Chromium hydride

Last updated

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.

Contents

The hydrogen in typical chromium hydride alloys may contribute only a few hundred parts per million in weight at ambient temperatures. Varying the amount of hydrogen and other alloying elements, and their form in the chromium hydride either as solute elements, or as precipitated phases, expedites the movement of dislocations in chromium, and thus controls qualities such as the hardness, ductility, and tensile strength of the resulting chromium hydride.

Material properties

Even in the narrow range of concentrations that make up chromium hydride, mixtures of hydrogen and chromium can form a number of different structures, with very different properties. Understanding such properties is essential to making quality chromium hydride. At room temperature, the most stable form of pure chromium is the body-centered cubic (BCC) structure α-chromium. It is a fairly hard metal that can dissolve only a small concentration of hydrogen.

It can occur as a dull brown or dark grey solid in two different crystalline forms: face-centered cubic with formula CrH~2 or a close packed hexagonal solid with formula CrH~1. Chromium hydride is important in chrome plating, being an intermediate in the formation of the chromium plate.

Unit cell of hexagonal close packed form of chromium hydride Hexagonal Chromium Hydride.jpg
Unit cell of hexagonal close packed form of chromium hydride
Unit cell of face-centered cubic CrH2 Fcc-CrH2.jpg
Unit cell of face-centered cubic CrH2

An apparent unusual allotrope of chromium in a hexagonal crystal form was investigated by Ollard and Bradley by X-ray crystallography; however they failed to notice that it contained hydrogen. [1] The hexagonal close packed crystalline substance they discovered actually contains CrHx with x between 0.5 and 1. [2] The lattice for the hexagonal form had unit cell dimensions a=0.271 nm and c=0.441 nm. [3] The crystal form has been described as anti-NiAs structure and is known as the β-phase. [4] Also known as ε-CrH, the space group is Fm3m with hydrogen only in octahedral sites. [5]

A face-centered cubic (fcc) phase of chromium hydride can also be produced when chromium is electrodeposited. Cloyd A. Snavely used chromate in sugar syrup cooled to about 5 °C and with a current density of 1290 amperes per square meter. The unit cell dimension in the material was 0.386 nm. The material is brittle and easily decomposed by heat. The composition is CrHx, with x between 1 and 2. [2] For current density above 1800 amps per square meter and at low temperatures, the hexagonal close-packed form was made, but if the current was lower or the temperature was higher, then regular body-centered cubic chromium metal was deposited. [6] The condition for preferring the formation of face-centered cubic chromium hydride is a high pH. [3] The fcc form of CrH has hydrogen atoms in octahedral sites in the P63/mmc spacegroup. [5]

Chart of effect of temperature and current density on chromium plate brightness. Brown and dark grey are the conditions for chromium hydride formation; pink is the area of no plating. Chromium electroplate result chart.png
Chart of effect of temperature and current density on chromium plate brightness. Brown and dark grey are the conditions for chromium hydride formation; pink is the area of no plating.

Face-centered cubic CrH had the composition CrH1.7. [3] But in theory it would be CrH2 if the substance was pure and all the tetrahedral sites were occupied by hydrogen atoms. The solid substance CrH2 appears as a dull grey or brown colour. Its surface is easily scratched, but that is due to the brittleness of the hydride. [3]

Face-centered cubic chromium hydride also forms temporarily when chromium metal is etched with hydrochloric acid. [7]

The hexagonal form spontaneously changes to normal chromium in 40 days, whereas the other form (face-centered cubic) changes to the body-centered cubic form of chromium in 230 days at room temperature. Ollard already noticed that hydrogen is evolved during this transformation, but was not sure that the hydrogen was an essential component of the substance, as electrodeposited chromium usually contained hydrogen. Colin G Fink observed that if the hexagonal form was heated in a flame that the hydrogen would quickly burn off. [6]

Electroplating chromium metal from a chromate solution involves the formation of chromium hydride. If the temperature is high enough the chromium hydride rapidly decomposes as it forms, yielding microcrystalline body-centered cubic chromium. Therefore, to ensure that the hydride decomposes sufficiently rapidly and smoothly, chromium must be plated at a suitably high temperature (roughly 60C to 75C, depending on conditions). As the hydride decomposes, the plated surface cracks. The cracking can be controlled and there may be up to 40 cracks per millimeter. Substances on the plating surface, mostly chromium sesquioxide, are sucked into the cracks as they form. The cracks heal over and newer electroplated layers will crack differently. When observed with a microscope the electroplated chromium will appear to be in the form of crystals with 120° and 60° angles, but these are the ghosts of the original hydride crystals; the actual crystals that finally form in the coating are much smaller and consist of body centered cubic chromium. [3]

