The chalcogenide compound Fe2SnSe4: Synthesis and crystal structure analysis by powder X-ray diffraction

aUniversidad Libre, Facultad de Ingeniería, Bogotá, Colombia bCentro de Tecnología para Aguas Profundas, Cátedras Conacyt-Instituto Mexicano del Petróleo, Veracruz, México cInstituto Politécnico Nacional, Escuela Superior de Ingeniería Química e Industrias Extractivas, Ciudad de México, México d Universidad de Los Andes, Departamento de Química, Facultad de Ciencias, Mérida, Laboratorio de Cristalografía, Venezuela


Introduction
The ternary chalcogenide semiconductors A II 2 -B IV -X VI 4 (II= Mn, Fe, Co; IV= Si, Ge, Sn; VI= S, Se, Te) have drawn wide interest for their magnetic, optoelectronic and thermoelectric properties 1-12 . The presence of transition metal and chalcogenide elements in the same compound enables unique interaction between the electron spins, which is one of the factors that give rise to potential applications 13 .
These type of materials generally belongs to the adamantine family of ternary chalcogenide crystallizing in the olivine type structure 1 with the anions forming a hexagonal close packing and the cations in tetrahedral and octahedral coordination. However, a distorted spinel structure with space group I4 1 /a has been reported for Fe 2 SnS 4 2 , and an orthorhombic structure with space group Cmmm for Mn 2 SnS 4 3 . It should be noted that the presence of a transition metal in these materials additionally introduces a magnetic behavior 4 .
From the point of view of their magnetic structure and properties, the chalcogenide olivines represent interesting examples of frustrated lattices of magnetic atoms determining complex magnetic excitations 14 . Materials belonging to this family of olivine-type compounds have been considered as good model systems to study multicritical phenomena 15 , and have found important new applications also for thermoelectric power generation 8 , and as cathode materials for batteries 16 .
In particular, for olivine-like structures containing iron atoms, recently it has been proven that the ternary chalcogenides Fe 2 GeS 4 and Fe 2 SiS 4 have the potential to overcome the limitations in the binary FeS 2 , which is considered as one of the promising light-absorbing material with high absorption coefficient to be used in thin film solar cells 17 . These materials are proposed to have high absorption coefficients and gap of 1.40 eV and 1.55 eV, respectively, which is more suitable for solar light absorption. Besides both phases are predicted to be more stable than the binary FeS 2 phase. These properties make Fe 2 GeS 4 and Fe 2 SiS 4 as a potential material to use in photovoltaics 17 . Meanwhile, Fe-content olivine compounds with Se ad Te anions emerge as promising candidates with good thermoelectric performances 8 .
The compound Fe 2 SnSe 4 , or more precisely Fe 2 Sn[] Se 4 , where [] denotes the cation vacancy which is included to maintain the same number of cations and anions sites, belongs to this family, II 2 -IV-VI 4 , of ternary chalcogenide semiconductors 18 . As regards to the physical properties of the ternary Fe 2 SnSe 4 , it was reported that it shows ferromagnetic behavior with a Curie temperature near to room temperature Tc= 301.4K 5 , and interesting transport properties for thermoelectric applications 8 . However, no complete X-ray crystal structure analysis has been reported and the only crystallographic information are the cell parameters: (a= 14.778 Å, b= 10.764 Å, c= 6.061 Å, V=964.0 Å 3 ) 5 and (a= 14.803 Å, b= 10.768 Å, c= 6.059 Å, V= 965.8 Å 3 ) 9 . Moreover, a search in the databases Powder Diffraction File PDF-ICDD 19 , Inorganic Crystal Structure Database (ICSD) 20 , and Springer Materials 21 showed no entries for this ternary chalcogenide compound.
It is very important to establish the crystal structure of a semiconductor because this is used to understand and explain *e-mail: gerzon@ula.ve the physical properties relevant to possible applications. In the hope of providing a full description of the Fe 2 SnSe 4 structure, in the present work, and as part of ongoing crystal structural studies on chalcogenide compounds [22][23][24][25][26][27] , we report the structural characterization of this ternary compound using powder X-ray diffraction techniques.

Synthesis
Fe 2 SnSe 4 was synthesized by the reaction of high purity elements, Fe, Sn, and Se with a nominal purity of at least 99.99% (Sigma-Aldrich), using the melt and annealing technique. Stoichiometric quantities of the three elements were charged in an evacuated quartz ampoule, previously subject to pyrolysis to avoid reaction of the starting materials with quartz. Then, the ampoule was sealed under vacuum (~10 -4 Torr) and the fusion process was carried out inside a furnace (vertical position) heated up to 1100 K at a rate of 20 K/h, with a stop of 48 h at 493 K (melting point of Se). The ampoule was shaking using a mechanical system during all the heating process to guarantee the complete mixing of all the elements. Then, the temperature was gradually decreased until 600 K and this temperature was maintained for 30 days. Finally, the furnace was turned off and the ingots were cooled to room temperature.

