Therefore, regarding the 4f moment of a light rare earth (J = L - S) and a heavy rare earth (J = L + S), the following important relationship exists: when R is a light rare earth element (Ce-Sm), the coupling between the moments 4f and 3d is ferromagnetic; when R is a heavy rare earth (Gd-Yb), this coupling is antiferromagnetic. The minima in the (Tcomp) curves reflect the mutual cancellation of the rare earth and 3d sublattice magnetization contributions.
Hydrogen sorption in intermetallic compounds 1. Pressure-composition isotherms
Sorption hysteresis
Also given in the table are data obtained by means of the quasi-elastic neutron scattering (QNS) technique. For the different measurement techniques indicated in the figure, see text (after Bowman et al., 1979).
Thermodynamic aspects
The high value of 0 E is consistent with the fact that the mass of hydrogen atoms is relatively low. A comparison of the free energy of formation of a given ternary hydride (with H2 gas and the corresponding intermetallic compound as the standard state) with the free energy of formation of binary hydrides of the corresponding constituent metals shows that ternary hydride formation is often not the most energetically favorable reaction (Buschow, 1977b).
Physical properties
Temperature dependence of the magnetization in ScMn 2 (solid line) and its hydride (dashed line) measured in a field of 3 kOe (after Buschow, 1982a). Temperature dependence of the magnetization (a, left scale, H = 3 and 9 kOe) and temperature dependence of the mutual sensitivity (Z-~, right scale) observed in GdAgH~.
- Hydrogen bonding, charge transfer and valeney changes
Results obtained by Cohen et al. 1980c) in various Laves phase compounds gives evidence of a strong decrease of the Dy isomer shift during loading. In view of the discussion on 3d band magnetism given in section 2.2.2 this is a very unlikely explanation. In the hydrides the R - F e distances are increased and a large part of the R - F e contacts are lost due to the absorbed H atoms.
This means that the presence of the H atoms has almost completely neutralized the unfavorable effect of the Sc atoms. A serious disadvantage in the study of the changes in physical properties due to hydrogen absorption is the fact that the material is pulverized during charging. They also checked the composition of the films and concluded from X-ray data that they were amorphous (Adachi et al., 1982).
In all cases it was found that the absorption of hydrogen gas leads to the disappearance or lowering of the superconducting state.
Technical applications
However, in most cases restoration of the hydrogen sorption properties can be achieved with caution. The pressure of the high purity hydrogen gas can be varied by changing the temperature. The coupling of the absorption-desorption cycles of two different hydrides (I and II) can be used to transfer heat from a low-temperature reservoir (T = Te) to a high-temperature reservoir (T = Th).
The lower part of the figure shows the flow of hydrogen gas (see arrows inside closed hydride systems) when hydride I is exposed to a high-temperature source (situation b) or when hydride II is brought into contact with a low-temperature source ( situation c). Important factors affecting efficiency are obviously the thermal conductivity and heat transfer of the hydride dust particles (Lynch, 1980; Töpler et al., . 1980a,b). A disadvantage of NJ-H2 cells is the continuous decline in their storage capacity (Holleck et al., 1980).
Closely related to the working principle of the hydrogen compressor is the working principle of a heat engine based on metal hydrides.
Concluding remarks
Also of particular importance is the mutual interaction of the H atoms in the ternary hydrides. The composition of the first hydride//1 is given under x(/31) and the associated plateau pressure under P(«-/~0- In both cases, the temperature (°C) at which this data was obtained is indicated in brackets. The type of the corresponding reaction (when such information was specified in the literature) is indicated in column 7.
Values of the sorption enthalpies and entropies are listed in the 4th and 5th columns. The composition of the ternary hydride is listed as RMùHx if no details about the hydrogen concentration are given. The values of Tcomp refer to the minimum in the temperature dependence of the magnetization, which corresponds to a cancellation of the R and M sublattice contributions.
The various compounds and hydrides have been listed in order of increasing rare earth concentration.
PARTHÉ and B. CHABOT
- Introduetion
- Survey of the compositions of ternary eompounds
- The different crystal chemical approaches to the classificaüon o f the ternary erystal structures
In almost all compounds of interest here, the rare earth content of a given type of ternary structure is generally fixed; however, the ratio of the number of T atoms to the number of M atoms can vary. The composition given in the literature can correspond to only one point in the field of homogeneity in a ternary diagram. The compositions of all RxTyMz ternary compounds which will be discussed in this paper are shown in the ternary diagram of fig.
