Neodymium(III) chloride or neodymium trichloride is a chemical compound of neodymium and chlorine with the formula NdCl3. This anhydrous compound is a mauve-colored solid that rapidly absorbs water on exposure to air to form a purple-colored hexahydrate, NdCl3·6H2O. Neodymium(III) chloride is produced from minerals monazite and bastnäsite using a complex multistage extraction process. The chloride has several important applications as an intermediate chemical for production of neodymium metal and neodymium-based lasers and optical fibers. Other applications include a catalyst in organic synthesis and in decomposition of waste water contamination, corrosion protection of aluminium and its alloys, and fluorescent labeling of organic molecules (DNA).
Appearance
NdCl3 is a mauve colored hygroscopic solid whose color changes to purple upon absorption of atmospheric water. The resulting hydrate, like many other neodymium salts, has the interesting property that it appears different colors under fluorescent light- In the chloride's case, light yellow (see picture).
Structure
Solid The anhydrous NdCl3 features Nd in a nine-coordinate tricapped trigonal prismatic geometry and crystallizes with the UCl3 structure. This hexagonal structure is common for many halogenated lanthanides and actinides such as LaCl3, LaBr3, SmCl3, PrCl3, EuCl3, CeCl3, CeBr3, GdCl3, AmCl3 and TbCl3 but not for YbCl3 and LuCl3.
Solution The structure of neodymium(III) chloride in solution crucially depends on the solvent: In water, the major species are Nd(H2O)83+, and this situation is common for most rare earth chlorides and bromides. In methanol, the species are NdCl2(CH3OH)6+ and in hydrochloric acid NdCl(H2O)72+. The coordination of neodymium is octahedral (8-fold) in all cases, but the ligand structure is different.
Properties NdCl3 is a soft paramagnetic solid, which turns ferromagnetic at very low temperature of 0.5 K. Its electrical conductivity is about 240 S/m and heat capacity is ~100 J/(mol·K). NdCl3 is readily soluble in water and ethanol, but not in chloroform or ether. Reduction of NdCl3 with Nd metal at temperatures above 650 °C yields NdCl2:
2 NdCl3 + Nd → 3 NdCl2 Heating of NdCl3 with water vapors or silica produces neodymium oxochloride:
NdCl3 + H2O → NdOCl + 2 HCl 2 NdCl3 + SiO2 → 2 NdOCl + SiCl4 Reacting NdCl3 with hydrogen sulfide at about 1100 °C produces neodymium sulfide:
2 NdCl3 + 3 H2S → 2 Nd2S3 + 6 HCl Reactions with ammonia and phosphine at high temperatures yield neodymium nitride and phosphide, respectively:
NdCl3 + NH3 → NdN + 3 HCl NdCl3 + PH3 → NdP + 3 HCl Whereas the addition of hydrofluoric acid produces neodymium fluoride:
NdCl3 + 3 HF → NdF3 + 3 HCl
Preparation
NdCl3 is produced from minerals monazite and bastnäsite. The synthesis is complex because of the low abundance of neodymium in the Earth's crust (38 mg/kg) and because of difficulty of separating neodymium from other lanthanides. The process is however easier for neodymium than for other lanthanides because of its relatively high content in the mineral – up to 16% by weight, which is the third highest after cerium and lanthanum. Many synthesis varieties exist and one can be simplified as follows: The crushed mineral is treated with hot concentrated sulfuric acid to produce water-soluble sulfates of rare earths. The acidic filtrates are partially neutralized with sodium hydroxide to pH 3–4. Thorium precipitates out of solution as hydroxide and is removed. After that the solution is treated with ammonium oxalate to convert rare earths into their insoluble oxalates. The oxalates are converted to oxides by annealing. The oxides are dissolved in nitric acid that excludes the main components, cerium, whose oxide is insoluble in HNO3. Neodymium oxide is separated from other rare-earth oxides by ion exchange. In this process, rare-earth ions are adsorbed onto suitable resin by ion exchange with hydrogen, ammonium or cupric ions present in the resin. The rare earth ions are then selectively washed out by suitable complexing agent, such as ammonium citrate or nitrilotracetate. This process normally yields Nd2O3; the oxide is difficult to directly convert to elemental neodymium, which is often the goal of the whole technological procedure. Therefore, the oxide is treated with hydrochloric acid and ammonium chloride to produce the less stable NdCl3:
Nd2O3 + 6 NH4Cl → 2 NdCl3 + 3 H2O + 6 NH3 The thus produced NdCl3 quickly absorbs water and converts to NdCl3·6H2O hydrate, which is stable for storage, and can be converted back into NdCl3 when necessary. Simple rapid heating of the hydrate is not practical for that purpose because it causes hydrolysis with consequent production of Nd2O3. Therefore, anhydrous NdCl3 is prepared by dehydration of the hydrate either by slowly heating to 400 °C with 4-6 equivalents of ammonium chloride under high vacuum, or by heating with an excess of thionyl chloride for several hours. The NdCl3 can alternatively be prepared by reacting neodymium metal with hydrogen chloride or chlorine, though this method is not economical due to the relatively high price of the metal and is used for research purposes only. After preparation, it is usually purified by high temperature sublimation under high vacuum.
Applications
Production of neodymium metal
Neodymium(III) chloride is the most common starting compound for production of neodymium metal. NdCl3 is heated with ammonium chloride or ammonium fluoride and hydrofluoric acid or with alkali or alkaline earth metals in vacuum or argon atmosphere at 300–400 °C.
2 NdCl3 + 3 Ca → 2 Nd + 3 CaCl2 An alternative route is electrolysis of molten mixture of anhydrous NdCl3 and NaCl, KCl, or LiCl at temperatures about 700 °C. The mixture melts at those temperatures, even though they are lower than the melting points of NdCl3 and KCl (~770 °C).
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