Lanthanum is achemical element with thesymbolLa and theatomic number 57. It is asoft,ductile, silvery-whitemetal that tarnishes slowly when exposed to air. It is the eponym of thelanthanide series, a group of 15 similar elements between lanthanum andlutetium in theperiodic table, of which lanthanum is the first and the prototype. Lanthanum is traditionally counted among therare earth elements. Like most other rare earth elements, its usualoxidation state is +3, although some compounds are known with an oxidation state of +2. Lanthanum has no biological role in humans but is used by some bacteria. It is not particularly toxic to humans but does show some antimicrobial activity.
Lanthanum usually occurs together withcerium and the other rare earth elements. Lanthanum was first found by the Swedish chemistCarl Gustaf Mosander in 1839 as an impurity incerium nitrate – hence the namelanthanum, from theancient Greekλανθάνειν (lanthanein), meaning 'to lie hidden'. Although it is classified as a rare earth element, lanthanum is the 28th most abundant element in the Earth's crust, almost three times as abundant aslead. In minerals such asmonazite andbastnäsite, lanthanum composes about a quarter of the lanthanide content.[10] It is extracted from those minerals by a process of such complexity that pure lanthanum metal was not isolated until 1923.
Lanthanum is the first element and prototype of the lanthanide series. In the periodic table, it appears to the right of thealkaline earth metalbarium and to the left of the lanthanide cerium. Lanthanum is generally considered the first of the f-block elements by authors writing on the subject.[11][12][13][14][15] The 57 electrons of a lanthanum atom are arranged in theconfiguration [Xe]5d16s2, with three valence electrons outside the noble gas core. In chemical reactions, lanthanum almost always gives up these three valence electrons from the 5d and 6ssubshells to form the +3 oxidation state, achieving the stable configuration of the preceding noble gasxenon.[16] Some lanthanum(II) compounds are also known, but they are usually much less stable.[17][18] Lanthanum monoxide (LaO) produces strong absorption bands in somestellar spectra.[19]
Among the lanthanides, lanthanum is exceptional as it has no 4f electrons as a single gas-phase atom. Thus it is only very weaklyparamagnetic, unlike the strongly paramagnetic later lanthanides (with the exceptions of the last two,ytterbium andlutetium, where the 4f shell is completely full).[20] However, the 4f shell of lanthanum can become partially occupied in chemical environments and participate in chemical bonding.[21][22] For example, the melting points of the trivalent lanthanides (all buteuropium and ytterbium) are related to the extent of hybridisation of the 6s, 5d, and 4f electrons (lowering with increasing 4f involvement),[23] and lanthanum has the second-lowest melting point among them: 920 °C. (Europium and ytterbium have lower melting points because they delocalise about two electrons per atom rather than three.)[24] This chemical availability of f orbitals justifies lanthanum's placement in the f-block despite its anomalous ground-state configuration[25][26] (which is merely the result of strong interelectronic repulsion making it less profitable to occupy the 4f shell, as it is small and close to the core electrons).[27]
The lanthanides become harder as the series is traversed: as expected, lanthanum is a soft metal. Lanthanum has a relatively highresistivity of 615 nΩm at room temperature; in comparison, the value for the good conductor aluminium is only 26.50 nΩm.[28][29] Lanthanum is the least volatile of the lanthanides.[30] Like most of the lanthanides, lanthanum has ahexagonal crystal structure at room temperature (α-La). At 310 °C, lanthanum changes to aface-centered cubic structure (β-La), and at 865 °C, it changes to abody-centered cubic structure (γ-La).[29]
As expected fromperiodic trends, lanthanum has the largestatomic radius of the lanthanides. Hence, it is the most reactive among them, tarnishing quite rapidly in air, turning completely dark after several hours and can readily burn to formlanthanum(III) oxide,La 2O 3, which is almost asbasic ascalcium oxide.[31] A centimeter-sized sample of lanthanum will corrode completely in a year as its oxidespalls off like ironrust, instead of forming a protective oxide coating likealuminium, scandium, yttrium, and lutetium.[32] Lanthanum reacts with thehalogens at room temperature to form the trihalides, and upon warming will formbinary compounds with the nonmetals nitrogen, carbon, sulfur, phosphorus, boron, selenium, silicon and arsenic.[16][17] Lanthanum reacts slowly with water to formlanthanum(III) hydroxide,La(OH) 3.[33] In dilutesulfuric acid, lanthanum readily forms the aquated tripositive ion[La(H 2O) 9]3+ : This is colorless in aqueous solution sinceLa3+ has no d or f electrons.[33] Lanthanum is the strongest andhardest base among therare earth elements, which is again expected from its being the largest of them.[34]
Some lanthanum(II) compounds are also known, but they are much less stable.[17] Therefore, in officially naming compounds of lanthanum its oxidation number always is to be mentioned.