In the event that a particular range of homogeneity has been given, or that different atoms randomly occupy a given crystallographic site, the composition given is that found in the literature and for which the structure has been determined. In this paper, we use the four-digit compound code to simplify the comparison between different formulas and to help quickly find a specific ternary compound in a ternary diagram. In the proposed four-digit composition code, the first two digits correspond to the (rounded) atomic percentage of the non-rare earth elements, and the second two digits to the (rounded) atomic percentage ratio of the M element to the sum of the T and M elements.
In the case of a compound where the T and M atoms randomly occupy a position in the structure, the last two digits of the composition code must be placed between brackets.
Structures and stoichiometries appear very complex in certain cases; However, when described 'correctly', they are much easier to understand and remember. Classification according to the types of polyhedra around the smallest atoms. This approach was developed in particular by Kripyakevich and Gladyshevskii (1971). If the polyhedra are simple and there are not too many different ones, structural drawings with drawn polyhedra are very useful for showing geometric relationships between different structures.
These structures can be further classified based on the number of directions of the prism axes and the type of coupling between the prisms.
SRPd 0.8 ~12RAu
The arrangements of coordination polyhedra around the non-rare earth elements in the various structures of the series are shown in Fig. The geometric arrangement of the rare earth atoms at the interface corresponds to a (distorted) empty segment of the tungsten (A2) structure type. The variation of the m/n ratio with the LC value is shown graphically in the accompanying diagram.
M atoms are assigned to the phosphorus (or arsenic) sites and the T atoms are placed in the centers of the empty trigonal prisms. Of the atoms located in the (11~0) plane, only B atoms are indicated that form a triangle with a B atom in the (11~0) plane with the same z-value. For the Mn2Cu3A1 and MnInCu4 types, the experimental verification is incom-.
The differences in the X-ray diffraction patterns of the ZrNiAI and TiFeSi types are small. A ternary derivative of the ThSi2 type with the same unit cell is discussed with 6750:LaPtSi. The atoms in the centers of the trigonal rare earth prisms form an infinite zig-zag chain.
MgCuA[ 2 type
Thus one can expect that in the structures of the members of this structural series all A1B: sheets have composition RT2 and all BaA14 sheets have composition RM 4. In the publication on Sc4Mn4Si7 the Wyckoff position of the Si(2) is incorrectly given as 8j stated) instead of 8i). The structures differ in the R/T ratio of the atoms that form the M-centered antiprisms and in the type of coupling of the centered octahedron columns with other octahedron columns (coupling by means of common octahedron edges).
With the exception of Sc2Cr4Si 5, all T atoms participating in the formation of the antiprisms are always octahedrally coordinated by Si atoms. Structural data of the structural sequences R m + Ts,ù + 3nM2n, where the structures are composed of m CaCus-type plates and n CeCo3B2-type plates. In the structures listed in the middle of Table 13 and shown in FIG. There are 38, stacked on each MgZn2 type plate, n CeCo3B2 type plates.
The structures listed in the right part of Table 13 consist of one ordered Laves-type ternary sheet (i.e. Mg2Cu3Si-type) and n CaCus-type sheets.
PrCoßa
In Fe3C and YPd2Si the centered prisms are connected to form nets (two prisms perpendicular to b, only one shown in Fig. 41), but in Re3B they form isolated infinite columns of prisms. The close relationship between the binary structure types suggests the possible existence of a ternary Re3B-type derived RT2M whose segment (0 < z < ½) is shown on the right side of Fig. 41 (four columns in parallel with one per unit cell, only two of which are shown in Fig. 41).
14 and treated with 6067:Sc2CoSi» These characteristic columns are the same as those found in 6750:ScRhSi with the TiNiSi type, where the Rh atoms are in the centers of the silicon tetrahedra. The structure of ScRhSi 2 is related to the structure of NbCoB2 (Ku, 76), shown in the lower part of fig. However, the much shorter B-B bonds in zigzag chains not only lead to different unit cell ratios, but also to significant changes in the tunable parameters of atom positions.
43 indicates that the columns in the boride not only have a different shape, but are also rotated relative to each other.
