Structure of the quaternary alloy Zn_0.6Mn_0.4In₂S₄ by Synchrotron Powder Diffraction and Electron Transmission Microscopy

Rosario Ávila-Godoy¹, Asiloé J. Mora², Dwight Acosta-Najarro³, Andrew N. Fitch⁴,

Gerzon E. Delgado², Andrés E. Mora^1,5, John Steeds⁵

¹Departamento de Física, Universidad de Los Andes, Mérida-Venezuela

²Departamento de Química, Universidad de Los Andes, Mérida-Venezuela

³Instituto de Física, Universidad Nacional Autónoma de México, DF, México

⁴European Synchrotron Radiation Facility, Grenoble Cedex, France

⁵University of Bristol, UK

When the solid solution of Zn_0.6Mn_0.4In₂S₄ is formed, the material departs from the stoichiometry of the parent compound ZnIn₂S₄ [1], a defect-type layered semiconductor that has octahedral and tetrahedral sites in which the cations can be accommodated. Therefore, it is possible that some cationic positions lose their point symmetry, because it becomes necessary to accommodate different proportions of the Zn, In and Mn cations in these sites. Hence, a change of crystalline symmetry from Rm to R3m is possible. Also, taking into account the ionic size, oxidation state and coordination number of Mn²⁺, it is probable that the magnetic ions occupys either octahedral or tetrahedral positions. Optical and magnetic measurements are contradictory in this matter [2]. The objective of the present work was to determine the structure of the quaternary alloy Zn_0.6Mn_0.4In₂S₄, and to locate in a precise way the positions of the Mn²⁺ ion in the crystalline cell. This was accomplished by means of two complementary techniques: X-ray powder diffraction using synchrotron radiation and Electron Transmission Microscopy techniques, such as High Resolution Microscopy (HRM) and Convergent Beam Electron Diffraction (CBED).

In spite of collecting the diffraction data in a spinning borosilicate capillary with the powder diffractometer of beamline ID31, ESRF, prefered orientation along the [001] direction due to the crystal morphology was present. However, the presence of reflection 006 at 14.3º 2q, implied that the structure could be non-centrosymmetric R3m, and Rietveld refinements using different cationic arrengements were performed. A model in which the tetrahedral sites were occupied by a random distribution of Zn, Mn and In atoms, but with local 3m symmetry, gave the best results. The Rietveld refinement of this model led to figures of merit: R_wp = 9.9%, R_p= 9.2%, χ² = 11.21 and R(F²) = 0.1146. The final Rietveld plot showing the observed, calculated and difference patterns of the Zn_0.6Mn_0.4In₂S₄ is shown in Figure 1. Selected Area Electron Diffraction (SAED) patterns and High Resolution Micrography along [001] showed the rhombohedral configuration (Figure 2a). From CBED patterns perpendicular to [001] the 6mm symmetry breaking to the 3m symmetry associated to the R3m space group can be observed in Figure 2b.

Fig. 1. Final Rietveld plot showing the observed, calculated and difference patterns of the Zn_0.6Mn_0.4In₂S₄

Fig. 2: (a) High-resolution micrograph, (b) CBED pattern of Zn_0.6Mn_0.4In₂S₄ along [001] showing the 3m symmetry that validate the R3m space group.

Acknowledgments

W. Giriat and A. López-Rivera, which kindly prepared the samples. CDCHT-ULA, FONACIT (Lab-97000821) and ESRF (France).

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