Th3P4 Type

The cubic unit cell of the Th3P4 type structure with space group I43d-Tg (No. 220), see Fig. 2, p. 36, contains four M3 xDxSe4 ( = cation vacancy). The ideal composition M3Se4 (x = 0) does not contain vacancies. Each M is surrounded by 8 Se atoms at the apices of a triangular dodecahedron. This can be resolved Into two distorted interlocked tetrahedra, one being 

Lattice constants a, in A 0.006, of nearly stoichiometric M2Se3 with Th3P4 structure and coordination number 8 for M are  

A Debye-Scherrer diagram of La2Se3 is given, Guittard etal. [7], also see Flahaut etal. [3], Suchet et al. [12], Lashkarev, Paderno [25], Lashkarev et al. [28], Miller etal. [26], Benacerraf, Guittard [8]. Additional data for the lattice constants for M2Se3 are summarized in the following table  

A plot of the lattice constant a against the ionic radius is linear with positive deviations at the heaviest M (Tm, Yb, Lu). The occurrence of the structure is apparently determined by a combination of size effect and 4f electron bonding [20]. The influence of the interatomic distances (determined experimentally and calculated from ionic radii) on bond character and bond strength is discussed by [25], also see Lashkarev etal. [27] and Miller etal. [26]. 

The calculated percentage of covalent bond character is given as follows [26]  

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U3X4 (X = P, As, Sb, Bi, Se, Te), Sr4Bi3, Ba4Bi3, Ba4As 2.6 are isostructural and crystallize as anti-Th3P4-type structure. 

From a structural standpoint, the rare earth sulfides have several polymorphic forms (20), whose stability regions are represented in Figure 3. The high temperature form (y) exists from lanthanum to dysprosium. It is cubic and is of the Th3P4 type, with a defect structure. In each unit cell, there are 102/3 metal atoms which are distributed at random among the 12 sites of the metal lattice. The structures of the low temperature a and f forms are not yet known. The structure of the 8 form, which is peculiar to dysprosium, yttrium, and erbium, is monoclinic (20). The three last forms have low crystal symmetry, and certainly have no vacant lattices. 

We were not able to observe solid solutions with the other structural forms of the L2S3 sulfides, so we present only compounds having the Th3P4 type in this paper. 

It may be seen from Table IV, where the rare earth elements, L, and the divalent metals, M, are listed in order of their ionic radii, that the incidence of the Th3P4-type structure is related to the ionic sizes of the elements. [The radii of the rare earth cations, L+3, were calculated from the sulfides, LS, which have NaCl-like structures. These sulfides do not have purely ionic bonds, but the values of the radii calculated by this way are in good accord with those published by Templeton and Dauben (23).] Table IV seems to indicate two necessary conditions ... 

As with the Th3P4 -type solid solutions, we can see that the NaCl-type solid ... 

Rare-earth sesquichalcogenides are insulators at room temperature but at higher temperatures their resistivities fall in the same way as those of semiconductors. These compounds have lower melting points than the monochalcogenides and they decompose on melting. Many sesquichalcogenides have the defect structure of the Th3P4 type [10]. 

Deformation potential coupling constants are of the order of fip, (Ziman 1960). To observe deformation potential effects in the temperature dependence of elastic constants several conditions have to be met as discussed above dpA(,(0) must be large and - Eq has to be of the order of k T. This excludes normal metals and only d-band metals with rather narrow bands can exhibit this behavior. Typical examples have been given above. In intermetallic rare-earth compounds simple density of states arguments show why elastic constant effects can be observed only for CsCl-type and Th3P4-type materials. In table 4 electronic specific heat values are listed for various rare earth compounds. This is an updated list of a previous work, see Liithi et al. (1982). This table indicates that monopnictides and monochalcogenides have smaller values of y than CsCl- and Th3P4-structure materials, i.e., the 5d band of the former structure is more hybridized than in the latter. 

By high pressure (77 kbars) high temperature (2000°C) treatment, Eatough et al. obtained the Th3P4-type for all the elements up to lutetium, in the R2S3 sulfide series (1969) and in the R2Se3 selenides series (1969, 1970). 

Dashed line non-stoichiometric phases a orthorhombic Gd2S3-type P tetragonal PrioSi40-type y cubic Th3P4-type S monoclinic Er2S3-type e rhombohedral AljOs-type ip orthorhombic ScaSs-type t cubic Tl203-type. 

Some thermal properties of the Th3P4-type compounds were determined... 

In this chapter we have to consider the only derivatives in which the rare earths are purely trivalent, and these have a metallic behavior. The properties of the Eu monochalcogenides are described in ch. 19, and chalcogenides which have valence changes are covered in ch. 20. Moreover, we have previously described the R3X4 compounds of the Th3P4 type. 

For the R2S3 compounds y. cubic Th3P4-type S monoclinic Y2S3-type e rhombohedral AI2O3 a-type... 

T-. cubic Th3P4-type A orthorhombic Er2MnS4-type A orthorhombic Yb3S4-type B orthorhombic CaFc204-type... 

For the R2Se3 compounds y cubic Th3P4-type 7j orthorhombic U2S3-type

[Pg.29]

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