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Boletín de la Sociedad Chilena de Química

Print version ISSN 0366-1644

Bol. Soc. Chil. Quím. vol.45 n.4 Concepción Dec. 2000

http://dx.doi.org/10.4067/S0366-16442000000400013 

A NEW STRUCTURAL MODIFICATION OF STANNITE

Jaime Llanosa, Myriam Tapiaa, Carlos Mujicaa,
Judith Oró-Soleb, and Pedro Gómez-Romerob

aDepartamento de Química, Facultad de Ciencias, Universidad Católica del Norte,
Casilla 1280, Antofagasta, Chile
bInstitut de Ciència de Materials de Barcelona (CSIC), Campus de la Universitat Autónoma
de Barcelona, 08193 Bellaterra, Barcelona, España.
(Received: July 6, 2000 - Accepted: August 14, 2000)

ABSTRACT

The crystal structure of the phase a-SnCu2FeS4, has been examined by EDX and electron and X-ray difraction techniques. This compound crystallizes in the tetragonal space group P4 (No. 81) with a=c= 541.4 (4) pm, Z=1, and it is a derivative structure of the basic sphalerite structure. The refinement of the structure converged to the final agreement index R(F)=0.060.

Keywords: chalcogenides, electron diffraction, electron microscopy, X-ray diffraction

RESUMEN

La estructura cristalina de la fase a-SnCu2FeS4 ha sido analizada por técnicas de difracción de rayos-X, electrones y Energía Dispersiva de Rayos-X. Este compuesto cristaliza en el sistema tetragonal, grupo espacial P4 (No. 81) with a=c=541,4(4) pm, Z=1 y puede ser descrita como una estructura derivada de la esfalerita. El refinamiento final de la estructura convergió a R(F)=0,060.

Palabras Claves: calcogenuros, difracción de electrones, microscopía electrónica, difracción de rayos-X.

INTRODUCTION

Ternary and quaternary chalcogenides have attracted growing interest in recent years due to their physical and chemical properties, which are very promising in important technologies such as nonlinear optics, solar energy conversion, cathodes in electrochemical cells (1-4).

In previous papers, we have reported on the solid state chemistry of alkali or alkali earth metals copper-iron-chalcogenides (5-8). As a result of an attempt to prepare substitutional solid solutions in the Sn-Cu-Fe-S system we have found a new structural modification of the stannite SnCu2FeS4 (9).

EXPERIMENTAL

Crystal of a-SnCu2FeS4 were obtained by solid state reaction of SnS, Aldrich, ( 0.37 g) and CuFeS2 (0.92 g). The mixture was heated in a tightly sealed graphite crucible at 1323K in a vertical furnace for 24 h under an Ar atmosphere. The sample was then allowed to cool over a period of 50 h at room temperature. The components of the sample were determined by qualitative XEDS analysis and the crystal structure determination by both electron and X-ray diffraction.

Chalcopyrite was synthesized by sulfurization of a stoichiometric mixture of copper oxide (Baker Reagents) and iron (Merck ) with CS2 as sulfiding agents carried by Ar. The mixture was pressed into a pellet and heated at 775 K by 12 h.

Electron diffraction patterns and chemical analyses were obtained using a JEOL JEM 1210 microscope operating at 120kV equipped with a side-entry 60º/30º double tilt GATHAN 646 analytical specimen holder and a link QX2000 EDX element analysis system. The specimen for electron microscopy were prepared by grinding the powder sample, dispersing it in n-butanol and depositing a droplet of this suspension on a carbon coated film supported on an alumina grid.

The data for the crystal structure determination of a-SnCu2FeS4 were obtained with the four circles diffractometer Enraf-Nonius CAD4 using graphite monochromated Mo Ka (l =71.069 pm) radiation.

The EDX measurements confirm the presence of the four elements, Sn, Cu, Fe, and S in the a-SnCu2FeS4 single crystal. Figure 1 shows and EDX curve for a-SnCu2FeS4 single crystal.


Electron diffraction studies of this sample and the subsequent reconstruction of the reciprocal lattice indicates that there not any systematic absences, thus corresponding to an extinction symbol P_ . Figure 2 shows the electron diffraction pattern for a-SnCu2FeS4 single crystal.


On the other hand, a crystal of a-SnCu2FeS4 was selected and mounted into a glass capillary for X-ray analysis with a four circles diffractometer. Cell parameters were refined from 25 centered reflections. Intensities were measured in the w-2q scan mode and the absorption correction was done empirically by y-scanning.

The structure was solved by the direct method in the space group and subsequently refined by the full least-squares method using the SHELXL-97 program system (10). The anisotropic refinement converged to R=0.060.

The crystallographic data as well as details of the structure analysis of a- SnCu2FeS4 are given in Table 1.


RESULT AND DISCUSSION

The refined atomic coordinates, equivalent displacement factors and site occupation are given in Table 2. Selected interatomic distances and angles are given in Table 3. The crystal structure, plotted using the program CrystalMaker 4.0 (11), is shown in Fig. 3.



The structure of a-SnCu2FeS4, is a normal tetrahedral compound which may be regarded as a derivative of the basic sphalerite structure. As it is expected for this type of structure, each S atom is surrounded by four cations (two Cu, one Fe and one Sn), and each cation is coordinated by four sulfur atoms. The tetrahedra surrounding the Sn atoms are the most regular.


Taking into account the metric of the unit cell of a-SnCu2FeS4 (a=b=c=541.3 pm) , the structure was first intended to solve in the following space groups: Pm3¯ (No. 221), P43¯m(No. 215), P432 (No. 207), Pm 3¯ (No. 200), and P23 (No. 195). All results shown either six-fold coordination for the copper, iron, tin, and sulfur atoms or very high residual factors.

Finally, is known that the symmorphic space group for the sphalerite structure is Td2 (F4 3m), a study of the subgroups of the crystallographic Td (4 3m) point group indicated that the lower possible symmetry was 4. Due to the electron diffraction results verify that there not any systematic absences, the structure was solved in the space group P4¯.

ACKNOWLEDMENTS

This work was supported by FONDECYT contract 1960372 and the Programa de Intercambio Científico CONICYT (Chile) and CSIC (Spain), grant 96022.

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