Zuckermann, Transport properties (electrical resistivity, thermoelectric power and thermal conductivity) of rare earth intermetallic compounds 117. Information on the holohedral symmetry of the rare earth site was obtained by Durville et al.
Intensities of absorption and emission
Broer, G o r t e r and Hoogschagen (B r o e r et ah, 1945) analyzed this situation, based on a fairly complete set of P values for aqua ions between 9000 and 30,000 cm-1, and concluded that the main mechanism for transitions is induced electrical energy is. dipole resulting from a very weak order-of-magnitude mixing of the ground state 4f q wave function (being odd for q odd, and even for q even) with functions of the opposite parity. There is one well-characterized exception to the selection rules (Broer et al., 1945) of the Ju d d - Of e l t treatment.
Luminescence
The mirrors can be placed separately or vaporized directly on the ends of the rod. This equation combines the requirements for the qualities of the resonator design (L and 7) and the gain medium related to the population inversion AN and the radiative transition probabilities reflected in the absorption coefficient k(v)0.
Nonradiative processes
Temperature dependence and lifetime of the excited states
Their model assumes that the phonons with single highest frequencies are active in the non-radiative transition. Experimentally, the multiphonon emission rate from an excited state i to an adjacent lower-lying state j in the absence of energy transfer is given by.
Multipolar interaction
Phonon-assisted transfer
A practical consequence of the energy transfer between UO~2 and Nd(III) can be applied to LSC, since the existence of two ions in a glass increases the absorption range of the spectrum (Reisfeld and Kalisky, 1980b). Estimation of energy transfer between Cr(III) and Nd(III), by both static and dynamic methods, is presented by Reisfeld and Kisilev (1985).
Recent nonoxide materials
Sulfide glasses
In addition to elemental selenium, some glasses containing germanium, arsenic, selenium, and tellurium are known (Reisfeld et al., 1977b), but they are of limited interest for lanthanide luminescence because they are transparent only in the far infrared. The two main categories are RxLal_xAI3S 6 and R , La1_xGa3S6, being transparent in yellow and with an absorption edge that rises rapidly in green and blue. EXCITED STATE PHENOMENA IN GLASSY MATERIALS 73 monomeric complexes of sulfur-containing ligands are also striking in d-groups (J0rgensen, 1962d, 1968).
Both Yb2S 3 and BaYb2S 4 are bright yellow due to simple electron transfer bands in the near ultraviolet (J0rgensen 1962c) as in the lemon yellow Yb(III) dithiocarbamates.
Chemical properties of excited states
In view of the notorious variability of cerium(IV) electron transfer bands with the neighboring atoms, it is not certain that this species differs greatly from Ce(IV), in the dark with a band at 240 nm. The interpretation by Okada et al is that during the short lifetime of the excited state in the crystal, one water molecule is detached by a photochemical reaction, and that the observed emission is due to Ce(OH2)~ 3. Although not perfectly is not. certain that the luminescence of Ce(III) in the ennea-aqua-ethyl sulfate is emitted by the same Ce(OH2)~ 3 that produces the weak absorption band at 33800 cm -a in the aqueous solution, it is possible to make a reliable estimate gives the concentration of the latter species.
It is therefore not surprising that the excited state of an aqua ion can be much more acidic than the ground state, being highly oxidizing and chemically polarizable (Pearson-soft) and perhaps having a lower coordination number N when realized (JCrgensen , 1984) that water ions of Fe(III), Cu(II), Pd(II), Au(III), Hg(II) and TI(III) deprotonate much more easily than water ions with the same oxidation state and with comparable ionic radii.
Conclusions
We have to recognize (Jørgensen, 1983) that internuclear distances are optimized in such a way that they give the lowest possible energy of the compound; they are not the quantum mechanical properties of the individual atom. The purpose of this chapter is to point out the spectroscopic and other physical properties of isotropic glasses as an interesting alternative to crystalline compounds (including substituted doped crystals with colorless lanthanum or yttrium compounds as the main constituent). In one way, it is surprising how close many of the similarities are, and also how technologically attractive glasses can be as laser materials compared to crystals.
Cross-white, 1977, Energy level structure and transition probabilities of the trivalent lanthanides in LaF 3 (Argonne National Laboratory Report).
Rare earth phosphates and related phosphor compounds Introduction
Anhydrous rare earth orthophosphates have been prepared by the fusion method (Duboin, 1888) and by chemical reactions (Mooney, 1950; Schwarz, 1963f). Single crystals were obtained by hydrothermal crystallization from a mixture of rare earth hydroxide and phosphoric acid (Anthony, 1957; Carron et al., 1958) and by flux growth from lead phosphate (Feigelson, 1964; Smith and Wanklyn, 1974). Rare earth phosphates in the middle of the array containing several water molecules have also been reported to be hexagonal (Hezel and Ross, 1967; . Kuznetsov et al., 1969).
When the rare earth phosphates are precipitated with sodium phosphate, not only binary and ternary phosphates but also hydroxide phosphates can be obtained (Tananaev and Vasileva, 1963).
YbPsOtq
A perspective, view along the a-axis showing four phosphate ribbons linked by neodymium in the structure of NdPsO14 (Albrand et al., 1974). The product is not always pure, but may contain ortho- and ultraphosphates (Tsuhako et al., 1979). The smaller rare earth cations form compounds with a composition R(PO3) 3 corresponding to metapthiophate, but according to their crystal structure the correct formula is U4(P4012)3 (Bagieu-Beucher, 1976; Smolin et al., 1978).
