View source for Lithium hydride

{{chembox
| Verifiedfields = changed
| Watchedfields = changed
| verifiedrevid = 476998437
| Name = Lithium hydride
| ImageFile = Lithium-hydride-3D-vdW.png
| ImageSize = 
| ImageName = Space-filling model of part of the crystal structure of lithium hydride
| ImageCaption = {{legend|rgb(192,128,256)|Lithium cation, {{chem2|Li+}}}}{{legend|white|Hydrogen anion, {{chem2|H−}}}}
| ImageFile1 = NaCl polyhedra.png
| ImageCaption1 = <span style="color:#C0C0C0; background-color:#C0C0C0;">__</span>{{chem2|H-}} <span style="color:#00FF00;background-color:#00FF00;">__</span>{{chem2|Li+}}<br>Structure of lithium hydride.
| ImageFile2 = File:Lithium_hydride.png
| ImageName2 = Space-filling model of the lithium hydride molecule
|Section1={{Chembox Identifiers
| ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}
| ChemSpiderID = 56460
| InChI = 1/Li.H/q+1;-1
| InChIKey = SRTHRWZAMDZJOS-UHFFFAOYAZ
| SMILES = [H-].[Li+]
| StdInChI_Ref = {{stdinchicite|changed|chemspider}}
| StdInChI = 1S/Li.Hssss
| StdInChIKey_Ref = {{stdinchicite|changed|chemspider}}
| StdInChIKey = SIAPCJWMELPYOE-UHFFFAOYSA-N
| CASNo = 7580-67-8
| CASNo_Ref = {{cascite|correct|CAS}}
| UNII_Ref = {{fdacite|correct|FDA}}
| UNII = 68KF447EX3
| PubChem = 62714
| RTECS = OJ6300000
 }}
|Section2={{Chembox Properties
| Formula = LiH
| Li=1|H=1
| Appearance = colorless to gray solid<ref name=crc/>
| Density = 0.78 g/cm<sup>3</sup><ref name=crc/>
| Solubility = reacts
| SolubleOther = slightly soluble in [[dimethylformamide]]<br>reacts with [[ammonia]], [[diethyl ether]], [[ethanol]]
| MeltingPtC = 688.7
| MeltingPt_ref = <ref name=crc>{{RubberBible86th|page=4.70}}</ref>
| BoilingPtC = 900–1000
| BoilingPt_notes= (decomposes)<ref>{{cite book|author1=David Arthur Johnson|author2=Open University|title=Metals and chemical change|url=https://books.google.com/books?id=dzjxzsKjZGUC&pg=PA167|access-date=1 November 2011|date=12 August 2002|publisher=Royal Society of Chemistry|isbn=978-0-85404-665-2|pages=167–}}</ref>
| RefractIndex = 1.9847<ref name="Smith">{{cite book|url=https://ntrs.nasa.gov/search.jsp?R=19720066808&hterms=lithium+hydride+fuel&qs=Ntx%3Dmode%2520matchallpartial%2520%26Ntk%3DAll%26N%3D0%26Ntt%3D%2522lithium%2520hydride%2522%2520fuel|author1=Smith, R. L. |author2=Miser, J. W. |publisher=NASA|title=Compilation of the properties of lithium hydride|date=1963}}</ref>{{rp|43}}
| MagSus = &minus;4.6·10<sup>−6</sup> cm<sup>3</sup>/mol
 }}
|Section3={{Chembox Structure
| CrystalStruct = [[Cubic crystal system|fcc]] ([[Halite structure|NaCl-type]])
| LattConst_a = 0.40834 nm<ref name="Smith" />{{rp|56}}
| Dipole = 6.0 D<ref name="Smith" />{{rp|35}}
 }}
|Section4={{Chembox Thermochemistry
| DeltaHf = −90.65 kJ/mol
| DeltaHc = 
| DeltaGf = −68.48 kJ/mol
| Entropy = 170.8 J/(mol·K)
| HeatCapacity = 3.51 J/(g·K)
 }}
|Section7={{Chembox Hazards
| ExternalSDS = [http://www.inchem.org/documents/icsc/icsc/eics0813.htm ICSC 0813]
| MainHazards = extremely strong irritant, highly toxic, highly corrosive
| GHSPictograms = {{GHS02}}{{GHS05}}{{GHS06}}
| GHSSignalWord = Danger
| HPhrases = {{H-phrases|260|301|314}}
| PPhrases = {{P-phrases|P223|P231+P232|P260|P264|P270|P280|P301+P316|P301+P330+P331|P302+P335+P334|P302+P361+P354|P304+P340|P305+P354+P338|P316|P321|P330|P363|P370+P378|P402+P404|P405|P501}}
| NFPA-H = 3
| NFPA-F = 2
| NFPA-R = 2
| NFPA-S = W
| AutoignitionPtC = 200
| AutoignitionPt_notes = 
| LD50 = 77.5 mg/kg (oral, rat)<ref>{{cite web|url=https://chem.nlm.nih.gov/chemidplus/rn/7580-67-8|title=ChemIDplus - 7580-67-8 - SIAPCJWMELPYOE-UHFFFAOYSA-N - Lithium hydride - Similar structures search, synonyms, formulas, resource links, and other chemical information.|first=Michael|last=Chambers|website=chem.sis.nlm.nih.gov|access-date=10 April 2018}}</ref>
| IDLH = 0.5 mg/m<sup>3</sup><ref name=PGCH>{{PGCH|0371}}</ref>
| LC50 = 22 mg/m<sup>3</sup> (rat, 4 h)<ref>{{IDLH|7580678|Lithium hydride}}</ref>
| PEL = TWA 0.025 mg/m<sup>3</sup><ref name=PGCH/>
| REL = TWA 0.025 mg/m<sup>3</sup><ref name=PGCH/>
 }}
|Section8={{Chembox Related
| OtherCations = [[Sodium hydride]]<br/>[[Potassium hydride]]<br/>[[Rubidium hydride]]<br/>[[Caesium hydride]]
| OtherCompounds = [[Lithium borohydride]]<br/>[[Lithium aluminium hydride]]
 }}
}}

