Wickleder described the synthesis and structures of many types of rare earth halides. ERogl, Phase equilibria in ternary and higher systems with rare earth elements and silicon 52.
ELECTRONIC EXCITATION IN ATOMIC SPECIES
Classification of lanthanides according to the number of 4f electrons in the atom and in the solid. Another intriguing question is: why should the spectrum of the middle group (D) closely resemble that of Sm S and, to a lesser extent, that of vapor (the structure is similar, but the width and intensity of Mrv and Mv lines diverge in fig 10. We can now discuss the above observations in the light of the double well potential model (Connerade 1978 a,b originally studied in the context of atomic 4 f orbital collapse (Connerade 1978 a,b see also sect 2 of this chapter) Interpretation of the present experimental observations requires that two types of 4 f orbitals coexist This possibility was first suggested by Gschneidner (1971) for the rare earth metals on the basis of their melting points and sublimation energy data Band and Fomichev (1980) and Band et al (1988) made a similar suggestion for atoms of sequence 4 f Schliiter and Varma (1983) approached the problem in a way more suited to the description of the condensed phase Model details double valley potential and the role of centrifugal barrier effects in valency changes in rare earths has been previously discussed by Connerade and Karnatak (1990) In the context of R Ox, this model was further discussed by Karnatak (1993) and coexistence of ions and covalents.
For covalent features in Ce, Pr and Tb spectra with higher oxides, a double-well potential barrier is applied to account for the delocalization of the 4f electron in the valence band.
SIMPLE AND COMPLEX HALIDES
In this article, we will begin by reviewing the structures of the binary rare earth halides, R Xz (where RZ+ is the cation of the rare earth elements scandium, yttrium, lanthanum, and cerium via lutetium and the X halide ion) Since the binary halides have been the subject of many excellent reviews, they we will mention it only briefly in this handbook (Haschke 1979, Eick 1994). Monocrystals are also often obtained when exploring special paths. Halides with rare earth elements in an oxidation state of less than + 3 can be obtained by reducing the respective (tri-)halides with their rare earth metals (ratio.
The trifluorides of lanthanum and cerium through samarium adopt the tysonite type structure with elevenfold coordination of the metal ion (Wells 1975) The lRF 11 l unit,. Depending on the standard electrode potentials E° of the R 2+/R 3 + half-cells (see Meyer 1988), two groups of dihalides, RX 2, of the rare-earth elements can be distinguished:. The crystal chemistry of the ternary rare-earth halides with monovalent cations A+ of the general composition Aw Ry X 3y+w has evolved significantly in the past sesquidecade.
The occupation of the trigonal prismatic site is strongly reflected by the increasing length of the c-axis. The structures of these halides strongly depend on the size of the A+ ion: If A+ is large enough (e.g. Cs+), the closest packings of A+ and X-spheres are found (e.g. elpsolite). The padded Li Sb F 6 -type structure can also be understood as a derivative of the Rh F 3 -type structure that occurs as the high-pressure form of several binary rare-earth halides of the Fe CI 3 (Bi I 3 ) -type According to 2 x RX 3= R 2X 6 (AR)X 6 A 2(AR)X 6.
The (A,R)6C 13 6, polyhedral "clusters" can be connected in two different ways leading to two different crystal structures, one with tetragonal (14/m) and the other with trigonal rhombic (R 3) symmetry with the approximate compositions (A,R)15X 34+x (- A 1 0R 5X 35=A 2RX 7) and (A,R)14X 32+x, respectively (fig 36) Based on X-ray examinations alone it often cannot be assessed whether or not an additional anion occupies the cuboctahedron Because A 2+ and R 3+ ions occupy, at least partially, equal crystallographic sites, electroneutrality can be achieved in any case. Very thorough research by Birnighausen and co-workers on isotypic mixed-valent rare-earth halides (A=R) has provided evidence for partial occupation of the cuboctahedron with oxide or chloride in some cases (see sect. 4 3) Recent research in the Ba X 2/RX 3 and Sr C 12/R C 13 systems led to compounds which can be formulated, for example, as Sr 1o Sm 4C 13 2 and Ball Pr 4Br 34 or, if additional oxygen is involved, as Sr 8Sm 6C R 14X 33") and Sak Pr 6Br R 1 5X 35"), respectively The occupation of the cuboctahedral site remains at least doubtful. For the rare-earth elements, dysprosium and thulium mixed valence compounds of the type A 5R 3X 12 have been observed with A=K, Rb (Hohnstedt and Meyer 1993, B 6 cker 1996), although K 5Dy 3112 has been described as K 513Dy. 311 2 initially They are isotypic with some transition metal compounds of which Na 5Ti 3C 11 2 was characterized quite recently (Hinz 1994) The structure consists of chains of different polyhedra running along the hexagonal c-axis: lRX 6l octahedra and lAX 7 l monocapped trigonal prisms are connected via common edges and lAX 6l trigonal prisms share common planes (fig 45).
