X-ray powder diffraction data and rietveld refinement of the ternary semiconductor chalcogenides AgInSe₂ and AgInTe₂

Gerzon E. Delgado*¹, Asiloé J. Mora¹, Carlos Pineda², Rosario Ávila-Godoy³, Solange Paredes-Dugarte⁴

1: Laboratorio de Cristalografía, Departamento de Química. Facultad de Ciencias, Universidad de Los Andes, Mérida, Venezuela,

2: Núcleo Universitario Alberto Adriani, Universidad de Los Andes, El Vigía, Venezuela.

3: Laboratorio de Análisis Químico y Estructural de Materiales, Departamento de Física, Facultad de Ciencias, Universidad de Los Andes, Mérida, Venezuela.

4: Laboratorio de Caracterización de Materiales, Instituto de Investigaciones en Biomedicina y Ciencias Aplicadas, Universidad de Oriente, Cumaná, Venezuela.

*e-mail: gerzon@ula.ve

ABSTRACT

The ternary chalcogenides AgInSe₂ and AgInTe₂ were studied by X-ray powder diffraction structure refinement using the Rietveld method. Both compounds crystallizes with a chalcopyrite structure in the space group I 2d (N° 122), Z = 4, and unit cell parameters a = 6.0988(2) Å, c = 11.7086(6) Å, V = 435.51(3) ų for AgInSe₂ and a = 6.4431(4) Å, c = 12.6362(9) Å, V = 524.57(6) ų for AgInTe₂. Improved X-ray powder diffraction data are reported with figures of merit M₁₉ = 84.0, F₁₉ = 40.7 (0.0071, 66) for AgInSe₂, and M₂₀ = 80.8, F₂₃ = 39.0 (0.0075, 79) for AgInTe₂.

Keywords: Semiconductors, Chalcogenides, Rietveld refinement, X-ray powder diffraction data.

Datos de difracción de rayos-X y refinamiento rietveld de los semiconductores calcogenuros ternarios AgInSe₂ y AgInTe₂

RESUMEN

Los calcogenuros ternarios AgInSe₂ y AgInTe₂ se estudiaron mediante refinamiento Rietveld utilizando datos de difracción de rayos-X en muestras policristalinas. Ambos compuestos cristalizan con una estructura tipo calcopirita en el grupo espacial I 2d, (N° 122), Z = 4, y parámetros de celda unidad a = 6.0988(2) Å, c = 11.7086(6) Å, V = 435.51(3) ų para AgInSe₂ y a = 6.4431(4) Å, c = 12.6362(9) Å, V = 524.57(6) ų para AgInTe₂. Se reportan mejores datos de difracción de polvo con figuras de mérito M₁₉ = 84.0, F₁₉ = 40.7 (0.0071, 66) para AgInSe₂, y M₂₀ = 80.8, F₂₃ = 39.0 (0.0075, 79) para AgInTe₂.

Palabras Claves: Semiconductores, Calcogenuros, Refinamiento Rietveld, Datos de difracción de polvo.

Recibido: 20-01-2014; Revisado: 09-05-2014

Aceptado: 08-07-2014; Publicado: 20-07-2014

1. INTRODUCTION

Chalcogenide semiconductors of the type Ag-In-VI (VI = S, Se, Te) have been studied because of their possible technological applications as photo-voltaic detectors, solar cells, light emitting diodes, modulators, filters and their use in nonlinear optics [1-2]. Following the rules of formation of semiconductor compounds [3], in the system Ag-In- VI, stoichiometric compounds with three compositions can be form: AgInVI₂, AgIn₃VI₅ and AgIn₅VI₈. These materials belong to the normal structure compounds (I-III-VI₂) and the defect structure compounds (I-III₃-□-VI₅, I-III₅-□₂-VI₈), respectively [4].

From the crystallographic point of view, in the Ag- In-S system, the AgInS₂ ternary compound crystallizes in two polymorphs: a tetragonal chalcopyrite-type phase (space group I 2d) and an orthorhombic wurtzite-like phase (space group Pna2₁). These phases were simultaneously characterized from a unique X-ray powder diffraction pattern [5]. In the Ag-In-Se and Ag-In- Te systems, the ternaries AgInSe₂ [6] and AgInTe₂ [7] were reported, at room temperature, in undistorted chalcopyrite-type structures. Both materials have been studied under high pressure and temperature and have been reported to have a firstorder structural phase transition from chalcopyrite structure to a cation-disorder NaCl-like structure [8]. In particular, for these ternaries chalcopyrite phases appear reported only poor quality X-ray powder diffraction data in the Powder Diffraction File PDFICDD [9], as shown in Table 1.

Therefore, in this work, we present the detailed structural characterization of the AgInSe₂ and AgInTe₂ compounds using Rietveld refinement and reported better X-ray powder diffraction data.