Superhexagonal[ when defined as? ] chromium hydride has also been produced by exposing chromium films to hydrogen under high pressure and temperature. [8]

In 1926 T. Weichselfelder and B. Thiede claimed to have prepared solid chromium trihydride by reacting hydrogen with chromium chloride and phenylmagnesium bromide in ether, forming a black precipitate. [9] [10]

Solid hexagonal CrH can burn in air with a bluish flame. It is ignitable with a burning match. [11]

The hydrogen content of chromium hydride is between zero and a few hundred parts per million in weight for plain chromium-hydrogen alloys. These values vary depending on alloying elements, such as iron, manganese, vanadium, titanium [12] and so on.

Alloys with significantly higher than a few hundred parts per million hydrogen content can be formed, but require extraordinarily high pressures to be stable. Under such conditions, the hydrogen content may contribute up to 0.96% of its weight, at which point it reaches what is called a line compound phase boundary. As the hydrogen content moves beyond the line compound boundary, the chromium-hydrogen system ceases to behave as an alloy, and instead forms a series of non-metallic stoichiometric compounds, each succeeding one requiring still higher pressure for stability. The first such compound found is dichromium hydride (Cr
2H), where the chromium-to-hydrogen ratio is 1/0.5, corresponding to a hydrogen content of 0.96%. Two of these compounds are metastable at ambient pressures, meaning that they decompose over extended lengths of time, rather than instantaneously so. The other such compound is Chromium(I) hydride which is several times more stable. Both these compounds are stable at cryogenic temperatures, persisting indefinitely. Although precise details are not known. [13] -

Other materials are often added to the chromium/hydrogen mixture to produce chromium hydride alloy with desired properties. Titanium in chromium hydride make the β-chromium form of the chromium-hydrogen solution more stable.[ citation needed ]

Related Research Articles

Nickel hydride is either an inorganic compound of the formula NiHx or any of a variety of coordination complexes.

<span class="mw-page-title-main">Lithium aluminium hydride</span> Chemical compound

Lithium aluminium hydride, commonly abbreviated to LAH, is an inorganic compound with the chemical formula Li[AlH4] or LiAlH4. It is a white solid, discovered by Finholt, Bond and Schlesinger in 1947. This compound is used as a reducing agent in organic synthesis, especially for the reduction of esters, carboxylic acids, and amides. The solid is dangerously reactive toward water, releasing gaseous hydrogen (H2). Some related derivatives have been discussed for hydrogen storage.

<span class="mw-page-title-main">Cubic crystal system</span> Crystallographic system where the unit cell is in the shape of a cube

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.

Zirconium alloys are solid solutions of zirconium or other metals, a common subgroup having the trade mark Zircaloy. Zirconium has very low absorption cross-section of thermal neutrons, high hardness, ductility and corrosion resistance. One of the main uses of zirconium alloys is in nuclear technology, as cladding of fuel rods in nuclear reactors, especially water reactors. A typical composition of nuclear-grade zirconium alloys is more than 95 weight percent zirconium and less than 2% of tin, niobium, iron, chromium, nickel and other metals, which are added to improve mechanical properties and corrosion resistance.

<span class="mw-page-title-main">Zirconium hydride</span> Alloy of zirconium and hydrogen

Zirconium hydride describes an alloy made by combining zirconium and hydrogen. Hydrogen acts as a hardening agent, preventing dislocations in the zirconium atom crystal lattice from sliding past one another. Varying the amount of hydrogen and the form of its presence in the zirconium hydride controls qualities such as the hardness, ductility, and tensile strength of the resulting zirconium hydride. Zirconium hydride with increased hydrogen content can be made harder and stronger than zirconium, but such zirconium hydride is also less ductile than zirconium.

