Rare earth element oxides

The lanthanides, distributed widely in low concentrations throughout the earth s cmst (2), are found as mixtures in many massive rock formations, eg, basalts, granites, gneisses, shales, and siUcate rocks, where they are present in quantities of 10—300 ppm. Lanthanides also occur in some 160 discrete minerals, most of them rare, but in which the rare-earth (RE) content, expressed as oxide, can be as high as 60% rare-earth oxide (REO). Lanthanides do not occur in nature in the elemental state and do not occur in minerals as individual elements, but as mixtures. 

Cerium was the first rare-earth element discovered, and its discovery came in 1803 by Jons Jakob Berzelius in Vienna. Johann Gadohn (1760—1852) also studied some minerals that were different from others known at that time. Because they were different from the common earth elements but were all very similar to each other, he named them rare-earth elements. However, he was unable to separate or identify them. In the 1800s only two rare-earths were known. At that time, they were known as yttria and ceria. Carl Gustav Mosander (1797—1858) and several other scientists attempted to separate the impurities in these two elements. In 1839 Mosander treated cerium nitrate with dilute nitric acid, which yielded a new rare-earth oxide he called lanthanum. Mosander is credited with its discovery. This caused a change in the periodic table because the separation produced two new elements. Mosander s method for separating rare-earths from a common mineral or from each other led other chemists to use... 

Symbol Nd atomic number 60 atomic weight 144.24 a rare earth lanthanide element a hght rare earth metal of cerium group an inner transition metal characterized by partially filled 4/ subshell electron configuration [Xe]4/35di6s2 most common valence state -i-3 other oxidation state +2 standard electrode potential, Nd + -i- 3e -2.323 V atomic radius 1.821 A (for CN 12) ionic radius, Nd + 0.995A atomic volume 20.60 cc/mol ionization potential 6.31 eV seven stable isotopes Nd-142 (27.13%), Nd-143 (12.20%), Nd-144 (23.87%), Nd-145 (8.29%), Nd-146 (17.18%), Nd-148 (5.72%), Nd-150 (5.60%) twenty-three radioisotopes are known in the mass range 127-141, 147, 149, 151-156. 

Mosander extracted from the mineral lanthana a rare earth fraction, named didymia in 1841. In 1879, Boisbaudran separated a rare earth oxide called samaria (samarium oxide) from the didymia fraction obtained from the mineral samarskite. Soon after that in 1885, Baron Auer von Welsbach isolated two other rare earths from didymia. He named them as praseodymia (green twin) and neodymia (new twin) after their source didymia (twin). The name praseodymium finally was assigned to this new element, derived from the two Greek words, prasios meaning green and didymos meaning twin. 

The discovery of samarium is credited to Boisbaudran, who in 1879 separated its oxide, samaria from Mosander s didymia, the mixture of rare earth oxides from which cerium and lanthanum were isolated earher. Demarcay in 1901 first identified samaria to be a mixture of samarium and europium oxides. The element got its name from its mineral, samarskite. The mineral, in turn, was named in honor of the Russian mine official Col. Samarki. 

If one adds to this the other applications, the consunption was probably between 2 000 - 3 000 tons of oxides. On the other hand, at the same time, about 7 500 tons of thorium nitrate were needed for Auer incandescent mantles. If one assumes that monazite contains 6 % thorium oxide and 60 % rare earth elements, then 30 000 tons of rare earth oxides were produced during this period of vhich only about 10 % was consumed. 

Period of Wide Technical Application. In the time between 1930 and 1940 work was done on various applications for the rare earth elements. Particularly successful was the production of sunglasses ("Neophan"), polishing media from rare earth oxides to replace iron oxide, decolorization of glass using cerium oxide, pure cerivim oxide as opacifier in ceramic glazes, use of cerium... 

For particular applications the rare earth elements are used in various purities according to vhether the general properties of the rare earth elements or specific properties of individual elements are needed. The rare earth industry distinguishes in general three grades of purity the grou of unseparated rare earth elements in the composition vhich occurs naturally in ores concentrates producible by simple chemical precipitation reactions ich in general contain 60 - 90 % of the individual element desired and the pure rare earth elements v ich contain between 98 and 99.999 % of a rare earth oxide. 

World demand for the rare earth elements is established in the range of 25 000 tons per year - calculated as rare earth oxides. 

