Na2CO3);
• H: on a complex containing 5 atoms (Na2SO4);
• L: alloys (amalgams);
• M: mixed crystals (NaCl/AgCl);
• S: silicatesAl
  2Yes
  5).

For example, C4 is the structural type of rutile (TiO
2) and E2 than the from ilmenite (FeTiO
3), which can be obtained from the structural type of α-Al2O3, substituting aluminum layers alternately with iron layers and titanium layers.

In the case of minerals, the classifications used are the Strunz classification and the Dana classification.