SOLID ELECTROLYTES
Typical solid electrolytes containing rare earth elements Mobile ion Solid electrolytes Notes: crystal structure, etc. Rare earth elements are vital components of many prominent solid electrolyte systems such as the oxygen-ion conductor in fluorite structures (Dell and Hooper 1978), the fluorine-ion conductor in trifluorides of tysonite- type (Reau and Portier 1978), the protonic conductor in doped perovskite phases (Iwahara et al. 1981 a) and the trivalent cationic conductor in hexagonal compounds of the O-alumina type (Verstegen et al. 1973) Typical solid state electrolytes containing rare earth elements are listed in Table 1. It is worth noting that rare earth elements have a high profile in conferences directly concerned with solid electrolytes. locations around the world), of the 190 oral presentations and 260 posters in the last conference held in Singapore in December 1995, and the 360 oral presentations in the more recent conference held in Hawaii in November 1997. Thus, the addition of rare earth oxides introduces oxygen-ion conductivity by affecting the defect equilibria. Increasing the concentration of doping agent leads to an increase in
In stabilized zirconia doped with a rare earth oxide such as yttria, the vacancy concentration is determined by the dopant concentration, for a wide range of temperature and oxygen partial pressure, as given by Eq. The addition of heavy earth or yttrium oxides stabilizes the high-temperature cubic phase of Bi 203 and, as shown in Fig. 9 for the Bi 203-Y 203 system, the oxide-ion conductivity can be maintained at a high level even at temperatures well below 1050 K (Takahashi et al 1975 a, Harwig and Gerards 1978 and Verkerk et al 1982) Even if trivalent rare earth ions are partially substituted for Bi, ¼ of the oxygen ion sites in the fluorite species is still gray. vacancy as in the case of pure Bi 203 In order to maintain this structure at low temperatures, it is necessary for the ion group to be distorted By partially replacing part of Bi with Y, it has been shown, using neutron diffraction analysis, that the oxygen ions are displaced from their ideal fluorite sublattice position toward the oxygen vacancy site (Infante et al 1987). This type of solid solution exhibits relatively high conductivity at elevated temperatures (Takahashi et al 1975 b, Iwahara et al 1981 b) The crystal structure is formed by slightly compressing the fluorine-type defect structure along the diagonal axis, and the conductivity is based on a void mechanism of similar to that of the fluorite oxides defect Substitution of Bi by rare earth cations of medium ionic radii also leads to the formation of the rhombohedral phase in the composition range 10-30 mol%, but at.
The small size of the F ion and its single charge make it a good candidate for an ionic conductor. Rare earth metals can act as a dopant or as a guest cation. Rare-earth fluorides exist either as hexagonal (LaF 3 ) or as orthorhombic structures (1-YF 3 ), as shown in Fig. 17 for rare-earth fluorides with La to Lu (Thomas and Brunton 1966). Fluorides of rare earth elements La to Nd crystallize in the tysonite structure at all temperature ranges below the melting point. Sm F 3 to Gd F 3 is orthorhombic at lower temperatures. As described in the previous section, light rare-earth trifluorides such as LaF3.
Li 3Sc 2(PO 4)3 in its orthorhombic form (y-phase) was reported as a new lithium-ion conductor (Bykov et al 1990) at temperatures above 518 K The lithium sites in the y-phase. Solid electrolytes in interface with suitable electrodes can quantitatively transduce the relationship between the activities of the ionically conductive component at the two electrodes directly into an electrical potential. This forms the basis for using solid electrolytes in the development of. One of the major advantages of using the oxygen sensor in the stoichiometric mode to control the air/fuel ratio at A= 1 is that there is a step change in emf at stoichiometric air/fuel ratio, as shown in fig. 36 This occurs because Po 2 changes strongly at A= 1, the main reason for referring to the sensors as lambda sensors.
The sensitivity factor at a given oxygen concentration is clearly a function of the applied potential, as can be seen from the results shown in Fig. 41 for oxygen in the concentration range of 2-14%. Lower applied potentials (e.g. greater sensitivity, while higher values (e.g. 50 mV) have yielded a linear response. Rare-earth fluorides are good conductors of fluorine ions (see section 5 2) Addition of aliovalent cations increases fluorine conductivity ions considerably further For example, single crystals of LaF3 doped with EuF2 are widely used in commercial ion-selective electrode (ISE) applications as F-specific electrodes (Frant and Ross 1966) In the field of ISE, only the pH-sensitive glass electrode is more widely used. If the oxygen pressures in the two electrodes are kept identical, the equation is simplified. If one of the humidity values is known, the other can be measured by the emf signal.
Incorporation of impurities or other defects in the host crystal introduces allowed energy levels in the forbidden gap A schematic diagram showing such defect levels is given in fig 1 Levels in the forbidden gap near CB (N levels) act as electron traps. Electrons in the N levels cannot fall into the full VB and so they remain trapped. Possible ways in which some of the trapped electrons can leave the N levels without heating the sample will be given when discussing TL fading in section 2 1 5 below Defect levels in the lower half of the forbidden gap (M levels and M' in fig 1) are usually filled with electrons. Exposing the sample to ionizing radiation lifts VB electrons to CB leaving behind holes (missing electrons) in VB (transition 1) CB electrons are free to move and some of them will be trapped in N levels (transition 3) Some M electrons will now settle to radiation-depleted VB states, leaving holes in the M levels.