Naturally occurring lanthanum is made up of two isotopes, the stable139 La and theprimordial long-lived radioisotope138 La.139 La is by far the most abundant, making up 99.910% of natural lanthanum: it is produced in thes-process (slowneutron capture, which occurs in low- to medium-mass stars) and ther-process (rapid neutron capture, which occurs in core-collapsesupernovae). It is the only stable isotope of lanthanum.[35] The very rare isotope138 La is one of the few primordialodd–odd nuclei, with a long half-life of1.05×1011 years. It is one of the proton-richp-nuclei which cannot be produced in thes- orr-processes.138 La, along with the even rarer180m Ta, is produced in the ν-process, whereneutrinos interact with stable nuclei.[36] All other lanthanum isotopes aresynthetic: With the exception of137 La with a half-life of about 60,000 years, all of them have half-lives less than two days, and most have half-lives less than a minute. The isotopes139 La and140 La occur asfission products of uranium.[35]
Lanthanum oxide is a white solid that can be prepared by direct reaction of its constituent elements. Due to the large size of theLa3+ ion,La 2O 3 adopts a hexagonal 7-coordinate structure that changes to the 6-coordinate structure ofscandium oxide (Sc 2O 3) andyttrium oxide (Y 2O 3) at high temperature. When it reacts with water,lanthanum hydroxide is formed:[37] a lot of heat is evolved in the reaction and a hissing sound is heard. Lanthanum hydroxide will react with atmosphericcarbon dioxide to form the basic carbonate.[38]
Lanthanum fluoride is insoluble in water and can be used as aqualitative test for the presence ofLa3+ . The heavier halides are all very solubledeliquescent compounds. The anhydrous halides are produced by direct reaction of their elements, as heating the hydrates causes hydrolysis: for example, heating hydratedLaCl 3 producesLaOCl.[38]
Lanthanum reacts exothermically with hydrogen to produce the dihydrideLaH 2, a black,pyrophoric, brittle, conducting compound with thecalcium fluoride structure.[39] This is a non-stoichiometric compound, and further absorption of hydrogen is possible, with a concomitant loss of electrical conductivity, until the more salt-likeLaH 3 is reached. LikeLaI 2 andLaI,LaH 2 is probably anelectride compound.[38]
Due to the large ionic radius and great electropositivity ofLa3+ , there is not much covalent contribution to its bonding and hence it has a limitedcoordination chemistry, like yttrium and the other lanthanides.[40]Lanthanum oxalate does not dissolve very much in alkali-metal oxalate solutions, and[La(acac) 3(H 2O) 2] decomposes around 500 °C. Oxygen is the most commondonor atom in lanthanum complexes, which are mostly ionic and often have high coordination numbers over 6 : 8 is the most characteristic, formingsquare antiprismatic anddodecadeltahedral structures. These high-coordinate species, reaching up to coordination number 12 with the use ofchelating ligands such as inLa 2(SO 4) 3· 9(H 2O), often have a low degree of symmetry because of stereo-chemical factors.[40]
Lanthanum chemistry tends not to involveπ-bonding due to the electron configuration of the element: thus its organometallic chemistry is quite limited. The best characterized organolanthanum compounds are thecyclopentadienyl complexLa(C 5H 5) 3, which is produced by reacting anhydrousLaCl 3 withNaC 5H 5 intetrahydrofuran, and its methyl-substituted derivatives.[41]
In 1751, the Swedish mineralogistAxel Fredrik Cronstedt discovered a heavy mineral from the mine atBastnäs, later namedcerite. Thirty years later, the fifteen-year-oldWilhelm Hisinger, from the family owning the mine, sent a sample of it toCarl Scheele, who did not find any new elements within. In 1803, after Hisinger had become an ironmaster, he returned to the mineral withJöns Jacob Berzelius and isolated a new oxide which they namedceria after thedwarf planetCeres, which had been discovered two years earlier.[42] Ceria was simultaneously independently isolated in Germany byMartin Heinrich Klaproth.[43] Between 1839 and 1843, ceria was shown to be a mixture of oxides by the Swedish surgeon and chemistCarl Gustaf Mosander, who lived in the same house as Berzelius and studied under him: he separated out two other oxides which he namedlanthana anddidymia.[44][45] He partially decomposed a sample ofcerium nitrate by roasting it in air and then treating the resulting oxide with dilutenitric acid.[b][47] That same year,Axel Erdmann, a student also at the Karolinska Institute, discovered lanthanum in a new mineral from Låven island located in a Norwegian fjord.
