Cr23C6 is the prototypical compound of a common crystal structure, discovered in 1933 [1] as part of the chromium-carbon binary phase diagram. Over 85 known compounds adopt this structure type, [2] which can be described as a NaCl-like packing of chromium cubes and cuboctahedra. [3]
The space group of this structure is called Fm3m (in Hermann–Mauguin notation) or "225" (in the International Tables for Crystallography). It belongs to a cubic crystal system, with Pearson symbol cF116.
The shortest interatomic distances are between carbon and chromium atoms, which is expected on the basis of atomic size. The carbon atoms are in positions that cap each face of the chromium cubes and their coordination environment can be thought of as distorted square antiprisms formed from chromium atoms of both the cubes and the cuboctahedra.
The closest Cr-Cr contacts are between members of a cuboctahedron, and the third closest are between members of a cube. The members of the cube, however, are closer to the 8 chromium atoms in the unit cell that are not part of either polyhedron. The coordination environment of these other atoms can be thought of as distorted Friauf polyhedra composed of chromium atoms, if next-nearest neighbors are included.
Examples of compounds that form in this structure type include Cr23C6, Mn23C6, and many ternary intermetallic carbides and borides. [4] A few phases of ternary silicides, germanides, and phosphides are also known to exist. [5] [6] In going from the binary to ternary systems, some of the transition metal atoms are substituted by a third element, which can be an alkali metal, alkaline earth metal, rare-earth element, main group element, or another transition metal. This leads to an empirical formula of the form A23−xBxC6. Materials of this kind continue to be studied for potentially interesting magnetic and physical properties. [7] [8]
A crystal or crystalline solid is a solid material whose constituents are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macroscopic single crystals are usually identifiable by their geometrical shape, consisting of flat faces with specific, characteristic orientations. The scientific study of crystals and crystal formation is known as crystallography. The process of crystal formation via mechanisms of crystal growth is called crystallization or solidification.
In chemistry, a carbide usually describes a compound composed of carbon and a metal. In metallurgy, carbiding or carburizing is the process for producing carbide coatings on a metal piece.
In crystallography, the cubiccrystal system is a crystal system where the unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals.
In chemistry, a nitride is an inorganic compound of nitrogen. The "nitride" anion, N3- ion, is very elusive but compounds of nitride are numerous, although rarely naturally occurring. Some nitrides have a found applications, such as wear-resistant coatings (e.g., titanium nitride, TiN), hard ceramic materials (e.g., silicon nitride, Si3N4), and semiconductors (e.g., gallium nitride, GaN). The development of GaN-based light emitting diodes was recognized by the 2014 Nobel Prize in Physics. Metal nitrido complexes are also common.
Chromium(II) carbide is a ceramic compound that exists in several chemical compositions: Cr3C2, Cr7C3, and Cr23C6. At standard conditions it exists as a gray solid. It is extremely hard and corrosion resistant. It is also a refractory compound, which means that it retains its strength at high temperatures as well. These properties make it useful as an additive to metal alloys. When chromium carbide crystals are integrated into the surface of a metal it improves the wear resistance and corrosion resistance of the metal, and maintains these properties at elevated temperatures. The hardest and most commonly used composition for this purpose is Cr3C2.
Complex metallic alloys (CMAs) or complex intermetallics (CIMs) are intermetallic compounds characterized by the following structural features:
Nickel aluminide refers to either of two widely used intermetallic compounds, Ni3Al or NiAl, but the term is sometimes used to refer to any nickel–aluminium alloy. These alloys are widely used because of their high strength even at high temperature, low density, corrosion resistance, and ease of production. Ni3Al is of specific interest as a precipitate in nickel-based superalloys, where it is called the γ' (gamma prime) phase. It gives these alloys high strength and creep resistance up to 0.7–0.8 of its melting temperature. Meanwhile, NiAl displays excellent properties such as lower density and higher melting temperature than those of Ni3Al, and good thermal conductivity and oxidation resistance. These properties make it attractive for special high-temperature applications like coatings on blades in gas turbines and jet engines. However, both these alloys have the disadvantage of being quite brittle at room temperature, with Ni3Al remaining brittle at high temperatures as well. To address this problem, has been shown that Ni3Al can be made ductile when manufactured in single-crystal form rather than in polycrystalline form.
A crystallographic database is a database specifically designed to store information about the structure of molecules and crystals. Crystals are solids having, in all three dimensions of space, a regularly repeating arrangement of atoms, ions, or molecules. They are characterized by symmetry, morphology, and directionally dependent physical properties. A crystal structure describes the arrangement of atoms, ions, or molecules in a crystal.
