Recent discoveries in the synthesis and characterization of properties of rare earth nanomaterials are systematically reviewed. Francois Nief, Molecular Chemistry of the Rare-Earth Elements in Uncommon Low-Valent States 241.
INTRODUCTION
In addition, the accommodation of rare earth elements in the periodic table is still an active research topic. This was achieved by placing the rare earth metals between two groups of the periodic table.
A SHORT HISTORY OF THE DISCOVERY OF THE RARE-EARTH ELEMENTS
Figures 1 and 2 illustrate the chronology of the separation of "ceria" and "yttria" into the various rare earth elements. Thus, the small Swedish village of Ytterby occupies a prominent place in the history of rare earth elements.
DMITRII IVANOVICH MENDELEEV 1 Mendeleev’s Attempted System
- Rare Earths as a Primary Group
- Mendeleev’s Natural System of Elements
- Homologous Accommodation Methodology
- Mendeleev’s Rare-Earth Research
Therefore, Mendeleev remained very doubtful about the new positions of the rare earth elements. Later he still published papers on the applications of the periodic law to the rare earths (Mendeleev, 1873; Mendeleev, 1873a,b), on the corrected atomic weight of yttrium (Mendeleev, 1872), and on scandium (Mendeleev, 1881; Mendeleev, 1881). ).
BOHUSLAV BRAUNER AND SIR WILLIAM CROOKES 1 Brauner’s Rare-Earth Research
Meta-Elements
Crookes' spectroscopic studies of radiant matter in rare earths seemed to point to the complexity of the elements. The mineral kingdom, on the other hand, also pointed towards the creation of the elements. In 1886, Crookes produced an alternative representation of the periodic table, his Mighty Pendulum (Figure 11), in which he modeled a possible scenario of elemental evolution (Crookes, 1886l).
Asteriod Hypothesis
First of all, the periodicity was no longer impaired by the presence of rare earths. Second, the striking similarities in the chemical and physical properties of the rare earths were highlighted by the placement of these metals in only one case. Ultimately, a violation of the concept of single occupancy can only be avoided by assuming that the rare earth elements are constituent meta-elements of the same rare earth elemental group.
Intraperiodic Accommodation Methodologies
Indeed, Brauner's asteroid hypothesis could be confirmed on the basis of Crookes' concept of metaelements, but Brauner himself never took this step. The first editions of the books by Jones (1903) and Smith (1906) placed the rare earths in group IV of the periodic table. Another six textbook authors switched to the methodology of intraperiodic accommodation between FIGURE 19 Meyer's periodic table (1918) with intraperiodic accommodation of the rare earths.
NIELS BOHR AND HENRY MOSELEY 1 Bohr’s Atomic Theory
Moseley’s Research on X-Ray Spectra of Elements
Moseley was still working in Rutherford's group at Manchester when he published his first paper on the high-frequency spectra of the elements in the December 1913 issue of the Philosophical Magazine (Moseley and Darwin, 1913). This integer N was the atomic number of the element and was identified with the number of. Moseley's research was not only important in establishing the correct order of the elements in the periodic table.
The Controversial Element 72
He therefore proposed to stick to the magnetic susceptibility of the rare-earth elements as a measure of their elementality. The Copenhagen school, on the other hand, accepted Bohr's theory of the atom with open arms. While the claim of the French scientists rested on two very faint lines in the X-ray spectrum, their claim to the discovery of element 72 rested on six pronounced X-ray lines.
The Elusive Element 61
They concluded that the element 61 differed in chemical behavior from the rare-earth metals or that it does not exist at all. Ida Noddack suspected that element 61 was radioactive and hypothesized that its half-life was shorter than the age of the Earth. So it turned out that during the fission process of uranium-235, isotopes of the element 61 could also be produced.
Intergroup Accommodation Methodologies
On the other hand, the name warned everyone about the "War Eagle". The discovery of promethium was first announced at a meeting of the American Chemical Society in New York in September 1947 (Marinsky et al., 1947). This was achieved by accommodating the rare earths between two groups of the periodic table. Note that the cobalt-nickel pair is an example of a deviation from increasing atomic weight in the periodic table.
SEABORG’S ACTINIDE CONCEPT
Both authors reserved spaces in their periodic tables for other members of the second series of rare earths, which were still undiscovered at the time. The actinide elements do not tend to occupy the 6d orbital, but there is gradual filling of the 5f shell through the actinide series. A detailed description of the development of the actinide concept can be found in Chapter 118 of this handbook (Seaborg, 1994).