Finally, Mosander explained his delay, saying that he had extracted a second element from cerium, and this he called didymium. Although he did not realise it, didymium too was a mixture, and in 1885 it was separated intopraseodymium and neodymium.
Since lanthanum's properties differed only slightly from those of cerium, and occurred along with it in its salts, he named it from theAncient Greekλανθάνειν [lanthanein] (lit.to lie hidden).[43] Relatively pure lanthanum metal was first isolated in 1923.[citation needed]
Lanthanum makes up 39 mg/kg of the Earth's crust,[48][49] behindneodymium at 41.5 mg/kg and cerium at 66.5 mg/kg. Despite being among the so-called "rare earth metals", lanthanum is thus not rare at all, but it is historically so-named because it is rarer than "common earths" such as lime and magnesia, and at the time it was recognized only a few deposits were known. Lanthanum is also ruefully considered a 'rare earth' metal because the process to mine it is difficult, time-consuming, and expensive.[17] Lanthanum is rarely the dominant lanthanide found in the rare earth minerals, and in their chemical formulas it is usually preceded by cerium. Rare examples of La-dominant minerals are monazite-(La) and lanthanite-(La).[50]
Production of Lanthanum from Monazite sand
TheLa3+ ion is similarly sized to the early lanthanides of the cerium group (those up tosamarium andeuropium) that immediately follow in the periodic table, and hence it tends to occur along with them inphosphate,silicate andcarbonate minerals, such asmonazite (MIIIPO4) andbastnäsite (MIIICO3F), where M refers to all the rare earth metals except scandium and the radioactivepromethium (mostly Ce, La, and Y).[51] Bastnäsite is usually lacking inthorium and the heavy lanthanides, and the purification of the light lanthanides from it is less involved. The ore, after being crushed and ground, is first treated with hot concentrated sulfuric acid, evolving carbon dioxide,hydrogen fluoride, andsilicon tetrafluoride: the product is then dried and leached with water, leaving the early lanthanide ions, including lanthanum, in solution.[52]
The procedure for monazite, which usually contains all the rare earths as well as thorium, is more involved. Monazite, because of its magnetic properties, can be separated by repeated electromagnetic separation. After separation, it is treated with hot concentrated sulfuric acid to produce water-soluble sulfates of rare earths. The acidic filtrates are partially neutralized withsodium hydroxide to pH 3–4. Thorium precipitates out of solution as hydroxide and is removed. After that, the solution is treated withammonium oxalate to convert rare earths to their insolubleoxalates. The oxalates are converted to oxides by annealing. The oxides are dissolved in nitric acid that excludes one of the main components,cerium, whose oxide is insoluble inHNO 3. Lanthanum is separated as a double salt with ammonium nitrate by crystallization. This salt is relatively less soluble than other rare earth double salts and therefore stays in the residue.[17] Care must be taken when handling some of the residues as they contain228 Ra, the daughter of232 Th, which is a strong gamma emitter. Lanthanum is relatively easy to extract as it has only one neighbouring lanthanide, cerium, which can be removed by making use of its ability to be oxidised to the +4 state; thereafter, lanthanum may be separated out by the historical method offractional crystallization ofLa(NO 3) 3· 2 NH 4NO 3· 4 H 2O, or byion-exchange techniques when higher purity is desired.[52]
