Crystal structure study of the semiconducting system Ag-In-VI (VI = S, Se, Te) by X-ray powder diffraction

Gerzon E. Delgado¹, Asiloé J. Mora¹, Tibaire Tinoco², Carlos Pineda²

¹Laboratorio de Cristalografía, Departamento de Química, Facultad de Ciencias,

²Centro de Estudios Avanzados en Óptica, Departamento de Física, Facultad de Ciencias,

Universidad de Los Andes, Mérida 5101, Venezuela.

Introduction

The ternary silver chalcogenides of the type Ag-In-VI (VI = S, Se, Te) form very stable stoichiometric compounds with potential application in non-linear optics and solar cells [1,2], due to its electro-physical properties independent of impurities [2]. Stoichiometric compounds with three compositions can be found: AgInVI₂, AgIn₃VI₅ and AgIn₅VI₈. These materials belong to the normal structure compounds (I III VI₂) and the defect structure compounds (I III₃VI₅, I III₅₂VI₈), respectively [3]. Compositions of the type AgIn₃VI₅ has not been reported in the literature. In this work, we present the structural characterization of some members of the system Ag-In-VI, using X-ray powder diffraction.

Experimental

The samples were synthesized by the melt and annealing technique. Stoichiometric quantities of Ag, In and X (S, Se, Te) were evacuated in sealed quartz ampoules and deposited into an one zone furnace, and then submitted to direct fusion. Chemical analysis of the resultant ingots were carried out with a Hitachi S-2500 SEM equipped with a Kevex EDX accessory. Small quantities of each the samples were ground mechanically in an agate mortar and pestle. The resulting fine powders were mounted on a flat zero-background holder. The X-ray powder diffraction data were collected at room temperature, in q/q reflection mode using a Siemens D5005 diffractometer (Bragg-Brentano) equipped with an X-ray tube (CuKα radiation; 40kV, 30mA) and a diffracted beam graphite monochromator. All the Rietveld refinements were carried out using the Fullprof program [4] available in the software package WinPlotr [5].

Results and discussion

In the Ag-In-S system, three phases were simultaneously characterized from a single X-ray powder diffraction pattern using the Rietveld method [6]. The X-ray powder pattern was composed of AgIn₅S₈ (42.7%), AgInS₂ tetragonal phase (32.7%) and AgInS₂ orthorhombic phase (24.6%) [7]. The AgIn₅S₈ crystallizes in the cubic space group Fd-3m, Z = 8, with a = 10.8265(2) Å, in a spinel-type structure. The description of the structure of AgIn₅S₈ in this space group implies a random distribution of the Ag and In atoms among the tetrahedral sites. The AgInS₂ ternary compound crystallizes in two polymorphs: a tetragonal chalcopyrite-type phase (I-42d), Z = 4, with unit cell parameters a = 5.8760(2) Å, c = 11.2007(7) Å, and an orthorhombic wurtzite-like phase (Pna2₁), Z =4, with a = 6.9972(6) Å, b = 8.2733(6) Å, c = 6.6939(6) Å. These structures can be described as a derivative of the sphalerite structure and the wurtzite structure, respectively. Ag-S and In-S average distances in both structures are 2.58(5) Å and 2.44(5) Å, respectively.

	AgInS2 (orthorhombic, Pna21) AgIn5S8 (cubic, Fd-3m)
a)	b)
Figure 1. (a) Final Rietveld refinement plot for the three phases of the Ag-In-S system. (b) Unit cell diagram of AgInS2 (orthorhombic) and AgIn5S8 (cubic) phases.

For the Ag-In-Se system, a chalcopyrite-type structure was found with composition AgInSe₂. This compound crystallizes with tetragonal symmetry (I-42d) and unit cell parameters: a = 6.1010(8) Å, c = 11.708(2) Å. The Ag-Se bond distance is 2.621(2) Å and In-Se 2.593(2) Å.

	AgInSe2 (tetragonal, I-42d)
Figure 2. Final Rietveld refinement plot for AgInSe2 and unit cell diagram.

For the Ag-In-Te system, AgIn₅Te₈ is isostructural with the AgIn₅Se₈ compound [8]. Crystallize in the tetragonal space group P-42m with a = 6.1952(2) Å, c = 12.419(4) Å, and consists of a three-dimensional arrangement of distorted AgTe₄ and InTe₄ tetrahedra connected by common faces [9]. The Ag-Te [2.890(6) Å] and In-Te [2.764(7) Å] bond distances agree well with those observed in other adamantane structures such as AgGaTe₂, LiInTe₂, CuInTe₂, MnIn₂Te₄ and CdIn₂Te₄.

	AgIn5Te8 (tetragonal, P-42m)
Figure 3. Final Rietveld refinement plot for AgIn5Te8 and unit cell diagram showing the tetrahedral coordination around the cations.

Acknowledgement

This study was supported by the CDCHT-ULA and FONACIT (Lab-97000821).

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