In the solid state, these compounds contain more than twenty water molecules (Petushkova et al., 1971).
Recently, Aslanov et al. 1975) have developed a method for synthesizing the hypophosphites in non-aqueous medium. The first structure contains three different hypophosphite groups, one of which is tridentate and two are bidentate bridging (Ionov et al., 1973a,b). According to Ionov et al. 1973a), the La and Eu compounds are similar but not isomorphic: some positional parameters of the oxygen atoms differ significantly.
The crystal structure of erbium hypophosphite, Er(H2POz)s, projected onto the yz plane (Aslanov et al., 1975).
ZOqJ
ZOSJ] ~
T1R(PO3) 4 is isomorphic with the first type of rubidium and cesium rare earth polyphosphates (Palkina et al., 1977). The rare earth atoms also have eight coordination, and the rubidium atoms have an irregular coordination polyhedron (fig. 29) (Koizumi and Nakano, 1977; Litvin et al., 1981b). The second and third structures differ from the first structure only in the coordination of the alkali metal (Masse et al., 1977; Palkina et al., 1981a;d).
The PO4 tetrahedra are isolated and the cations are arranged in an ordered manner (Salmon et al., 1978).
OIE 31)
Rare earth arsenates
- Physical properties
- Spectroscopical properties
Most rare earth arsenates (S m .. Lu) have been found to have the tetragonal zirconium structure (Durif, 1956), while the larger rare earths, like the corresponding phosphates, have the monoclinic monazite structure (Carron et al., 1958). The region of the first type in the rare-earth series is narrower in arsenates (L a. Ternary rare-earth arsenates with a hexagonal apatite structure have been prepared by Escobar and Baran (1982a).
Because the rare earth arsenates are highly insoluble, Na3AsO4 can be used in the quantitative determination of rare earth ions (Shakhtakhtinskaya and Iskenderov Shakhtakhtinskii et al., 1977).
Rare earth sulfites
- Ternary sulfites
45 shows that the solubility decreases with increasing temperature, as in the case of the rare earth sulfates. However, there is a recent report of the preparation of anhydrous Ndz(SO3) 3 by precipitation from aqueous solution at 95°C (Leskelfi et al., 1985); this may provide a route to products that have better crystallinity. Due to several competing reactions, the experimental conditions play an important role in the decomposition of Ce2(SO3) 3 • 3 H 2 0, as of the other lanthanide sulphites.
Again, experimental conditions play a central role and lead in some cases to unexpected shapes of TG curves (Leskel/i et al., 1985).
Rare earth sulfates
Koppel (1904) found that Ce2(SOn)3-12H20 crystallizes near 0°C; and a recent study on the crystal structure of La2(SeO4)3.12HzO provides indirect evidence for the existence of the dodecahydrate (Karvinen and Niinist6, 1986). Solubility diagram of the system (NH4)2SO4-Ce2(SO4)3-H20 showing the stability region of the binary cerium sulfate hydrates and the ternary phases (Schr6der, 1938). The table shows that in the case of the structure of the octahydrate (P r. - - L u, Y) several structural determinations have been carried out over the years.
Different coordination modes of the sulfato ligand in rare earth sulfate structures (after Ponomarenko et al., 1978).
The decomposition schemes and the observed trends in decomposition temperatures have been discussed to some extent by Wendlandt and colleagues and by Bukovec et al. Pokrovskii and Kovba (1976) have given decomposition temperatures for anhydrous sulfates, and more recently Niinist6 et al. Lu, Y) taking into account the experimental conditions, which can significantly affect the observed temperatures for reactions (2)-(4), in some cases by several hundreds of degrees (Fig. In general, the decomposition t e m p e r a t u r e s of the anhydrous rare earth sulfates are high in comparison with sulfates of main group or transition metal elements, as Tagawa (1984) has shown in a recent comparative study.
The study confirmed previous results (de Saja et al., 1981) on the three-step dehydration mechanism for Ce H20 with di- and monohydrates as intermediates.
IDD k
With the exception of LiSc(SO4)2.2H20 (Komissarova et al., 1970c), the known lithium compounds are anhydrous. Y b (Vazhnov et al., 1983), but only the structure of the monoclinic K3Yb(SO4) 3 compound (isostructural with Tm and Lu compounds) is known from X-ray studies (table 8). Characterization of the K3Sc(SO4) 3 crystals by optical crystallography indicated monoclinic symmetry (Ivanov-Emin et al., 1966).
The main structural features predicted on the basis of the IR spectra (Petrov et al., 1971).
CSz804
Square antiprismatic coordination around Pr 3+ in the CsPr(SO4) 2 structure (Bukovec et al., 1978). Perspective view of a CsLa(SO4) 2 unit cell showing the layered structure (Bukovec et al., 1980a). Like the CsR(SO4) 2 series, the Cs3R(SO4) 3 series comprises several structural types (Table 10), and the Sc compound also probably has its own monoclinic structure (Komissarova et al., 1970b).
4 H 2 0 was revealed to be isostructural with the corresponding rare earth series (Bullock et al., 1980).