'''Lithium hydride''' is an [[inorganic compound]] with the formula [[Lithium|Li]][[Hydride|H]]. This [[alkali metal]] [[hydride]] is a colorless solid, although commercial samples are grey. Characteristic of a [[Hydride#Ionic hydrides|salt-like (ionic) hydride]], it has a high melting point, and it is not soluble but reactive with all [[Polar solvent|protic]] [[organic solvents]]. It is soluble and nonreactive with certain [[molten salt]]s such as [[lithium fluoride]], [[lithium borohydride]], and [[sodium hydride]]. With a [[molar mass]] of 7.95 g/mol, it is the lightest [[ionic compound]].

==Physical properties==
LiH is a [[diamagnetism|diamagnetic]] and an [[Ionic conductivity (solid state)|ionic conductor]] with an [[Electrical resistivity and conductivity|electric conductivity]] gradually increasing from {{val|2|e=-5|u=Ω<sup>−1</sup>cm<sup>−1</sup>}} at 443&nbsp;°C to 0.18&nbsp;Ω<sup>−1</sup>cm<sup>−1</sup> at 754&nbsp;°C; there is no discontinuity in this increase through the melting point.<ref name="Smith" />{{rp|36}} The [[dielectric constant]] of LiH decreases from 13.0 (static, low frequencies) to 3.6 (visible-light frequencies).<ref name="Smith" />{{rp|35}} LiH is a soft material with a [[Mohs hardness]] of 3.5.<ref name="Smith" />{{rp|42}} Its [[Creep (deformation)|compressive creep]] (per 100 hours) rapidly increases from <&nbsp;1% at 350&nbsp;°C to >&nbsp;100% at 475&nbsp;°C, meaning that LiH cannot provide mechanical support when heated.<ref name="Smith" />{{rp|39}}

