O F Zmiy, L D Gulay, T A Ostapyuk - Interaction of the components in the ag2se snse2 as2se3 system - страница 1
Chemistry of Metals and Alloys
Chem. Met. Alloys 1 (2008) 115-119 Ivan Franko National University of Lviv www. chemetal-j ournal. org
Interaction of the components in the Ag2Se-SnSe2-As2Se3 system
O.F. ZMIY1*, L.D. GULAY2, T.A. OSTAPYUK1, O.S. KLYMOVYCH1
1 Department of General and Inorganic Chemistry, Volyn State University, Voli Ave 13, 43009 Lutsk, Ukraine
2 Department of Ecology and Protection of Environment, Volyn State University, Voli Ave 13, 43009 Lutsk, Ukraine * Corresponding author. E-mail: email@example.com
Received September 17, 2007; accepted June 18, 2008; available on-line September 10, 2008
The isothermal section of the quasi-ternary Ag2Se-SnSe2-As2Se3 system at 520 K was constructed using differential-thermal and X-ray phase analyses. The existence of a quaternary compound, Ag2SnAs6Se12 (space group R 3 m, a = 0.38118(1) nm, c = 3.9724(3) nm) with incongruent type of melting at 690 K, was established. The Se atoms in the structure of the Ag2SnAs6Se12 compound are stacked in a close-packed arrangement with the layers in the sequence ABC. A statistical mixture of cations (0.333Ag+0.167Sn+0.500As) occupy one half of the octahedral interstices. The remaining As atoms are distributed over the other octahedral interstices.
Semiconductors / Phase equilibria / Isothermal section / Crystal structure / X-ray powder diffraction
The binary compounds Ag2Se, SnSe2 and As2Se3 melt congruently at 1170, 930 and 648 K , respectively, and can be considered as the components of a quasi-ternary system. The Ag2Se-SnSe2 polythermal section of the ternary Ag-Sn-Se system has been investigated in [2,3], but information on the quasi-binary SnSe2-As2Se3 system is absent in the literature. The Ag2Se-As2Se3 system has been investigated in [4-6]. The authors of  confirmed the existence of the compounds Ag3AsSe3 and AgAsSe2 that were reported in [4,5] and discovered the compound AgAs3Se5 with unknown structure.
The isothermal section of the quasi-ternary Ag2Se-SnSe2-As2Se3 system at 520 K and the crystal structure of the new quaternary compound Ag2SnAs6Se12 are presented and discussed in this paper.
2. Experimental details
111 samples were prepared for the investigation of the interaction between the components in the Ag2Se-SnSe2-As2Se3 system. The alloys were prepared by sintering mixtures of the elemental constituents of purity better than 99.999 wt.% in an evacuated quartz ampoule. The synthesis was carried out in a tubular resistance furnace. The ampoules were first heated with a rate of 25 K per hour up to 1100 K and then kept at this temperature for 2 hours. Afterwards the samples were slowly cooled (5 K per our) down to 520 K and annealed at this temperature for 600 hours. Finally the ampoules were quenched in cold water.
X-ray powder diffraction patterns of the samples were recorded using a DRON-4-13 powder diffractometer (CuKa radiation, 10° < 20 < 90°, step scan mode with a step size of 0.05° and counting time of 1 s per data point). Phase analysis was carried out. The X-ray powder diffraction pattern of the Ag2SnAs6Se12 sample used for the crystal structure determination was recorded on a DRON-4-13 powder diffractometer (CuKa radiation, 10° < 20 < 100°, step scan mode with a step size of 0.05° and counting time of 20 s per data point). The crystal structure was determined using the CSD program .
The alloys of the SnSe2-As2Se3 system were investigated by differential-thermal analysis (Paulik-Paulik-Erdey system derivatograph, Pt/Pt-Rh thermocouple) (DTA) and partly by metallography ("Leica VMHT Auto" metal microscope).
3.1. The quasi-binary systems The lack of literature data on the SnSe2-As2Se3 system motivated our investigation of this section. Results of the investigation of 16 samples in the SnSe2-As2Se3 system are shown in Fig. 1 and Table 1. The diagram is of the eutectic type with the eutectic point situated at 19 mol.% SnSe2 and 640 K. No significant solid solubility ranges were detected for the initial components (<1 mol. %).
Table 1 DTA results of SnSe2-As2Se3 system.
Phase composition (mol.%)
20 40 60 80
Fig. 1 Phase diagram of the SnSe2-As2Se3 system (1 - L; 2 - L+SnSe2; 3 - L+As2Se3; 4 - SnSe2+As2Se3).
The formation of the Ag3AsSe3, AgAsSe2 and AgAs3Se5 compounds in the Ag2Se-As2Se3 system was observed, in good accordance with the results of . The formation of Ag8SnSe6 in the Ag2Se-SnSe2 system was also confirmed.
Fig. 2 Isothermal section of the quasi-ternary Ag2Se-SnSe2-As2Se3 system at 520 K.
Table 2 Results of the crystal structure determination of the Ag2SnAs6Se12 compound.
Number of formula units per unit cell
R 3 m (No. 166)
Cell volume (nm3)
Number of atoms per unit cell
Calculated density (g/cm3)
Radiation and wavelength (nm)
Mode of refinement
Number of atom sites
Texture axis and parameter
[0 0 1] 0.432(9)
Table 3 Atomic coordinates and isotropic displacement parameters for Ag2SnAs6Se12.