RARE-EARTH CRISIS ANNO 2010
The D-block has been torn apart in the long form, due to the insertion of the f-block. The placement of the elements in the periodic table is based on electronic configurations and the concept of the differentiating electron. This also implies that the preferred representation of the intermediate periodic table is 14LaAc (Figure 31).
CONCLUSIONS
He thus severed all relations with the other elements and consequently put the individuality of the rare earth elements in the spotlight. Contemporary discussions of the ``rare earth crisis'' have shifted from the accommodation of the rare earths as a whole to the placement of lanthanum and lutetium in particular. Thus, chemists argue about the methods to draw the boundaries of the rare earth island.
ACKNOWLEDGMENT
HISTORICAL INTRODUCTION
- The first detection and synthesis of endohedral metallofullerenes
The first production of macroscopic quantities of endohedral metallofullerenes was also reported by the Rice group (Chai et al., 1991). The specificity of La@C82fullerene was soon confirmed by Whetten and co-workers (Alvarez et al., 1991). Scuseria and co-workers made a similar theoretical prediction for Sc@C60 (Guo et al., 1994).
SYNTHESIS, EXTRACTION FROM SOOT, AND SEPARATION/PURIFICATION
- Synthesis of endohedral metallofullerenes
- Solvent extraction from primary soot containing metallofullerenes
In the laser furnace method (Figure 3), a target composite rod or disc for laser evaporation, consisting of metal oxide/graphite with a high-strength pitch binder, is placed in a furnace at 1200 C (Haufler et al. ., 1991 ). The laser furnace method is suitable for the study of growth mechanism of fullerenes and metallofullerenes (Curl and Smalley, 1991; Haufler et al., 1991). The fullerene smoke (soot) rising along a convection flow around the evaporation source has the maximum content of metallofullerenes ( Saito et al., 1996).
PURIFICATION AND ISOLATION OF METALLOFULLERENES 1 Purification by liquid chromatography
- HPLC purification on metallofullerenes
In the first HPLC stage, the toluene solution of the extracts was separated by a preparative recycling HPLC system (Japan Analytical Industry LC-908-C60) with a Trident-Tri-DNP column (Buckyclutcher I, 21 mm 500 mm: Regis Chemical) or a 5-PBB (pentabromobenzyl) column (20 mm250 mm, Nacalai Tesque) with CS2 eluent. It has been found (Inakuma et al., 1995; Yamamoto et al., 1994a,b) that most of the monometallofullerenes, M@C82, have at least two types of structural isomers (commonly called I and II), which can be separated by the two-stage HPLC technique. For ESR-active metal lofullerenes, the observation of the corresponding hyperfine structures can further confirm the identification and isolation.
MOLECULAR AND CRYSTAL STRUCTURES OF METALLOFULLERENES
- Endohedral or exohedral?
- Structural isomers obeying the so-called IPR
- Confirmation on endohedral structures as determined by synchrotron X-ray diffraction
- Crystal structures
The endohedral structure of Sc@C82 was also studied by synchrotron X-ray diffraction with MEM analysis (Takata et al., 1998). The charge state corresponds to an ultraviolet photoelectron spectroscopy (UPS) experiment (Hino et al., 1998). A synchrotron X-ray structural study on Sc3@C82 was recently reported based on Rietveld/MEM analysis (Takata et al., 1999).
CARBIDE METALLOFULLERENES
One of the main reasons for the prevalence of carbide di- or tri-metallofullerenes is that two or three positively charged metal atoms in fullerene cages can bind tightly together around a negatively charged C2 molecule (Inoue et al., 2004).
ELECTRONIC STATES AND STRUCTURES
- Intrafullerene electron transfer within the carbon cage
- ESR detection of structural isomers of metallofullerenes
- Electrochemistry on metallofullerenes
- Similarity of UV–Vis–NIR absorption spectra
- The Fermi level and the electronic structure
The temperature-dependent linewidths of the ESR hfs of La@C82, Sc@C82 and Gd@C82 have been discussed by Kato et al. The CV measurements indicate that the oxidation state of the yttrium atom in Y@C82 (Suzuki et al., 1996 ) was close to that of La@C82, probably 3þ. The absorption spectra of the three isomers (A, B, C) of Tm@C82 are almost exactly the same as three structural isomers (III, I, IV) of Ca@C82, respectively, suggesting that each isomer shares the same isomeric structure.