2. EXPERIMENTAL PART

The samples were synthesized by the melt and annealing technique. Stoichiometric quantities of Ag, In and Se (Te) powders with a nominal purity of 99.99 wt% (Fisher Scientific), were evacuated in sealed quartz ampoules and deposited into an one zone furnace, and then submitted to direct fusion. The mixture, en each case, was slowly heated up to 250ºC at a rate of 5ºC/hour. The ampoules were kept at this temperature for a period of 6 hours. Then, the temperature was gradually raised at the same rate up to 590°C. It was kept at this condition for 24 hours. Subsequently, it was heated at 800ºC at 10°C/hour and remained at this temperature for 3 hours. Then, the reacted mixtures were heated up to 1050°C at the same rate. Finally, the furnace was turned off and the ingots were cooled to room temperature in about a day.

Chemical analysis of the resultant ingots were carried out with a Hitachi S-2500 SEM equipped with a Kevex EDX accessory. Three different regions of each ingot were scanned. The error standardless analysis was around 5%.

Small quantities of each sample were ground mechanically in an agate mortar and pestle. The resulting fine powders were mounted on a flat zerobackground holder (a plate of single crystalline silicon cut parallel to the 510 lattice planes) covered with a thin layer of petroleum jelly. The X-ray powder diffraction data were collected at room temperature, in θ/θ reflection mode using a Siemens D5005 diffractometer (Bragg-Brentano geometry) equipped with an X-ray tube (CuKα radiation: λ= 1.54059 Å; 40kV, 30mA) and a diffracted beam graphite monochromator. Fix 1 mm aperture slit, 1 mm divergence slit and a 0.1 mm monochromator slit and a 0.6 mm detector slit were used. The specimens were scanned in the 2θ range of 10-100°, the scan step was 0.02°, and the time of counting in every step was 35s. Quartz was used as an external standard.

3. RESULTS AND DISCUSSION

3.1 Chemical analysis

The following average atomic percentages were obtained from the EDX study: Ag (24.4%), In (26.7%), Se (48.9%) for AgInSe₂, and Ag (23.5%), In (26.3%), Te (51.2%) for AgInTe₂. These results are in good quality agreement with the ideal composition 1:1:2 for each compound.

3.2 X-ray powder diffraction

The 19 (AgInSe₂) and 23 (AgInTe₂) peak positions measured from the patterns were input to the autoindexing program Dicvol04 [10]. Unique solutions were readily obtained in a tetragonal system. A comparison of the powder patterns taking into account the chemical composition, crystal system and cell parameters, showed that these materials are isostructural with the AgInS₂ chalcopyrite phase, which crystallize in the tetragonal space group I 2d (Nº 122) [5]. The complete powder diffraction dataset of both compounds were reviewed and the unit cell refined with the NBS*AIDS program [11] in the space group I 2d. From these analyses, the refined unit cell parameters given in the Tables 2 and 3 were obtained. These tables contain the observed and calculated X-ray powder diffraction data for AgInSe₂ and AgInTe₂, with the M_N [12] and F_N [13] figures of merit.

These powder data are improved that the three pattern with low quality (PDF 35-1099, PDF 38- 0952, PDF 75-0118) reported for AgInSe₂ and the two (PDF 49-1301, PDF 75-0119) reported for AgInTe₂ in the ICDD Powder Diffraction Files [9].

The crystal structure refinement was performed by means of Rietveld method [14], using the Fullprof program [15]. The starting parameters were taken from the AgInSe₂ and AgInTe₂ undistorted chalcopyrite-type structures [6,7] with the unit cell given in Tables 2 and 3. The structural refinement was carried out following the same procedure employed elsewhere [5, 16]. Details of the Rietveld refinement results for each compound are given in Tables 4 and 5. These tables contain the atomic coordinates, isotropic temperature factors and bond distances for each compound. Figures 1 and 2 shows the observed, calculated and difference profile for the final Rietveld refinements of AgInSe₂ and AgInTe₂. Figure 3 shows the unit cell diagram of the semiconducting compounds prepared with the program Diamond [17].

The ternary chalcogenides AgInSe₂ and AgInTe₂ are chalcopyrite-like compounds and can be described as derivatives of the sphalerite structure. In this type of materials, each cation is surrounded by four anions (Se or Te), and each anion is surrounded by two Ag and two In atoms. This array is expected for adamantane compounds [3]. The bond distances Ag-Se (Te) and In-Se (Te) are in good agreement with similar distances in other adamantane-type compounds found in the ICSD database [18], and agrees with the bond strength reported from ab initio calculations [19].

4. CONCLUSIONS

The crystal structure of the ternary semiconductor chalcogenides AgInSe₂ and AgInTe₂ were Rietveld refined using X-ray powder diffraction data. These compounds crystallize in the chalcopyrite-type structure and are reported improved X-ray powder diffraction data.

5. ACKNOWLEDGEMENTS

This work was supported by CDCHTA-ULA and FONACIT (Lab-97000821).

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