Th3P4 Type Phase

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 Eu and M are statistically distributed on the cationic sites within the entire range of solid solutions, including the composition EuM2Se4. The lattice constants for EuM2Se4 vary linearly 

The distribution of the Th3P4 phase in the YbSe-M2Se3 systems is shown in Fig. 5, p. 59. The variation of the lattice constant with composition n = M/(Yb + M) is shown in Fig. 7, from which the following values for n at the lower limit of the phase range were taken  

This structure type has also been established for M =Eu and M =Dy on polycrystalline material. However, single crystals of EuDy2Se4 made by spontaneous crystallization from a KCl-KI melt and appearing as black regular octahedra are reported to belong to the cubic Th3P4 type with a = 9.2104(1) A. Two polymorphic modifications probably exist, Kuz micheva et al. [4]. 

If compositions YbM Se4 prepared at 1300 C, with p-CaH02Se4 structure, are annealed at 1200°C, a transition to the orthorhombic Yb3S4 structure occurs for M = Y, Ho, Er, Yb, and Lu. The phases with this structure seem to have strictly stoichiometric compositions. Lattice constants are as follows  

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The sesquisulfide Cm2S3 forms a defect body-centered cubic phase of the Th3P4 type with lattice parameter 8.452(5) A [47]. The sesquiselenide was obtained by thermal dissociation of CmSe2 at 620°C, again yielding a Th3P4-type phase, with cell parameter a = 8.788 A [47]. Unlike gadolinium or plutonium, no other sesquiselenide forms were observed, even after thermal treatment at various temperatures. 

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. 

The silver atoms accept tetrahedral (AgErSe2), five-coordinated (AgYbS2) or octahedral (NaCl phases) environments. For the Th3P4-type solid solutions the preceding remarks are still of value. 

Solid solutions of the ThiPrtype. These are formed from R2S3 sulfides of this type, although their extent is not well-known. It is possible that they go to the compositional limit R2CdS4. However, Yim et al. (1973) observed only the Th3P4-type for the La2CdS4 phase, and obtained non-identified phases or mixtures of phases for the subsequent elements up to Dy. 

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). 

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

At elevated temperatures (about 1400°C), continuous solid solutions of the cubic Th3P4 structure appear between y-R2S3 and y-R2S3 sulfides of this same type. Moreover, the phases G and H are no longer stable and only the I and F phases persist (Vo Van et al., 1971). 

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