The [[thermal conductivity]] of LiH decreases with temperature and depends on morphology: the corresponding values are 0.125&nbsp;W/(cm·K) for crystals and 0.0695&nbsp;W/(cm·K) for compacts at 50&nbsp;°C, and 0.036&nbsp;W/(cm·K) for crystals and 0.0432&nbsp;W/(cm·K) for compacts at 500&nbsp;°C.<ref name="Smith" />{{rp|60}} The linear [[thermal expansion coefficient]] is 4.2{{e|-5}}/°C at room temperature.<ref name="Smith" />{{rp|49}}

==Synthesis and processing==
LiH is produced by treating [[lithium]] metal with [[hydrogen]] gas:
:{{chem2|2 Li + H2 → 2 LiH}}

This reaction is especially rapid at temperatures above 600&nbsp;°C. Addition of 0.001–0.003% carbon, and/or increasing temperature/pressure, increases the yield up to 98% at 2-hour residence time.<ref name="Smith" />{{rp|147}} However, the reaction proceeds at temperatures as low as 29&nbsp;°C. The yield is 60% at 99&nbsp;°C and 85% at 125&nbsp;°C, and the rate depends significantly on the surface condition of LiH.<ref name="Smith" />{{rp|5}}

Less common ways of LiH synthesis include [[thermal decomposition]] of [[lithium aluminium hydride]] (200&nbsp;°C), [[lithium borohydride]] (300&nbsp;°C), [[n-Butyllithium|''n''-butyllithium]] (150&nbsp;°C), or [[ethyllithium]] (120&nbsp;°C), as well as several reactions involving lithium compounds of low stability and available hydrogen content.<ref name="Smith" />{{rp|144–145}}

Chemical reactions yield LiH in the form of lumped [[powder]], which can be compressed into [[Pelletizing|pellets]] without a [[Binder (material)|binder]]. More complex shapes can be produced by [[casting]] from the [[Melting|melt]].<ref name="Smith" />{{rp|160 ff.}} Large single [[crystals]] (about 80&nbsp;mm long and 16&nbsp;mm in diameter) can be then grown from molten LiH powder in hydrogen atmosphere by the [[Bridgman–Stockbarger technique]]. They often have bluish color owing to the presence of [[colloidal]] Li. This color can be removed by post-growth [[Annealing (materials science)|annealing]] at lower temperatures (~550&nbsp;°C) and lower thermal gradients.<ref name="Smith" />{{rp|154}} Major impurities in these crystals are [[Sodium|Na]] (20–200 [[parts per million|ppm]]), [[Oxygen|O]] (10–100 ppm), [[Magnesium|Mg]] (0.5–6 ppm), [[Iron|Fe]] (0.5-2 ppm) and [[Copper|Cu]] (0.5-2 ppm).<ref name="Smith" />{{rp|155}}

[[File:LiHcrack.jpg|thumb|left|Cracking in cast LiH after machining with a [[fly cutter]]. Scale is in inches.]]
Bulk cold-pressed LiH parts can be easily machined using standard techniques and tools to [[micrometre|micrometer]] precision. However, [[Casting|cast]] LiH is [[brittle]] and easily cracks during processing.<ref name="Smith" />{{rp|171}}

A more energy efficient route to form lithium hydride powder is by [[ball milling]] lithium metal under high hydrogen pressure. To prevent [[cold welding]] of lithium metal (due to its high [[ductility]]), small amounts of lithium hydride powder are added during this process.<ref>Solvent-and catalyst-free mechanochemical synthesis of alkali metal monohydrides
IZ Hlova, A Castle, JF Goldston, S Gupta, T Prost… - Journal of Materials Chemistry A, 2016</ref>

