Th3P4-type structure

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

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 ... 

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 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... 

EuM2Se4 samples with M = La to Sm are mixed crystals with the cubic Th3P4 type structure of the M2Se3 or M3Se4, space group I43d-T (No. 220), and Z = 4. Lattice constants a ( 0.005 A) and X-ray densities Dcaic are ... 

The lattice constant a of the body-centered cubic Th3P4 type structure does not noticeably differ from that of La3 e4, which forms a continuous series of solid solutions with La2Se3. The lattice contains a high concentration of disordered cation defects, Bozorth etal. [15]. The following a values have been reported for La2Se3 ... 

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 were identified in several phase diagrams, described above. They have been proposed to be related to the cubic Th3P4-type structure (Pearson symbol c/28, space group lAM (No. 220), Z = 4), in which atoms occupy only two sites (12a and 16c). For i Bl2Te4, this implies disorder of the R and Bi atoms. For i Bi4Tey, partial occupancy would have to be involved as well. Transport properties have been measured for the Eu and Yb members, which are semiconductors with band gaps of 0.3-0.5 eV (Godzhaev et al., 1977 Aliev et al., 1986 Maksudova et al., 1988). 

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). 

In the EuS-CaS(SrS, BaS) systems there exists a continuous series of solid solutions with the NaQ-type structure (Vovan and Khodadad 1970). Samarium monosulphid forms a continuous series of solid solutions of the NaCl type with CaS, SrS at 1300°C, whereas with MgS it forms the ternary compound Mg Sm2 S4 (x= 0.33) with the Th3P4-type structure. The solid solutions with the NaCl-type structure decompose at 1000°C. At this temperature, the compound BaSm2S4 with the orthorhombic lattice and an assumed structure of the CaFe204 type is formed in the BaS-SmS system. Such a behaviour of the solid solutions is due to instability of SmS at a temperature of 1260°C, Sm3S4 is one of the decomposition products. 

Ternary tellurides of rare earth elements crystallize in structure types which are derivatives of the NaCl-, SC2S3- and EuEr2Tc4-type structure, as well as in the Th3P4-type structure (Eliseev et al. 1980, Sadovskaya et al. 1985) (table 16). 

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