Macrocycle complexes of the lanthanide(III), yttrium(III) and dioxouranium(VI) ions from metal-formed syntheses 443. Sundstr6m, Low temperature heat capacity of the rare earth metals 379 6. McEwen, Magnetic and transport properties of the rare earths 411.
Specific heats
A complete treatment of the specific heat of simple systems at low temperatures can be found in G o p a l (1966). Basic models for magnetically ordered systems, i.e. the Ising, X - Y and Heisenberg models allow us to evaluate ~//M at TN for simple magnetic structures.
Classification of the cerium binary compounds
When the models are extended beyond their specific limits, one finds that the characteristic energies (for example the K on do temperature TK) reach unphysical values, as for example in the case of Ce impurities diluted in the Th matrix (Luengo et al. 1975). The specific heat measurements show two peaks, at 1.3 K and at 6.5 K. The C~ contribution is shown in figure 2.
Region III: antiferromagnets with large Ce-Ce spacing
The entropy increase reaches a value of R In 2 in the paramagnetic phase, which is 75% of the value associated with the ferromagnetic phase. Under external pressure, measurements of the specific heat show a gradual disappearance of the modulated (antiferromagnetic) phase, without significant changes in the T c transition, but with an increase in the 7LT and 7m ~ members (250 mJ K - 2ICe atom at 11 kbar) (Peyrard 1980), see fig.
Region IV: compounds with a nonmagnetic ground state
Comparison of C%In specific heat (solid line) with Desgranges and Rasul (1985) model (dashed lines). While the maximum magnetic contribution to the specific heat decreases with increasing magnetic field for x = 0.3 and 0.35, see Dhar et al.
Miscellanea
Neutron scattering
Concluding remarks
However, as experimental information increases, the need for a more complete condensation of information becomes apparent. It is therefore necessary to have more experimental reviews regarding, e.g., magnetic and transport properties, in order to increase the applications of theoretical models to real systems.
Introduction
RARE EARTH CARBIDE 63 In addition, rare earths have been known to form a variety of stable gaseous carbides through pioneering work by Chupka et al. These stable gaseous carbides include the first known gaseous metal dicarbides, tetracarbides, and tricarbides, as well as recently discovered gaseous carbides with more than four carbon atoms, up to six for cerium (Gingerich et al. This will allow the prediction of the stability and possibly the molecular structural properties of a large number of still unknown gaseous metal carbides.
Ternary rare-earth carbon compounds are also gaining attention due to their unique magnetic and electrical properties and their potential importance in nuclear technology and in the construction of permanent magnets. A large number of researchers focus on the superconductivity of the rare earth (thorium or uranium) carbon systems, the magnetic properties of the rare earth transition metal carbon systems and the electrical properties of the rare earth transition metal carbon systems. -boron carbon systems. In this work, we will provide an overview of the basic research on the rare earth metal carbides by focusing on the phase diagram and thermodynamics involved in the formation of carbides, and on the crystal structures and the chemical and physical properties.
Binary rare-earth-carbon phase diagrams
The crystal structure of this compound was found to be body-centered cubic of the PuzC 3 type with the I743d space group (Atoji et al. 1958). On the basis of the review by Gschneidner and Calderwood (1986), this new phase was considered Y15Ca9 with the tetragonal structure of the ScasC19 type (Bauer and Nowotny 1971). On the basis of these data, the general characteristics of the phase diagrams of the heavy rare earth carbon systems can be summarized.
GdC 2 , then decreases as the atomic number of the heavy rare earth in the dicarbide increases. In addition, based on the yttrium-carbon phase diagram, the unknown information about the R-C phase diagrams could be deduced. The existence of the ytterbium sesquicarbide with the cubic Pu2Cs-type structure has not been established (Spedding et al. 1958) under general conditions.
All reports on the existence of rare earth monocarbides were published before the mid-1960s, e.g. However, after that no data was obtained to prove the existence of rare earth monocarbides.
Crystal structure of the binary rare earth carbides
Thus, the rCg2p contribution of the C2 group to the conduction bands should be smaller in sesquicarbides than in dicarbides, i.e. Atoji's (1981b) neutron diffraction study on the crystal and magnetic structures of cubic ErCo. The layer repeat unit of the trigonal structure is given by [AcB D Cb D BaC D ], where a, b, and c designate fully occupied carbon layers and [] indicates vacant layers.
Therefore, the cubic-to-trigonal transformation can be accomplished by a short-range transport of the carbon atoms. From these curves, the valence state of the rare earth elements, especially europium and ytterbium in the carbides can be derived. The a and c lattice parameters of the dicarbides as a function of the ionic radius (Schwetz et al. 1979).
The lattice parameter of the hypocarbide (cubic) as a function of the ionic radius (Schwetz et al. 1979). Pseudobinary system: mixed rare earth carbides and solid solutions of the rare earth carbides and the actinide carbides.