1 The site occupations were not refined.
3.2. Isothermal section of the quasi-ternary Ag2Se-SnSe2-As2Se3 system at 520 K
The isothermal section of the quasi-ternary Ag2Se-SnSe2-As2Se3 system at 520 K is shown in Fig. 2. The formation of a new quaternary compound, Ag2SnAs6Se12, was established. This compound melts congruently at 690 K. Two-phase equilibria exist between the quaternary Ag2SnAs6Se12 and the ternary Ag8SnSe6, AgAsSe2, and AgAs3Se5, and the binary SnSe2 and As2Se3 compounds. Two-phase Ag8SnSe6-Ag3AsSe3 and Ag8SnSe6-AgAsSe2 equilibria are also observed. Seven three-phase fields exist in the Ag2Se-SnSe2-As2Se3 system at 520 K. No significant solubility of the third (fourth) component in the binary (ternary) compounds was detected (<1-2 mol.%).
3.3. Crystal structure of the Ag2SnAs6Se12 compound The crystal structure of the Ag2SnAs6Se12 compound was investigated using X-ray powder diffraction. The peaks of the X-ray powder diffraction pattern were indexed on the basis of a hexagonal unit cell with the
lattice parameters listed in Table 2. The extinctions were found to be consistent with the space group R 3 m. By assuming space group R 3 m we were able to extract a plausible structural model from the powder X-ray intensities by means of direct methods and difference Fourier syntheses. Preferred orientation was taken into account during the refinement procedure. Two cation positions and three anion positions were found. All anion positions are fully occupied. The first cation position was found to be occupied by a random distribution of Ag, Sn and As atoms (0.333Ag+0.167Sn+0.500As), the second position by As atoms alone (0.5As). The composition of the statistical mixture (M) and the occupation factor of the As site were fixed according to the nominal composition of the sample. Some results of the crystal structure determination are summarized in Table 2, whereas the atomic coordinates and isotropic displacement parameters are given in Table 3. The observed, calculated and difference diffraction patterns for Ag2SnAs6Se12 are shown in Fig. 3.
Fig. 3 Observed, calculated and difference diffraction patterns for Ag2SnAs6Se12 (Cu Ka).
Table 4 Interatomic distances d (nm) and coordination numbers (c.n.) of the atom sites in Ag2SnAs6Se12.
Se1 Se2 Se3
0.2600(4) 0.2944(3) 0.2443(4) 0.2443(4) 0.2944(3) 0.2600(4)
3 6 6 3
' M = 0.333Ag+0.167Sn+0.500As
A projection of the crystal structure of the Ag2SnAs6Se12 compound onto the (1120) plane, the coordination polyhedra of the M (a), As (b), Se1 (c), Se2 (d) and Se3 (e) atoms and the decomposition of the structure into atomic layers of hexagonal topology formed by Se are shown in Fig. 4.
Interatomic distances and coordination numbers of the atoms in the Ag2SnAs6Se12 structure are given in Table 4. The interatomic distances agree well with the sum of the respective ionic radii .
The atoms of the statistical mixture M (0.333Ag+0.167Sn+0.500As) are surrounded by six Se atoms forming an octahedron. The As atoms from the As site have three neighbors. The coordination polyhedron is a triangle, non-coplanar with the As atom. The sites Se1 and Se2 are surrounded by six cations (octahedron), the site Se3 by three cations (non-coplanar triangle).
The Se atoms in the structure of the Ag2SnAs6Se12 compound are stacked in a close-packed arrangement with the layers in the sequence ...ABC... (cubic close packing) (Fig. 4). The atoms of the statistical mixture M (0.333Ag+0.167Sn+0.500As) occupy one half of the octahedral interstices. The other As atoms are distributed over the remaining octahedral interstices, occupying half of these. The M atoms are located close to the centres of the octahedra (Fig. 5a), whereas the As atoms are shifted from the centers of the octahedra. Since three Se atoms are located at significantly longer distances than the other three Se atoms, actually a triangular surrounding should be considered for the As atoms (Fig. 5b).
The packing of the M- and As-centered octahedra in the structure of the Ag2SnAs6Se12 compound is shown in Fig. 6. A similar arrangement of Ag- and As-centered octahedra is observed in the structure of the AgAsSe2 compound (space group R3m,
Fig. 4 Projection of the crystal structure of the Ag2SnAs6Se12 compound onto the (1120) plane, the coordination polyhedra of the M (a), As (b), Se1 (c), Se2 (d) and Se3 (e) atoms and the decomposition of the structure into Se layers of hexagonal topology.
Fig. 5 The polyhedra centered by M (a) and As alone (b) in the structure of the Ag2SnAs6Se12 compound.
a = 0.3915 nm, c = 2.0375 nm)  (Fig. 6). The structure of Ag2SnAs6Se12 is an a X b X 2c superstructure of AgAsSe2. Both crystal structures are superstructures of the NaCl-type structure (space group Fm 3 m).
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Fig. 6 The packing of M-, Ag- and As-centered octahedra in the structures of Ag2SnAs6Se12 and AgAsSe2.
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Proceeding of the X International Conference on Crystal Chemistry of Intermetallic Compounds,
Lviv, September 17-20, 2007.
Chem. Met. Alloys 1 (2008)