Native element minerals are those elements that occur in nature in uncombined form with a distinct mineral structure. The elemental class includes metals, intermetallic compounds, alloys, metalloids, and nonmetals. The Nickel–Strunz classification system also includes the naturally occurring phosphides, silicides, nitrides, carbides, and arsenides.
Binary compounds of silicon are binary chemical compounds containing silicon and one other chemical element. Technically the term silicide is reserved for any compounds containing silicon bonded to a more electropositive element. Binary silicon compounds can be grouped into several classes. Saltlike silicides are formed with the electropositive s-block metals. Covalent silicides and silicon compounds occur with hydrogen and the elements in groups 10 to 17.
Chromium hydrides are compounds of chromium and hydrogen, and possibly other elements. Intermetallic compounds with not-quite-stoichometric quantities of hydrogen exist, as well as highly reactive molecules. When present at low concentrations, hydrogen and certain other elements alloyed with chromium act as softening agents that enables the movement of dislocations that otherwise not occur in the crystal lattices of chromium atoms.
Chromium(II) selenide is an inorganic compound with the chemical formula CrSe. It crystalizes in a hexagonal structure with space group P63/mmc. It is one of many related Chromium-Selenium phases, including Cr7Se8, Cr3Se4, Cr0.68Se, Cr2Se3, and Cr5Se8. The compound has been described as an antiferromagnet, but its inverse magnetic susceptibility does not match the behavior expected for an antiferromagnet according to the Curie–Weiss law. One suggestion was that the Néel temperature is at 320 K, as the temperature where the compound has maximum specific heat. When synthesized as single atomic layer, CrSe is ferromagnetic, with a Curie Temperature of around 280 K.
Chromium(III) boride, also known as chromium monoboride (CrB), is an inorganic compound with the chemical formula CrB. It is one of the six stable binary borides of chromium, which also include Cr2B, Cr5B3, Cr3B4, CrB2, and CrB4. Like many other transition metal borides, it is extremely hard (21-23 GPa), has high strength (690 MPa bending strength), conducts heat and electricity as well as many metallic alloys, and has a high melting point (~2100 °C). Unlike pure chromium, CrB is known to be a paramagnetic, with a magnetic susceptibility that is only weakly dependent on temperature. Due to these properties, among others, CrB has been considered as a candidate material for wear resistant coatings and high-temperature diffusion barriers.
Topologically close pack (TCP) phases, also known as Frank-Kasper (FK) phases, are one of the largest groups of intermetallic compounds, known for their complex crystallographic structure and physical properties. Owing to their combination of periodic and aperiodic structure, some TCP phases belong to the class of quasicrystals. Applications of TCP phases as high-temperature structural and superconducting materials have been highlighted; however, they have not yet been sufficiently investigated for details of their physical properties. Also, their complex and often non-stoichiometric structure makes them good subjects for theoretical calculations.
A holmium–magnesium–zinc (Ho–Mg–Zn) quasicrystal is a quasicrystal made of an alloy of the three metals holmium, magnesium and zinc that has the shape of a regular dodecahedron, a Platonic solid with 12 five-sided faces. Unlike the similar pyritohedron shape of some cubic-system crystals such as pyrite, this quasicrystal has faces that are true regular pentagons.
In crystallography, a Strukturbericht designation or Strukturbericht type is a system of detailed crystal structure classification by analogy to another known structure. The designations were intended to be comprehensive but are mainly used as supplement to space group crystal structures designations, especially historically. Each Strukturbericht designation is described by a single space group, but the designation includes additional information about the positions of the individual atoms, rather than just the symmetry of the crystal structure. While Strukturbericht symbols exist for many of the earliest observed and most common crystal structures, the system is not comprehensive, and is no longer being updated. Modern databases such as Inorganic Crystal Structure Database index thousands of structure types directly by the prototype compound. These are essentially equivalent to the old Stukturbericht designations.
Chromium telluride (Cr2Te3) is an inorganic chemical compound. It is composed of the chromium(III) cation and the telluride anion. It has a shadowy gray color, and has a hexagonal crystal structure.
Carbohydrides are solid compounds in one phase composed of a metal with carbon and hydrogen in the form of carbide and hydride ions. The term carbohydride can also refer to a hydrocarbon.
Phosphide carbides or carbide phosphides are compounds containing anions composed of carbide (C4−) and phosphide (P3−). They can be considered as mixed anion compounds. Related compounds include the phosphide silicides, germanide phosphides, arsenide carbides, nitride carbides and silicide carbides.
Silicide carbides or carbide silicides are compounds containing anions composed of silicide (Si4−) and carbide (C4−) or clusters therof. They can be considered as mixed anion compounds or intermetallic compounds, as silicon could be considered as a semimetal.