==Reactions==
LiH powder reacts rapidly with [[air]] of low [[humidity]], forming [[lithium hydroxide|LiOH]], [[lithium oxide|{{chem2|Li2O}}]] and [[lithium carbonate|{{chem2|Li2CO3}}]]. In moist air the powder ignites spontaneously, forming a mixture of products including some nitrogenous compounds. The lump material reacts with humid air, forming a superficial coating, which is a viscous fluid. This inhibits further reaction, although the appearance of a film of "tarnish" is quite evident. Little or no [[Lithium nitride|nitride]] is formed on exposure to humid air. The lump material, contained in a metal dish, may be heated in air to slightly below 200&nbsp;°C without igniting, although it ignites readily when touched by an open flame. The surface condition of LiH, presence of oxides on the metal dish, etc., have a considerable effect on the ignition temperature. Dry [[oxygen]] does not react with crystalline LiH unless heated strongly, when an almost explosive combustion occurs.<ref name="Smith" />{{rp|6}}

LiH is highly reactive towards [[water]] and other [[protic]] reagents:<ref name="Smith" />{{rp|7}}
:{{chem2|LiH + H2O → Li+ + H2 + OH−}}

LiH is less reactive with water than Li and thus is a much less powerful reducing agent for water, [[alcohols]], and other media containing reducible [[solutes]]. This is true for all the binary [[Hydride#Ionic hydrides|saline hydrides]].<ref name="Smith" />{{rp|22}}

LiH pellets slowly expand in moist air, forming [[Lithium hydroxide|LiOH]]; however, the expansion rate is below 10% within 24 hours in a pressure of 2&nbsp;[[Torr]] of water vapor.<ref name="Smith" />{{rp|7}} If moist air contains [[carbon dioxide]], then the product is [[lithium carbonate]].<ref name="Smith" />{{rp|8}} LiH reacts with [[ammonia]], slowly at room temperature, but the reaction accelerates significantly above 300&nbsp;°C.<ref name="Smith" />{{rp|10}} LiH reacts slowly with higher [[alcohols]] and [[phenols]], but vigorously with lower alcohols.<ref name="Smith" />{{rp|14}}

LiH reacts with [[sulfur dioxide]] to give the [[dithionite]]:
:{{chem2|2 LiH + 2 SO2 → Li2S2O4 + H2}}
though above 50&nbsp;°C the product is [[lithium sulfide]] instead.<ref name="Smith" />{{rp|9}}

LiH reacts with [[acetylene]] to form [[lithium carbide]] and [[hydrogen]]. With anhydrous [[organic acids]], phenols and [[acid anhydrides]], LiH reacts slowly, producing hydrogen gas and the lithium salt of the acid. With water-containing acids, LiH reacts faster than with water.<ref name="Smith" />{{rp|8}} Many reactions of LiH with oxygen-containing species yield LiOH, which in turn irreversibly reacts with LiH at temperatures above 300&nbsp;°C:<ref name="Smith" />{{rp|10}}
:{{chem2|LiH + LiOH → [[Lithium oxide|Li2O]] + H2}}
Lithium hydride is rather unreactive at moderate temperatures with {{chem2|[[Oxygen|O2]]}} or {{chem2|[[Chlorine|Cl2]]}}. It is, therefore, used in the synthesis of other useful hydrides,<ref>{{Cite web|title=NCERT Chemistry Textbook|url=https://ncert.nic.in/textbook/pdf/kech202.pdf}}</ref> e.g.,

:{{chem2|8 LiH + [[Aluminium chloride|Al2Cl6]] → 2 [[Lithium aluminium hydride| Li[AlH4] ]] + 6 [[Lithium chloride|LiCl]]}}
:{{chem2|2 LiH + [[Diborane|B2H6]] → 2 [[Lithium borohydride|Li[BH4] ]]}}

