Lanthanide refers to element 57 in the periodic tablelanthanumTo element 71Lutetium15 elementGeneral term。Their chemical properties are similar and they form a series separately, occupying a special place in the periodic table of elements.lanthanumIt is a series of elements (La), scandium (Sc) and yttrium (Y). A total of 17 elements are called rare earth elements (RE).La (lanthanum), Ce (cerium), Pr (praseodymium), Nd (neodymium), Pm (promethium), Sm (samarium), Eu (europium) are called cerium group rare earth (light rare earth);Gd (gadolinium), Tb (terbium), Dy (dysprosium), Ho (holmium), Er (erbium), Tm (thulium), Yb (ytterbium), Lu (lutetium), Sc, Y are called yttrium group rare earth (medium heavy rare earth).[1]
Lanthanide elements are all active metals with very strong reduction ability. Their activity is only inferior to alkali metals and alkaline earth metals, and stronger than aluminum, zinc and other elements.Among the lanthanides, La is the most active.Lanthanide elements easily react with halogen, oxygen, acid, sulfur, nitrogen, hydrogen, etc.Therefore, in order to avoid the oxidation of lanthanide metal elements, the surface needs to be coated with wax during storage.
Oxalate, carbonate and phosphate of lanthanide elements are difficult to dissolve in water, while salts formed by lanthanide metal elements and strong acids of sulfuric acid, nitric acid and hydrochloric acid are easy to dissolve in water, and the crystallized salt generally contains crystal water.[2]
Rare earth elements are not rare, but they are scattered in the crust and similar in nature, so it is difficult to extract and separate them.The content of elements with even atomic numbers is generally higher than that of adjacent elements with odd atomic numbers[4]。
Mineral reserves
According to its existing form, there are three types of ore sources:
1. Rare earth symbiosis constitutes independent rare earth element minerals.
2. It is dispersed in calcite, apatite and other minerals in the form of isomorphism.
3. The adsorption state exists in clay ore, mica ore and other minerals.
our countryrare earth——Large reserves, wide distribution, complete minerals and easy exploitation.In the southHeavy rare earthMainly, Inner MongoliaLight rare earthMainly.
Rare earth minerals with important industrial significance includeXenotime（YPOfour）、Bastnasite（Ce（COthree）F) , limonite (YNbOfour）Etc.
grouping
According to the gradual change of rare earth element properties, the rare earth elements are divided into the following groups[4]：
one
According to the electronic layer configuration of atoms and their atomic weight, the rare earth elements are divided into two groups: the lanthanides before europium are called light rare earth elements or cerium group elements;Adding yttrium to lanthanides after europium is called heavy rare earth element or yttrium group element.
two
According to rare earth sulfate solution and NatwoSOfourThe solubility of rare earth element sulfate double salt in aqueous solution can be divided into three groups: namely, the rare earth element sulfate double salt from lanthanum to samarium is insoluble, called cerium group;Europium to dysprosium are slightly soluble in sulfate double salt, which is called terbium group;Yttrium and holmium to lutetium sulfate double salts are easily soluble, which is called yttrium group.Some people also call terbium group as medium rare earth element.
physical property
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Lanthanide metal is silver white, soft and ductile[5]。Its activity is second only to alkali metals andalkaline-earth metal, should be kept away from air.The order of metal activity increases from Sc, Y, La;It decreases from La to Lu, that is, La is the most active.The density of lanthanide metal increases with the atomic number, from La to Lu.But the density of Eu and Yb is small.Lanthanide metal is a strong reducing agent, its reduction ability is only inferior to Mg, and its reactivity is comparable to that of aluminum.Moreover, with the increase of atomic number, the reduction ability gradually weakened.La in acid solution2+Ion is a strong reducing agent, La4+Ions are strong oxidants.
chemical property
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Lanthanide metal is a strong reducing agent, its reduction ability is only inferior to Mg, and its reactivity is comparable to that of aluminum.Moreover, with the increase of atomic number, the reduction ability gradually weakens[5]。
La in acid solution2+Ion is a strong reducing agent, La4+Ions are strong oxidants.