Elements from selenium through the middle rare earths will be present in the mixed fission product population they exhibit a wide variety of volatilities (1). The elements Y, Zr, and Nb and the rare earth oxides are high boiling and condensable at low partial pressures, whereas the noble gases, and the alkali metals Mo, Tc, Pd, Ag, Cd, Sn, Sb, Te, Ru, and perhaps Rh, are very volatile in a relative sense Sr and Ba are predicted to be of refractory or intermediate behavior. 

RARE-EARTH ELEMENTS AND METALS. Sometimes referred to as the fraternal fifteen," because of similarities in physical and chemical properties, the rare-earth elements actually are not so rare. This is attested by Fig. 1, which shows a dry lake bed in California that alone contains well in excess of one million pounds of two of die elements, neodymium and praseodymium. The world s largest rare earth body and mine near Baotou, Inner Mongolia, China is shown in Fig. 2. It contains 25 million tons of rare earth oxides (about one quarter of the world s human reserves. The term rare arises from the fact that these elements were discovered in scarce materials. The term earth stems from die tact that the elements were first isolated from their ores in the chemical form of oxides and that the old chemical terminology for oxide is earth. The rare-earth elements, also termed Lanthanides, are similar in that they share a valence of 3 and are treated as a separate side branch of the periodic table, much like die Actinides. See also Actinide Contraction Chemical Elements Lanthanide Series and Periodic Table of the Elements. 

Until 1964, monazite, a thorium-rare-earth phosphate, REPO4TI13 (P04)4, was the main source for the rare-earth elements. Australia, India, Brazil. Malaysia, and the United Slates are active sources. India and Brazil supply a mixed rare-earth chloride compound after thorium is removed chemically from monazite. Bastnasite, a rare-earth fluocarbonate mineral REFCO3. is a primary source for light rare earths. From 1965 to about 1985. an open-pit resource at Mountain Pass, California, has furnished about two-thirds of world requirements for rare-earth oxides. In the early... 

Figure 1 Glow discharge mass spectrometry (GD-MS) spectrum of a mixture of rare earth oxides compacted in a tantalum host matrix. Oxide composition 5% by weight in disk, each element present at 110 ppm. (From Ref. 39.)...

As mentioned in section 6.2, binary rare-earth oxide fluorides, LnLn O vF,., where Ln and Ln are different rare-earth elements coming respectively from Ln203 and Ln F3... 

This chapter is organized in three main parts. In the first one the deposition and growth techniques generally used for the preparation of metal and metal oxide films and particles will be reported. The other two parts discuss the deposition of metals and metal oxides according to their classification as main group, transition metal and rare earth metal elements. 

Oxides of the lanthanide rare earth elements share some of the properties of transition-metal oxides, at least for cations that can have two stable valence states. (None of the lanthanide rare earth cations have more than two ionic valence states.) Oxides of those elements that can only have a single ionic valence are subject to the limitations imposed on similar non-transition-metal oxides. One actinide rare-earth oxide, UO2, has understandably received quite a bit of attention from surface scientists [1]. Since U can exist in four non-zero valence states, UO2 behaves more like the transition-metal oxides. The electronic properties of rare-earth oxides differ from those of transition-metal oxides, however, because of the presence of partially filled f-electron shells, where the f-electrons are spatially more highly localized than are d-electrons. 

All the actinide elements whose oxides have been studied can be made as fluorite-type dioxides. For most of them this is their most stable form in air at room temperature. The fluorite-related phases of each actinide element known will be discussed individually before a comparison is made with the rare earth oxides 45). 

Tetramethyl-3,5-heptanedionates. Ten millimoles of each of the rare-earth chelates were prepared by the method of Eisentraut and Sievers (9). H(thd) from the Pierce Chemical Co., Rockford, 111. was used without further purification. Five mmoles of the 99.9% rare earth oxide (Michigan Chemical Corp., Saint Louis, Mich.) were dissolved in the stoichiometric amount of 6N HNO3, and appropriate amounts of H2O and 95% EtOH were added to make 50 ml. of 50% ethanol solution containing the required amount of rare-earth nitrate. The dried product was sublimed at 180°C. in vacuo, recrystallized from n-hexane in vacuo, and vacuum dried. Although no elemental analyses were made on the final product, the melting points were taken on a Fisher-Jones melting point apparatus, and the results obtained were compared with the literature values shown in Table XII. The products were stored in evacuated desiccators. 

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