==Applications==

===Hydrogen storage and fuel===
With a hydrogen content in proportion to its mass three times that of NaH, LiH has the highest hydrogen content of any hydride. LiH is periodically of interest for hydrogen storage, but applications have been thwarted by its stability to decomposition. Thus removal of {{chem2|H2}} requires temperatures above the 700&nbsp;°C used for its synthesis, such temperatures are expensive to create and maintain. The compound was once tested as a fuel component in a model rocket.<ref>[http://www.astronautix.com/lvs/lex.htm Lex] {{webarchive|url=https://web.archive.org/web/20080723202520/http://www.astronautix.com/lvs/lex.htm |date=2008-07-23 }}. Astronautix.com (1964-04-25). Retrieved on 2011-11-01.</ref><ref>[https://ntrs.nasa.gov/search.jsp?R=19690026364&hterms=lithium+hydride+fuel&qs=Ntx%3Dmode%2520matchallpartial%2520%26Ntk%3DAll%26N%3D0%26Ntt%3D%2522lithium%2520hydride%2522%2520fuel Empirical laws for hybrid combustion of lithium hydride with fluorine in small rocket engines]. Ntrs.nasa.gov. Retrieved on 2011-11-01.{{closed access}} {{Password-protected}}</ref>

===Precursor to complex metal hydrides===
LiH is not usually a hydride-reducing agent, except in the synthesis of hydrides of certain metalloids. For example, [[silane]] is produced in the reaction of lithium hydride and [[silicon tetrachloride]] by the Sundermeyer process:
:{{chem2|4 LiH + SiCl4 → 4 LiCl + SiH4}}

Lithium hydride is used in the production of a variety of reagents for [[organic synthesis]], such as [[lithium aluminium hydride]] ({{chem2|Li[AlH4]}}) and [[lithium borohydride]] ({{chem2|Li[BH4]}}). [[Triethylborane]] reacts to give [[superhydride]] ({{chem2|Li[BH(CH2CH3)3]}}).<ref name=Ullmann>Peter Rittmeyer, Ulrich Wietelmann "Hydrides" in Ullmann's Encyclopedia of Industrial Chemistry 2002, Wiley-VCH, Weinheim. {{doi|10.1002/14356007.a13_199}}</ref>

===In nuclear chemistry and physics===
Lithium hydride (LiH) is sometimes a desirable material for the shielding of [[nuclear reactor]]s, with the isotope [[lithium-6]] (Li-6), and it can be fabricated by casting.<ref>{{cite book|author=Peter J. Turchi|title=Propulsion techniques: action and reaction|url=https://books.google.com/books?id=-o9TJa2F4qsC&pg=PA339|access-date=2 November 2011|date=1998|publisher=AIAA|isbn=978-1-56347-115-5|pages=339–}}</ref><ref>{{cite journal|first=Frank H. |last=Welch |title= Lithium hydride: A space age shielding material |journal=Nuclear Engineering and Design |volume=26 | issue=3 |date= February 1974 |pages=440–460|doi=10.1016/0029-5493(74)90082-X}}</ref>

====Lithium deuteride====
Lithium deuteride, in the form of [[lithium-7]] deuteride ({{chem2|^{7}Li^{2}H}} or <sup>7</sup>LiD), is a good [[Neutron moderator|moderator]] for [[nuclear reactor]]s, because [[deuterium]] (<sup>2</sup>H or D) has a lower [[neutron]] absorption [[Cross section  (physics)|cross-section]] than ordinary hydrogen or [[Isotopes of hydrogen#Hydrogen-1 (protium)|protium]] (<sup>1</sup>H) does, and the cross-section for <sup>7</sup>Li is also low, decreasing the absorption of neutrons in a reactor. <sup>7</sup>Li is preferred for a moderator because it has a lower neutron capture cross-section, and it also forms less [[tritium]] (<sup>3</sup>H or T) under bombardment with neutrons.<ref>{{cite web|last1=Massie|first1=Mark|last2=Dewan|first2=Leslie C.|title=US 20130083878 A1, April 4, 2013, NUCLEAR REACTORS AND RELATED METHODS AND APPARATUS|url=http://appft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PG01&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.html&r=1&f=G&l=50&s1=%2220130083878%22.PGNR.&OS=DN/20130083878&RS=DN/20130083878|website=U.S. Patent Office|publisher=U.S. Government|access-date=2 June 2016|ref=transatomic_patent}}</ref>