Lanthanum metal is obtained from its oxide by heating it withammonium chloride or fluoride and hydrofluoric acid at 300–400 °C to produce the chloride or fluoride:[17]
La 2O 3 +6 NH 4Cl→2 LaCl 3 +6 NH 3 +3 H 2O
This is followed by reduction with alkali or alkaline earth metals in vacuum or argon atmosphere:[17]
LaCl 3 +3 Li→La +3 LiCl
Also, pure lanthanum can be produced by electrolysis of molten mixture of anhydrousLaCl 3 andNaCl orKCl at elevated temperatures.[17]
The first historical application of lanthanum was in gas lanternmantles.Carl Auer von Welsbach used a mixture oflanthanum oxide andzirconium oxide, which he calledActinophor and patented in 1886. The original mantles gave a green-tinted light and were not very successful, and his first company, which established a factory inAtzgersdorf in 1887, failed in 1889.[53]
Modern uses of lanthanum include:
LaB 6 hot cathode
One material used for anodic material ofnickel–metal hydride batteries isLa(Ni 3.6Mn 0.4Al 0.3Co 0.7). Due to high cost to extract the other lanthanides, amischmetal with more than 50% of lanthanum is used instead of pure lanthanum. The compound is anintermetallic component of theAB 5 type.[54][55]NiMH batteries can be found in many models of theToyota Prius sold in the US. These larger nickel-metal hydride batteries require massive quantities of lanthanum for the production. The 2008 Toyota Prius NiMH battery requires 10 to 15 kilograms (22 to 33 lb) of lanthanum. As engineers push the technology to increase fuel efficiency, twice that amount of lanthanum could be required per vehicle.[56][57][58]
Hydrogen sponge alloys can contain lanthanum. These alloys are capable of storing up to 400 times their own volume of hydrogen gas in a reversible adsorption process. Heat energy is released every time they do so; therefore these alloys have possibilities in energy conservation systems.[29][59]
Lanthanum trifluoride (LaF 3) is an essential component of a heavy fluoride glass namedZBLAN. This glass has superior transmittance in the infrared range and is therefore used for fiber-optical communication systems.[62]
Cerium-dopedlanthanum bromide andlanthanum chloride are the recent inorganicscintillators, which have a combination of high light yield, best energy resolution, and fast response. Their high yield converts into superior energy resolution; moreover, the light output is very stable and quite high over a very wide range of temperatures, making it particularly attractive for high-temperature applications. These scintillators are already widely used commercially in detectors ofneutrons orgamma rays.[63]
Carbon arc lamps use a mixture of rare earth elements to improve the light quality. This application, especially by themotion picture industry for studio lighting and projection, consumed about 25% of the rare-earth compounds produced until the phase out of carbon arc lamps.[29][64]
Small amounts of lanthanum added tosteel improves itsmalleability, resistance to impact, andductility, whereas addition of lanthanum tomolybdenum decreases its hardness and sensitivity to temperature variations.[29]
Small amounts of lanthanum are present in many pool products to remove the phosphates that feed algae.[66]
Various compounds of lanthanum and other rare-earth elements (oxides, chlorides,triflates, etc.) are components of various catalysis, such aspetroleum crackingcatalysts.[69]
Lanthanum-bariumradiometric dating is used to estimate age of rocks and ores, though the technique has limited popularity.[70]
Lanthanum fluoride is used in phosphor lamp coatings. Mixed with europium fluoride, it is also applied in the crystal membrane offluoride ion-selective electrodes.[17]
Lanthanum-modified bentonite (orphoslock) is used to remove phosphates from water in lake treatments.[73]
