18 | Relationships between Structures#

Word count: 0 words
Reading time: 0 minutes

Note

This section is under construction.

Antistructure#: A structure type where the positions (and there stoichiometry) of the cations and anions are swapped relative to the parent structure type. For example fluorite with the formula CaF₂ is an antistructure of the antifluorite structure type, as exemplified by Na₂O.
Vacancy ordering#: A crystal structure where all of the atoms at one position in the unit cell are systematically removed in relation to a parent structure. For example, Cs₂SnI₆ is related to perovskite by a vacancy ordering of half of the Sn atoms.
Double structure#: A crystal structure with where the unit cell, and therefore fomula unit are doubled in relation to a parent structure. Double structures are often related to parent structures by a slight structure distortion or small change in the space group symmetry. For example, many of the lanthanide elements have a double hexagonal close packed structure (dhcp) that is very similar to hcp but with a doubled the number of atoms in the unit cell.
Heterovalent substitution#: A means of doubling the formula unit and unit cell of a structure by replacing one element of a given valence with two different elements of different valences that average to the original valence. For example, elpasolite (K₂Na^IAl^IIIF₆) is a double structure by heterovalent substitution of the perovskite structure type with the generic formula ABX₃ where A is a monovalent cation, B is a divalent cation, and X is a monovalent anion.
Polymorphism#: A chemical phenomenon where a particular stoichiometry of elements in a compound can adopt multiple crystal structures. For example, sphalerite and wurtzite are two polymorphs of ZnS.
Allotropy#: Analogous to polymorphism, allotropy is exclusively used to describe polymorphism in elemental structure. Diamond and graphite are allotropes of carbon.
Dimensional Reduction#: A conceptual chemical reaction where a salt (AX) is reacted with a parent compound (e.g. ABX₃) to from a new crystal structure where the bond network dimensionality is reduced. For example 2D slices of the parent 3D perovskite structure (ABX₃) can be formed by the reaction \(AX + ABX_3 \rightarrow A_2BX_4\) where the 2D layers in A₂BX₄ are isostructural to a 2D slice of the parent perovskite structure.
Intergrowth#: The crystallization of one crystal structure on the face of another crystal structure.
Recurrent Intergrowth#: A special case of intergrowth where two sublattices (often layered) are intergrown to form a new single crystal structure with alternating layers of the two parent structures. The Aurivillius family of layered perovskites are an example of recurrent intergrowth.
Twinning#: A case of intergrowth where two identical crystals (in composition and structure) are intergrown along a common plane. Twinned crystals are ofter related by a symmetry element at the twin boundary such as a mirror plane, inversion center, or rotation axis. Snowflakes are a common example of twinned H₂ crystals.
Zeolite#: A class of many aluminosilicate structure types that are characterized by their porous networks of vertex sharing Si/Al tetrahedra. Zeolites are often used as catalysts and adsorbents due to their high surface area and variable pore sizes.
Doping#: The intentional addition of impurities to a crystal structure to change its properties. Doping is often used to change the electrical, optical, or mechanical properties of a material. For example, the addition of In to SnO can change the material from a transparent insulator to a transparent conductor.
Solid Solution#: A solid solution describes a random distribution of two or more elements or compounds within a single crystal structure with well defined lattice parameters. Solid solutions are often formed in non-stoichiometric compounds. Many commercial “golds” are sold as a solid solutions of gold, copper, and silver (e.g. 18-22 carat golds). 24 carat gold is sold as pure gold.
Alloying#: The mixing of two or more elements or compounds to form a solid solution with a persistent crystal structure type largely independent of the ratio component compounds. Alloying can be used to change the mechanical, electrical, or optical properties of a material. For example the soft metals copper and tin in about a 8:1 ratio form the hard metal bronze. Both elements are distributed randomly with the fcc crystal structure type (same as copper).
Vegard’s Law#: An empirical observations that often the lattice parameters of a solid solution change linearly with the composition of the solid solution. For example, the lattice parameters of the solid solution system made between KBr and KI form a line between the lattice parameters of the two end members (KBr and KI).