Th3P4-type compounds

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

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. 

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. 

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... 

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

[Pg.29]

R2MgS4 compounds of the spinel type observed for R = Tm, Yb, Lu and Sc (Patrie et al., 1965)- A transition to the Th3P4-type is observed by high pressure high-temperature treatment (Hirota et al., 1976). 

In contrast to the preceding cases T1 or Pb only exist in ternary compounds with their lower valencies, T1(I) and Pb(II). These compounds, of general formulae TIRX2 and PbR2X4, have classical structures, rhombohedral NaFe02-type. and cubic Th3P4-type or orthorhombic CaFe204-type respectively. These compounds have been described in sections 4 and 5. 

PbTe-R2Tei-RiTeA systems. Aloman et al. (1968, 1971) describe R2PbTe4 compounds, which form Th3P4-type solid solutions with R2Tej and R3Te4. However, the existence of these compounds has not been confirmed by other studies (Patrie et al., 1969b). 

The known lanthanide and actinide compounds with this M/X ratio have the Th3p4 type structure, except ThjN. The Va group elements P, As, Sb, and Bi and the Via group elements (chalcogens) S, Se, and Te can form cubic ThjP -type compounds with the lanthanides and actinides. Each lanthanide or actinide atom has eight equidistant nearest X neighbors in these compounds. Figure 22 shows the variation of the lattice parameters in the actinide series for the known compounds of this type. 

A large number of metaffic compounds with the NaCl-type structure are found at the AnX stoichiometry. These form when X is a chalcogenide, a pnictide or carbon. An3X4 compounds having the cubic Th3P4-type structure frequently coexist with the NaCl-type compounds in systems of the actinides with the chalcogenides and pnictides. 

The higher values for Sm2Se3 as compared to 0q values in the M series La to Nd are due to partial divalency, Dudniketal. [2]. A universal value 0o 25O Kfor M2Se3 compounds is given in Takeshita et al. [8]. Debye temperatures for the Th3P4 type sesquiselenides estimated with use of crystal chemical data, see Kuz micheva et al. [9]. 

Dy3Se4 is pseudo-cubic of the CaHo2Se4 structure type with a = 5.69 A. A Th3P4 type structure with a = 8.67 A is assigned to products Dy3Se4.o5 [1.2] the value a = 8.64 A is given by Slovyanskikh et al. [3]. The composition is believed to represent an individual compound [2]. 

These compounds were prepared with divalent rare earths (R = Sm, Eu, Yb) by reaction of elements or binaries (Hulliger, 1979b). They adopt the anti-Th3P4-type structure (Pearson symbol c/28, space group I4M (No. 220), Z = 4) and are assumed to be semiconducting. Eu4Bi2Te orders antiferromagnetically at Tn = 36 K. 

AgRjSg Cubic lattice of the Th3P4 type. The phases containing AgRjSg are imperfect. The compound formula may be written ... 

The structures of the polymorphic modifications of the compounds CaR2S4 (R = Ho-Lu, Y) are as follows a low-temperature orthorhombic (a) structure of the Yb3S4 type and a high-temperature (j8) structure of the MnY2S4 type (Flahaut et al. 1961, Patrie and Flahaut 1967). The cubic structure of the Th3P4 type was observed for the compounds CaR2S4 (R = La-Dy, Ho) (Flahaut et al. 1965, Yim et al. 1973). 

The compounds SrR2S4 (R = La-Dy) have the Th3P4-type structure (Flahaut et al. 1962b, 1965, Sallavuard and Rene 1971), in the case of R = Tb-Lu, Y, they have the CaFe204-type structure (Patrie et al. 1964b, Vovan et al. 1966). 

---