Since the elements of the lanthanide and actinide seriesAtomic numberThe increase of is only filled with electrons in the inner orbits (corresponding 4f and 5f orbits), and its outer orbits (corresponding6s、5dand7s6d track)Electronic layoutBasically the same, so the lanthanides andActinide elementNot only are the chemical properties similar, but also the chemical properties of the elements in each series are similar.Most actinides have the following properties: they can formComplex ionAnd organic chelatescation；Produce trivalent insoluble compounds, such ashydroxide、fluoride、carbonateandoxalateEtc;Produce trivalent soluble compounds, such assulfate、Nitrate、PerchlorateAnd some halides, etc.stayaqueous solutionMost actinides are in the+3 oxidation state. The first and last actinides have different oxidation states, such asProtactiniumThere is+5 oxidation state;uranium、Neptunium、AmericiumThere are+5 and+6 oxidation states,NeptuniumAnd plutonium have+7 oxidation state, which can be MOMr、MO卂MO 幯 exists in the form of plasma (the highest oxidation state of lanthanide is+4);CaliforniumEinsteinium、Of、ExtraandNanometerAnd other elements have+2 oxidation state.The difference between actinides and lanthanides is that the energy required for 5f electrons in light actinides to be excited to the 6d orbit is less than the energy required for 4f electrons in corresponding lanthanides to be excited to the 5d orbit, which makes actinides have more bonding electrons than lanthanides, resulting in a higher oxidation state;The heavy actinides are just the opposite.
Electron absorption spectrum
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characteristic
1. Most La3+Ions are absorbed in the visible light region[6]。
2. Rare earth ions with the same unpaired f electrons have similar colors.
3.La3+Ions are produced by f-f transition.The f-f transition belongs to the forbidden transition, and its absorption spectrum isMolar extinction coefficientVery small (about 0.51 · mol-1·cm-3）。
4. Its absorption spectrum is atom like linear spectrum.
5. La can also occur3+InterligandCharge transfer spectrumAt this time, the molar extinction coefficient is large (about 50~1000l · mol-1·cm-3）。
Ion magnetism
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The magnetic properties of lanthanides are complex. Because 4f electrons can be well shielded by 5s and 5p electrons, lanthanides are less affected by the external electric field, and the contribution of orbital motion to the magnetic moment is not suppressed by the electric field effect of surrounding coordination atoms. Therefore, when calculating their magnetic moments, the influence of both electron spin and orbital motion on the magnetic moment must be considered[4]。
The large number of unpaired electrons in lanthanide elements and compounds, together with the contribution of electronic orbital movement to the magnetic moment, make them have good magnetism and can be used to make goodmagnetic material Rare earth alloys can also be used as permanent magnetic materials.
application
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application area
Lanthanides are widely used.It is mainly used as a catalyst in the chemical industry.For example, chlorides and phosphates mixed with lanthanides are used as catalysts to accelerate the cracking and decomposition of petroleum.blendrare earth oxide Widely used as glassPolishing materialIt can also be used to manufacture radiation resistant glass andLaser glass。useYttrium trioxideAnd Dysprosium trioxide to produce high temperature resistant transparent ceramics, which are used in rockets, lasersvacuumAnd other technical projects.In addition, the phosphor widely used in the TV industry is some rare earth compounds, and this phosphor is used to make TV screens.
Adding a small amount of rare earth elements in steel can greatly improve theMechanical propertiesTherefore, rare earth elements can be called "vitamins" of steel.For example, adding cerium into pig iron can obtain nodular cast iron, making pig iron tough and wear-resistant. Iron can replace steel, and casting can replace forging[7]。
In addition, the use of rare earth elements in agriculture can increase grain production by 10%~20%, cabbage production by 29%, soybean production by 50%, and also can increase the yield and sweetness of watermelon, so it can be used as efficient micro fertilizer.
Oxidized state
Lanthanides in solid statewaterCharacteristics in solution or other solventsOxidized stateIs+3.Because the lanthanide elementGaseousWhen two 6s electrons and one 5d electron or two 6s electrons and one 4f electron are lostionization energyIt is relatively low, so it can generally form a stable+3 oxidation state.In addition to the+3 characteristic oxidation state, lanthanides also have some unusual oxidation states.For example:cerium、praseodymium、neodymium, terbium and dysprosium exist in the+4 oxidation state, because their 4f layer is relatively stable when it is kept or close to the full empty, half full or full full state, but only the+4 oxidation state of cerium can exist in the solution, which is a strong oxidant.
Similarly, cerium, neodymium, samarium, europium, thulium and ytterbium also exist in the+2 oxidation state.