Plutonium hydride is a non-stoichiometric chemical compound with the formula PuH2+x. It is one of two characterised hydrides of plutonium, the other is PuH3. PuH2 is non-stoichiometric with a composition range of PuH2 – PuH2.7. Additionally metastable stoichiometries with an excess of hydrogen (PuH2.7 – PuH3) can be formed. PuH2 has a cubic structure. It is readily formed from the elements at 1 atmosphere at 100–200 °C: When the stoichiometry is close to PuH2 it has a silver appearance, but gets blacker as the hydrogen content increases, additionally the color change is associated with a reduction in conductivity.

<span class="mw-page-title-main">Titanium hydride</span> Chemical compound

Titanium hydride normally refers to the inorganic compound TiH2 and related nonstoichiometric materials. It is commercially available as a stable grey/black powder, which is used as an additive in the production of Alnico sintered magnets, in the sintering of powdered metals, the production of metal foam, the production of powdered titanium metal and in pyrotechnics.

<span class="mw-page-title-main">Hydrogen storage</span> Methods of storing hydrogen for later use

Several methods exist for storing hydrogen. These include mechanical approaches such as using high pressures and low temperatures, or employing chemical compounds that release H2 upon demand. While large amounts of hydrogen are produced by various industries, it is mostly consumed at the site of production, notably for the synthesis of ammonia. For many years hydrogen has been stored as compressed gas or cryogenic liquid, and transported as such in cylinders, tubes, and cryogenic tanks for use in industry or as propellant in space programs. Interest in using hydrogen for on-board storage of energy in zero-emissions vehicles is motivating the development of new methods of storage, more adapted to this new application. The overarching challenge is the very low boiling point of H2: it boils around 20.268 K (−252.882 °C or −423.188 °F). Achieving such low temperatures requires expending significant energy.

<span class="mw-page-title-main">Magnesium hydride</span> Chemical compound

Magnesium hydride is the chemical compound with the molecular formula MgH2. It contains 7.66% by weight of hydrogen and has been studied as a potential hydrogen storage medium.

Binary compounds of hydrogen are binary chemical compounds containing just hydrogen and one other chemical element. By convention all binary hydrogen compounds are called hydrides even when the hydrogen atom in it is not an anion. These hydrogen compounds can be grouped into several types.

Cadmium hydride is an inorganic compound with the chemical formula (CdH
2)
n. It is a solid, known only as a thermally unstable, insoluble white powder.

<span class="mw-page-title-main">Iron hydride</span> Index of articles associated with the same name

An iron hydride is a chemical system which contains iron and hydrogen in some associated form.

Cr<sub>23</sub>C<sub>6</sub> crystal structure

Cr23C6 is the prototypical compound of a common crystal structure, discovered in 1933 as part of the chromium-carbon binary phase diagram. Over 85 known compounds adopt this structure type, which can be described as a NaCl-like packing of chromium cubes and cuboctahedra.

Scandium hydride, also known as scandium–hydrogen alloy, is an alloy made by combining scandium and hydrogen. Hydrogen acts as a hardening agent, preventing dislocations in the scandium atom crystal lattice from sliding past one another. Varying the amount of hydrogen controls qualities such as the hardness of the resulting scandium hydride. Scandium hydride with increased hydrogen content can be made harder than scandium.

<span class="mw-page-title-main">Chromium(I) hydride</span> Chemical compound

Chromium(I) hydride, systematically named chromium hydride, is an inorganic compound with the chemical formula (CrH)
n. It occurs naturally in some kinds of stars where it has been detected by its spectrum. However, molecular chromium(I) hydride with the formula CrH has been isolated in solid gas matrices. The molecular hydride is very reactive. As such the compound is not well characterised, although many of its properties have been calculated via computational chemistry.

Chromium(II) hydride, systematically named chromium dihydride and poly­(dihydridochromium) is pale brown solid inorganic compound with the chemical formula (CrH2)n. Although it is thermodynamically unstable toward decomposition at ambient temperatures, it is kinetically metastable.

Iron(II) hydride, systematically named iron dihydride and poly(dihydridoiron) is solid inorganic compound with the chemical formula (FeH
2)
n (also written ([FeH
2])n or FeH
2). ). It is kinetically unstable at ambient temperature, and as such, little is known about its bulk properties. However, it is known as a black, amorphous powder, which was synthesised for the first time in 2014.

