Synthesis, thermal analysis and structural characterization of the ternary compound Ag₂SnTe₃

Síntesis, análisis térmico y caracterización estructural del compuesto ternario Ag₂SnTe₃

Ternary compounds belonging to the family Cu₂-IV-VI₃ (IV= Ge, Sn, VI = S, Se, Te) have interesting semiconducting and optoelectronic properties, mainly in applications as photovoltaic and acoustic-optic devices in the near infrared [1,2]. These materials belong to the normal structure compounds (I₂-IV-VI₃) derivatives of the II-VI binary semiconductors [3] and have low melting points which diminish with increments of the atomic number of the anions.

The crystal structure of the ternaries Cu₂GeS₃ [4], Cu₂GeSe₃ [5], Cu₂GeTe₃ [6], Cu₂SnS₃ [7], Cu₂SnSe₃ [8] and Cu₂SnTe₃ [9] have been investigated by powder and single-crystal X-ray diffraction. These materials have received considerable attention recently for acoustooptic applications due to their low band gaps, low melting points, high mean atomic weights and high refractive indices [10-15]. On the other hand, silver-containing ternary compounds have been very little studied structurally and the limited information found in the literature is concerning some of their physical properties [16-20]. In particular for the ternary Ag₂SnTe₃., the synthesis, electrical and optical properties were reported [17], however its crystal structure was not characterized.

Therefore, in this work a complete structural analysis of the ternary compound Ag.SnTe. is performed by using X-ray powder diffraction data.

Figure 1 shows the DTA curve for the ternary compound Ag.SnTe.. A sharp endothermic peak observed at 343 °C corresponds to the compound melt (Tf). At a temperature 416 °C, one sharp endothermic peak occurred corresponding to the binary SnTe.

The X-ray diffractogram of Ag.SnTe. is shows in Figure 2. A search in the ICDD-PDF database [21] using the software available with the diffractometer was performed, and one known phase present in small quantities were readily identified: SnTe (PDF N° 46-1210). The first intense peaks corresponding to the phase of interest were indexed in a monoclinic cell using the Dicvol04 program [22], with unit cell parameters a = 7.450 Å, b = 12.840 Å, c = 7.415 Å, β = 109.4 °. A detailed pattern examination of the main phase, taking into account the sample composition and unit cell parameters, established that this material is isomorphic with Cu₂SnSe₃ compound [8], which crystallize in a monoclinic cell, space group Cc (Nº 9).

The Rietveld refinement [23] of the Ag₂SnTe₃ structure was carried out using the Fullprof program [24]. Initial positional parameters were taken from those of Cu₂SnSe₃ With the diffraction data available it was only possible to described the thermal motion of the atoms by one overall isotropic temperature factor. The refinement converged to the final profile agreement factors summarized in Table 1. The Rietveld semi-quantitative analysis [28] converged to the following weight fraction percentages: Ag.SnTe. (86.7 %) and SnTe (13.3 %). The final Rietveld plot is shown in Figure 2. Unit cell parameters, atomic coordinates, isotropic temperature factor, bond distances and angles are shown in Table 2. Figure 3 shows the unit cell diagram of Ag₂SnTe₃.

The structure of Ag.SnTe. can be described as derivative of the sphalerite structure. As expected for adamantane structure compounds [3], each anion is coordinated by four cations (Te1 by two Ag and two Sn, Te2 and Te3 by three Ag and one Sn) located at the corners of a slightly

Rp = 100 ∑|yobs - ycalc| / ∑|yobs|

Rwp = 100 [∑w|yobs - ycalc|2 / ∑w|yobs|2]1/2

Rexp = 100 [(N+C) / ∑.(y_obs.)]^1/2

[8] and unit cell parameters were those obtained above. Atomic positions of the binary SnTe compound [25] were included as secondary phase in the refinement. The angular dependence of the peak full width at half maximum (FWHM) was described by the Cagliotti’s formula (FWHM= (Utan.θ+Vtanθ+W)^1/2) [26]. Peak shapes were described by the parameterized Thompson-Cox-Hastings pseudoVoigt profile function [27]. The background variation was described by a polynomial with six coefficients.

R. = 100 ∑.|I. -Ic .| / ∑k |I.|

S = [Rwp / Rexp] N-P+C= degrees of freedom

distorted tetrahedron. Ag and Sn cations are similarly coordinated by four anions. Figure 3 shows the tetrahedral coordination around the cations. The Ag-Te and Sn-Te bond distances compare quite well with those observed in other adamantane structures, such as AgGaTe. [29], AgIn.Te. [30], Cu.SnTe. [9], Mn.SnTe. [31], AgFe.GaTe. [32] and AgInTe. [33].

Symmetry codes: ⁽ⁱ⁾ 0.5+x, 0.5-y, 0.5+z; ⁽ⁱⁱ⁾ x, y, 1+z; ⁽ⁱⁱⁱ⁾ -0.5+x, 0.5-y, 0.5+z; ^(iv) 0.5+x, 0.5+y, z; ^(v) 0.5+x, -0.5+y, z; ^(vi) -0.5+x, 0.5-y,

-0.5+z; ^(vii) 0.5+x, 0.5-y, -0.5+z; ^(viii) -1+x, y, z; ^(ix) x, -y, -0.5+z; ^(x) -0.5+x, -0.5+y, z; ^(xi) x, 1-y, -0.5+z; ^(xii) -0.5+x, 0.5+y, z.

The ternary compound Ag.SnTe. was synthesized by using the direct fusion technique. The thermal differential analysis indicates a melting point of 343 ºC for this compound. The refinement of the crystal structure of Ag₂SnTe₃ by Rietveld method from X-ray powder diffraction confirms that this compound crystallizes in the monoclinic space group Cc, and can be described as derivative of the sphalerite structure. The structure consists of a three-dimensional arrangement of slightly distorted AgTe₄ and SnTe4 tetrahedra.

This work was supported by CDCHT-ULA (Grants C-188514-05-B and NURR-C-606-15-05-B) and FONACIT (Grants LAB-97000821, PEII-1697 and project No 2011001341).