META-CAGE VIBRATION WITHIN METALLOFULLERENES
The peaks around 150 cm1 can be attributed to internal vibrational modes, probably metal-to-cage vibrations. The frequency of the metal-to-cage vibration is only slightly affected by the isomerization of the fullerene cage, but it strongly depends on the type of metal ion inside the fullerene. In general, the metal-to-cage peaks around 100 cm1 are almost invariant among the isomers, but are much smaller than those for M@C82(M¼La, Y, Ce, Gd) described above.
STM STUDIES ON METALLOFULLERENES
- STM studies of metallofullerenes on clean surfaces
- Metallofullerenes as superatoms
The STM image of the Sc2C2@C82 molecules shows a slight deviation (within about 10%) from the perfectly round shape. The white contrasts correspond to the Sc2C2@C82 fullerenes slightly ahead of the other close packed fullerenes. In a Y@C82–Y@C82fullerene interaction, the positively charged (Z¼ þ3) yttrium nucleus on one side of Y@C82 attracts the negatively charged (Z¼ 3) C82 cage of the other Y@C82 molecule.
MAGNETISM OF METALLOFULLERENES
In fact, a synchrotron X-ray diffraction study on a Y@C82 powder sample (Takata et al., 1995) reveals the presence of such a charge-transfer-type interaction from an analysis of the entire Y@C82 microcrystal electron density distribution map. The apparent discrepancy in results between this magnetic ESR measurement (metallic) and UPS studies on La@C82 (insulator) (Hino et al., 1993; Poirier et al., 1994), as described in Section 6.5, may be due to sample preparation , that is, the purity of the sample along with the thickness of the sublimated samples. This is consistent with their study of the small electronic coupling between fans and peelectrons and the shallow potentials of the C82 fullerene cage surrounding the Ce ion.
Interestingly, when heated, almost all of the various ring isomers spontaneously convert to endohedral metallofullerenes, trapping the metal atom within the cage with high efficiency (>98%) (Clemmer et al., 1994b). Since pyridine and aniline are not suitable HPLC solvents (eluents) for purification, the purification and isolation of M@C60-type metallofullerenes has been extremely difficult. Despite these difficulties, Ogawa et al. 2000) performed the first isolation of an M@C60-type metallofullerene, Er@C60.
APPLICATIONS TO RADIOCHEMISTRY AND MRI CONTRAST AGENTS
The commercially available MRI contrast agents usually use Gd3þ(S¼7/2) to enhance the relaxation rate of water protons. Purified Gd@C82fullerene was organically functionalized to obtain a water-soluble property and was then evaluated for use as an MRI contrast agent in terms of relaxation measurements. 1997) reported relatively low relaxivity at 20 MHz compared to other conventional Gd-containing MRI contrast agents. This is encouraging for the prospects of future use of this water-soluble Gd-metallofullerene.
CONCLUSIONS AND FUTURE PROSPECTS
Organic functionalizations of the metallofullerenes (Akasaka et al., 1995a,b,c; Kato et al., 1996; Suzuki et al., 1995c) will be an important direction for the synthesis of further new materials based on endohedral metallofullerenes, because as Akasaka et al. 1995a,b,c) have found that metallofullerenes are generally more reactive, either thermally or photochemically, than the corresponding empty fullerenes due to the small HOMO-LUMO holes. As described in the previous section, physiological and medicinal applications of the endohedral metallofullerenes will become extremely important in relation to trace element chemistry in biological systems and await further studies. In any case, the endohedral metallofullerenes will continue to tempt physicists, chemists and materials scientists for years to come.
ACKNOWLEDGMENTS
SYNTHESIS CONDITIONS
Direct synthesis of the elemental constituents in evacuated quartz ampoules has been used in most reported cases. The ampoules with the mixtures of elements were heated to different predetermined maximum temperatures, for example 1420 K, and were kept at this temperature for several hours. The ampoules were then slowly cooled to a respective annealing temperature, for example 870 K, and annealed at this temperature for many hours.
TERNARY SYSTEMS
- Ternary R–Si–X (X ¼ S, Se, Te) systems
- Ternary R–Ge–X (X ¼ S, Se, Te) systems
- Ternary R–Sn–X (X ¼ S, Se, Te) systems
- Ternary R–Pb–X (X ¼ S, Se, Te) systems
An investigation of the Eu–Sn–X (X¼S, Se) systems led to five different chalcogenides that adopt orthorhombic symmetry. They crystallize in the cubic body-centered and orthorhombic primitives, and face-centered unit cells. Two chalcogen-poor La5Pb3S and La5Pb3Se compounds (SGP63/mcm) were synthesized in the R–Pb–X (X¼S, Se) systems.