Lanthanum telluride (La 3Te 4) is considered to be applied in the field of radioisotope power system (nuclear power plant) due to its significant conversion capabilities. The transmuted elements and isotopes in the segment will not react with the material itself, thus presenting no harm to the safety of the power plant. Though iodine, which can be generated during transmutation, is suspected to react withLa 3Te 4 segment, the quantity of iodine is small enough to pose no threat to the power system.[74]
Lanthanum has no known biological role in humans. The element is very poorly absorbed after oral administration and when injected its elimination is very slow.Lanthanum carbonate (Fosrenol) was approved as aphosphate binder to absorb excess phosphate in cases ofend stage renal disease.[71]
While lanthanum has pharmacological effects on several receptors and ion channels, its specificity for theGABA receptor is unique among trivalent cations. Lanthanum acts at the same modulatory site on theGABA receptor aszinc, a known negativeallosteric modulator. The lanthanum cationLa3+ is a positive allosteric modulator at native and recombinant GABA receptors, increasing open channel time and decreasing desensitization in a subunit configuration dependent manner.[75]
Lanthanum is a cofactor for themethanol dehydrogenase of themethanotrophic bacteriumMethylacidiphilum fumariolicum SolV, although the great chemical similarity of the lanthanides means that it may be substituted with cerium, praseodymium, or neodymium without ill effects, and with the smaller samarium, europium, or gadolinium giving no side effects other than slower growth.[76]
Lanthanum has a low to moderate level of toxicity and should be handled with care. The injection of lanthanum solutions produceshyperglycemia, low blood pressure, degeneration of thespleen andhepatic alterations.[citation needed] The application in carbon arc light led to the exposure of people to rare earth element oxides and fluorides, which sometimes led topneumoconiosis.[78][79] As theLa3+ ion is similar in size to theCa2+ ion, it is sometimes used as an easily traced substitute for the latter in medical studies.[80] Lanthanum, like the other lanthanides, is known to affect human metabolism, lowering cholesterol levels, blood pressure, appetite, and risk of blood coagulation. When injected into the brain, it acts as a painkiller, similarly tomorphine and other opiates, though the mechanism behind this is still unknown.[80] Lanthanum meant for ingestion, typically as a chewable tablet or oral powder, can interfere with gastrointestinal (GI) imaging by creating opacities throughout the GI tract; if chewable tablets are swallowed whole, they will dissolve but present initially as coin-shaped opacities in the stomach, potentially confused with ingested metal objects such as coins or batteries.[81]
The price for a (metric) ton [1000 kg] ofLanthanum oxide 99% (FOB China in USD/Mt) is given by the Institute of Rare Earths Elements and Strategic Metals (IREESM) as below $2,000 for most of the period from early 2001 to September 2010 (at $10,000 in the short term in 2008); it rose steeply to $140,000 in mid-2011 and fell back just as rapidly to $38,000 by early 2012.[82] The average price for the last six months (April–September 2022) is given by the IREESM as follows:Lanthanum Oxide - 99.9%min FOB China - 1308 EUR/mt and forLanthanum Metal - 99%min FOB China - 3706 EUR/mt.[83]
^The thermal expansion of α-La isanisotropic: the parameters (at 20 °C) for each crystal axis are αa = 2.9×10−6/K, αc = 9.5×10−6/K, and αaverage = αV/3 = 5.1×10−6/K.[3]
"L'oxide de cérium, extrait de la cérite par la procédé ordinaire, contient à peu près les deux cinquièmes de son poids de l'oxide du nouveau métal qui ne change que peu les propriétés du cérium, et qui s'y tient pour ainsi dire caché. Cette raison a engagé M. Mosander à donner au nouveau métal le nom deLantane."
[ The oxide of cerium, extracted from cerite by the usual procedure, contains almost two fifths of its weight in the oxide of the new metal, which differs only slightly from the properties of cerium, and which is held in it so to speak "hidden". This reason motivated Mr. Mosander to give to the new metal the nameLantane. ][46]
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