![Table 5 also summarized other pertinent crystallographic data: the Debye Waller temperature factor coefficients, B in exp[-2B(sinO/2) 2] at 3 0 0 K and 5 K; the carbon positional parameters, z in (000, ±±!~222f - - -~ ( 0 0](https://thumb-ap.123doks.com/thumbv2/123dok/10272257.0/93.702.94.466.724.923/summarized-pertinent-crystallographic-waller-temperature-coefficients-positional-parameters.webp)
Pseudobinary system: mixed rare earth carbides and solid solutions of the rare earth carbides and the actinide carbides
The theoretical values of the lattice constants and the energies of the vacancy formation were determined from the total energies. Despite this theoretical research, a systematic investigation of the relationship of the chemical bonds to the formation and structural stability of the binary rare earth carbides has yet to be conducted. According to this model, the reduction in Tt was related to the difference in the unit cell volumes AV of the dicarbides, ATt = K(AV) 2 , for 50 mol% solid solutions.
As in the rare-earth mixed systems, the rare-earth carbides also form a solid solution with uranium dicarbide and there is a compositional dependence of the cubic-to-tetragonal phase transformation for the UC2 LaC2 or the UC2 -CeC2 solid solutions ( McColm et al. 1972), as well as for the solid solution U Cz - GdC2 (Wallace et al. 1964). 14, Perspective view of the carbon-rich part of the ternary GdUC system (Wallace et al. The limiting solubilities of the rare earth elements in the uranium carbides and of uranium in the rare earth carbides have been determined, which are of Ce in UC and U in CezC3 4.5 and 5.5 atomic % total metal, respectively (McColm et al. 1972).
The mechanism for the decrease in the tetragonal-to-cubic transformation temperature by adding rare earth atoms to UC2 has been discussed in terms of a strain energy model, which was also applied to the mixed rare earth dicarbide system, see Sect. In the two ternary CaCz-type compounds, the cubic NaCl-type structure can be arrested down to 4 K due to the large strain energy of the disordered lattice.
Thermodynamic properties of binary rare earth carbides 1. Thermodynamic stability of gaseous rare earth carbides
The first rare earth gaseous monocarbide, CeC, was observed in the study of chemical reaction equilibria. The phase diagram of the ternary system Y B-C in the quenched state is shown in fig. Smith and Gilles (1967) prepared this phase for Nd, Gd, Tb, Dy, Ho, Er and Yb from the reaction between graphite and the corresponding rare earth tetraboride, identified the structure of the RB2C 2 phases based on a primitive tetragonal cell. , and proved that the previously reported phases RB~ (R = La, Pr, Gd, Er, Yb, Y) (Post et al. 1956, Binder are also members of the RB2C 2 series.
The stacking of the boron-carbon network and the rare-earth metal layer was also deduced from this atomic arrangement. The lattice parameters of the R 3 Sn and R 3 SnC compounds and the quantities of the fcc phases have been measured. The liquidus projection and the isothermal portion at 900°C of the Sm Co - C ternary phase diagram have been determined (Stadelmaier and Liu 1985).
The powder pattern of the latter compound (Stadelmaier and P a r k 1981) corresponds to that found for GdFeC2 in the work of Jeitschk and Gerss (1986). A projection of the structure and coordination polyhedra and the position parameter can be found in Gerss et al. Relatively large differences in lattice parameters were observed in samples of R 2 Mn17C3_ x compounds.
Apparently, the stability of both structure types depends on the size of the rare earth elements.
Carbides of rarc-earth-metal-(oxygen, nitrogen, hydrogen, halogens)
This may be regarded as an indication of the essentially covalent nature of the m e t a l - c a r b o n bonds. The rare earth oxycarbides are usually produced by carboreduction of the oxides and are found to coexist with the rare earth carbides or oxides in the samples. However, in many cases the formation and yield of the rare earth oxycarbides strictly depends on the starting materials and synthesis conditions (temperature, stoichiometry and CO partial pressure) (Hfijek et al. 1984a), e.g.
However, the reported phase diagrams of either the C e - C - O or the Y C - O system gave no indication of the presence of this phase. Similar to the studies on the production of the rare earth oxycarbide, the thermodynamics of the formation of the compounds N d 2 0 2 C2 (Butherus et al. 1966), Ce202C2 (Pialoux 1988) and Y b 2 O C (Haschke and Eick 1970a) , b) have been investigated in detail. It is clear that the homogeneity region of the NaCl-type phase will expand with increasing temperature, at least in the direction towards ScO (Karen et al. 1986).
The formation of the rare-earth-nitrogen-carbon compounds was investigated only for the light lanthanide systems with emphasis on the existence of the. However, the reviewers suggest that this factor is probably minor while the size factor of the rare earth atom is essential.
Physical properties of the rare earth carbides
The change in the slope of the resistivity-temperature curve occurred at about 18 K, which is close to the N6el temperature reported by Atoji. This may be related to the fact that the direction of the ferromagnetic moment is parallel to the c axis in Tb 2 C. The superconductivity of rare earth carbides was first discovered by the Krupka-Giorgi-Krikorian group (Krupka et al.
The maximum transition temperature was observed in the range of Th:R atomic ratio of 8 : 2 to 7 : 3. Specific heat and critical magnetic field curves for the rare earth and rare earth thorium sesquicarbides. This may be the result of the transfer of 5d electrons from the rare earth atoms to the 3d nickel band.
Curie temperature, Tc, and saturation magnetization, M~, of the REFea4C compounds in a field B = 8 T (Gueramian et al. 1987). Wagman, 1973, in: Selected values of the thermodynamic properties of the elements (American Society for Metals, Metal Park, OH).
Compounds and structures 1. Discrete clusters