<span class="mw-page-title-main">Iron–hydrogen alloy</span>

Iron–hydrogen alloy, also known as iron hydride, is an alloy of iron and hydrogen and other elements. Because of its lability when removed from a hydrogen atmosphere, it has no uses as a structural material.

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.

In chemistry, a hydridonitride is a chemical compound that contains hydride and nitride ions in a single phase. These inorganic compounds are distinct from inorganic amides and imides as the hydrogen does not share a bond with nitrogen, and contain a larger proportion of metals.

References

  1. ↑ Bradley, A. J.; E. F. Ollard (1926). "Allotropy of Chromium". Nature. 117 (2934): 122. Bibcode:1926Natur.117..122B. doi: 10.1038/117122b0 . ISSN   0028-0836. S2CID   4131298.
  2. 1 2 Snavely, Cloyd A.; Dale A. Vaughan (January 1949). "Unit Cell Dimension of Face-centered Cubic Chromium Hydride and Space Groups of Two Chromium Hydrides". Journal of the American Chemical Society. 71 (1): 313–314. doi:10.1021/ja01169a085. ISSN   0002-7863.
  3. 1 2 3 4 5 Snavely, Cloyd A. (1947). "A Theory for the Chromium Plating;A Theory for the Physical Characteristics of Chromium Plate". Journal of the Electrochemical Society. 91 (1): 537–577. doi:10.1149/1.3071841. Archived from the original on June 22, 2017.
  4. ↑ Poźniak-Fabrowska, J; B Nowak; M Tkacz (2001). "Magnetic properties of cubic and hexagonal chromium hydrides: a comparison of the magnetic susceptibility with the 53Cr NMR Knight shift". Journal of Alloys and Compounds. 322 (1–2): 82–88. doi:10.1016/S0925-8388(01)01266-X. ISSN   0925-8388.
  5. 1 2 Antonov, V.E.; A.I. Beskrovnyy, V.K. Fedotov, A.S. Ivanov, S.S. Khasanov, A.I. Kolesnikov, M.K. Sakharov, I.L. Sashin, M. Tkacz (2007). "Crystal structure and lattice dynamics of chromium hydrides". Journal of Alloys and Compounds. 430 (1–2): 22–28. doi:10.1016/j.jallcom.2006.05.021. ISSN   0925-8388.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  6. 1 2 Sasaki, Kumazo; Sinkiti Sekito (24 February 1931). "Three Crystalline Modifications of Electrolytic Chromium". Journal of the Electrochemical Society. 59 (1): 437–444. doi:10.1149/1.3497824. full text available
  7. ↑ Smith, W. H. (1956). "The Fracture of Brittle Chromium by Acid Etching". Journal of the Electrochemical Society. 103 (1): 51. doi:10.1149/1.2430232. ISSN   0013-4651.
  8. ↑ Pan, Y; M Takeo; J Dash (1993). "Formation of superhexagonal chromium hydride by exposure of chromium thin films to high-temperature, high-pressure hydrogen in a ballistic compressor". International Journal of Hydrogen Energy. 18 (6): 491–504. doi:10.1016/0360-3199(93)90006-V. ISSN   0360-3199.
  9. ↑ mellor. "The Chemical Properties of Chromium" (PDF). A Comprehensive Treatise on Inorganic and Theoretical Chemistry. p. 160.
  10. ↑ Weichselfelder, Theodor; Bruno Thiede (1926). "Über die Hydride der Metalle Nickel, Kobalt, Eisen und Chrom". Justus Liebigs Annalen der Chemie. 447 (1): 64–77. doi:10.1002/jlac.19264470107.
  11. ↑ Raub, Christoph J. (September 1993). "Hydrogen in Electrodeposits: of Decisive Importance, But Much Neglected" (PDF). Plating and Surface Finishing: 35.
  12. ↑ Johnson, John R.; Reilly, James J. (November 1978). "Reaction of hydrogen with the low-temperature form (C15) of titanium-chromium (TiCr2)". Inorganic Chemistry. 17 (11): 3103–3108. doi:10.1021/ic50189a027.
  13. ↑ Snavely, Cloyd A.; Vaughan, Dale A. (1949). "Unit Cell Dimension of Face-centered Cubic Chromium Hydride and Space Groups of Two Chromium Hydrides". Journal of the American Chemical Society. 71 (1): 313–314. doi:10.1021/ja01169a085. ISSN   0002-7863.

Further reading

This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.