The corresponding [[lithium-6]] [[deuterium|deuteride]] ({{chem2|^{6}Li^{2}H}} or <sup>6</sup>LiD) is the primary [[nuclear fusion|fusion]] fuel in [[thermonuclear weapon]]s.{{Citation needed|date=July 2024}} In hydrogen warheads of the [[History of the Teller–Ulam design|Teller–Ulam design]], a [[nuclear fission]] trigger explodes to heat and compress the lithium-6 deuteride, and to bombard the <sup>6</sup>LiD with [[neutron]]s to produce tritium in an [[exothermic]] reaction:
:{{chem2|^{6}LiD + n → ^{4}He + T + D}}
The deuterium and tritium then fuse to produce [[helium]], one neutron, and 17.59&nbsp;MeV of free energy in the form of [[gamma ray]]s, [[kinetic energy]], etc.  Tritium has a favorable reaction [[cross section (physics)|cross section]].  The helium is an inert byproduct.{{Citation needed|date=July 2024}}
:{{nuclide|hydrogen|3}} + {{nuclide|hydrogen|2}} → {{nuclide|helium|4}} + [[Neutron|n]]. 

Before the [[Castle Bravo]] [[nuclear weapons test]] in 1954, it was thought that only the less common isotope <sup>6</sup>Li would breed tritium when struck with fast neutrons. The Castle Bravo test showed (accidentally) that the more plentiful <sup>7</sup>Li also does so under extreme conditions, albeit by an [[endothermic]] reaction.

==Safety==
LiH reacts violently with water to give hydrogen gas and LiOH, which is caustic. Consequently, LiH dust can explode in humid air, or even in dry air due to static electricity. At concentrations of {{nobr|5–55 mg/m<sup>3</sup>}} in air the dust is extremely irritating to the mucous membranes and skin and may cause an allergic reaction. Because of the irritation, LiH is normally rejected rather than accumulated by the body.<ref name="Smith" />{{rp|157,182}}

Some lithium salts, which can be produced in LiH reactions, are toxic. LiH fire should not be extinguished using carbon dioxide, carbon tetrachloride, or aqueous fire extinguishers; it should be smothered by covering with a metal object or graphite or [[Dolomite (rock)|dolomite]] powder. Sand is less suitable, as it can explode when mixed with burning LiH, especially if not dry. LiH is normally transported in oil, using containers made of ceramic, certain plastics or steel, and is handled in an atmosphere of dry argon or helium.<ref name="Smith" />{{rp|156}} Whilst nitrogen can be used, it will react with lithium at elevated temperatures.<ref name="Smith" />{{rp|157}} LiH normally contains some metallic lithium, which corrodes steel or [[silica]] containers at elevated temperatures.<ref name="Smith" />{{rp|173–174, 179}}

== References ==
{{reflist|30em}}

== External links ==
{{Wiktionary}}
* [https://web.archive.org/web/20080226213021/https://www.mcis.soton.ac.uk/Site_Files/pdf/nuclear_history/Working_Paper_No_5.pdf University of Southampton, Mountbatten Centre for International Studies, Nuclear History Working Paper No5.]
* [https://www.cdc.gov/niosh/npg/npgd0371.html CDC - NIOSH Pocket Guide to Chemical Hazards]

{{Lithium compounds}}
{{Hydrides by group}}

{{DEFAULTSORT:Lithium Hydride}}
[[Category:Lithium compounds]]
[[Category:Metal hydrides]]
[[Category:Nuclear materials]]
[[Category:Nuclear fusion fuels]]
[[Category:Superbases]]
[[Category:Rock salt crystal structure]]