Chemical analysis
The stoichiometry of the sample was determined by energy-dispersive X-ray spectroscopy (EDS) analysis using a JMS-6400 scanning electron microscope (SEM). The average composition of Fe: Sn: Se sample, taken from the central part of the ingots, was 14.0: 28.6: 57.4 at. % that led to the formula close to the ideal value composition 2: 1: 4. The error in the standardless analysis was around 5%.

X-ray powder diffraction
For the X-ray analysis, a small quantity of the sample was ground mechanically in an agate mortar and pestle. The resulting fine powder was mounted on a flat holder. The X-ray powder diffraction data were collected at room temperature, in reflection mode using a Panalytical X'pert diffractometer using CuKα radiation (λ = 1.5418 Å). The specimen was scanned from 10 to 80° 2θ, with a step size of 0.02° and counting time of 20 s. Silicon (SRM-640) was used as an external standard.

Results and Discussion
For pattern indexing and unit cell parameter refinement, the precise determination of peaks positions was carried out using the Highscore Plus v3.0 analytical software. The X-ray diffractogram of Fe 2 SnSe 4 is shown in Figure 1. A search in the PDF-ICDD database 19 was performed and no binaries are present. Therefore, the powder X-ray pattern corresponds to a single phase.
The 20 first measured reflections were completely indexed using the program DICVOL04 28 , which gave a unique solution in an orthorhombic cell with unit cell parameters a = 13.202 Å, b = 7.675 Å, c = 6.357 Å, and indexed figures of merit M 20 = 26.7 29 and F 20 = 40.7(0.0100, 49) 30 . Systematic absences indicate a P-type cell, which suggested along with the sample composition and cell parameter dimensions that this material is isostructural with the olivine type compounds with orthorhombic space group Pnma (N° 62) as the recently reported compound Mn 2 SnSe 4 [ 27 ]. So the space group Pnma and the atomic position parameters of Mn 2 SnSe 4 were taken as the staring values to refine the structural parameters of Fe 2 SnSe 4 .
The crystal structure refinement, employing the Rietveld method 31 , was performed using the Fullprof program 32 available in the software package Winplotr 33 . The indexing results were taken as the starting unit cell parameters. The angular dependence of the peak full width at half maximum (FWHM) was described by the Caglioti's formula, FWHM 2 =Utan 2 θ+Vtanθ+W, where U, V, and W are fitting parameters 34 . Peak shapes were described by the parameterized Thompson-Cox-Hastings pseudo-Voigt profile function 35 . The background was described by the automatic interpolation of 66 points throughout the whole pattern. The thermal motion of the atoms was described by one overall isotropic temperature factor. Details of the Rietveld refinement of Fe 2 SnSe 4 are summarized in Table 1, and the atomic positions and thermal displacement factors are presented in Table 2. The observed, calculated, and residual powder XRD patterns of Fe 2 SnSe 4 are shown in Figure 1.
It should be noted that the previously reported unit cell parameters for this material 5,9 will be used to reproduce the experimental diffraction pattern, however, it does not produce good results. On the other hand, the unit cell volume obtained in this work agrees in the order of magnitude with those reported for similar compounds (see Table 3).
The structure of the ternary chalcogenide Fe 2 SnSe 4 can be described as an olivine type structure which consists of a hexagonal close packing of Se -2 anions with the Fe +2 cations occupying half of the octahedral sites and the Sn +4 cations occupying an eighth of the tetrahedral sites. As expected for these materials each anion is coordinated by four cations (three Fe and one Sn) located at the corners of a slightly distorted tetrahedron. The polyhedral coordination of the cations and anions are presented in Figure 2, showing the FeSe 6 octahedra, SnSe 4 tetrahedra and Se(SnFe 3 ) tetrahedra formed. Figure 3 shows the unit cell diagram of Fe 2 SnSe 4 with the octahedra and tetrahedra coordination around the cations. Details of the olivine type structure description are published elsewhere 21 .
Unit cell parameter values of Fe 2 SnSe 4 are very similar to those reported in the crystal structures of the Fe-content compounds with II 2 -IV-VI 4 compositions as shown in Table 3.   and CuFe 2 (Al,Ga,In)Se 4 46,47 . All the bond angles in the structure of Fe 2 SnSe 4 are close to the ideal tetrahedral and octahedral bond angle values.

Conclusions
The crystal structure of the ternary magnetic compound Fe 2 SnSe 4 was refined using powder X-ray diffraction through the Rietveld method. This material was synthesized by the melt and annealing technique and crystallizes with an olivine structure in the orthorhombic space group Pnma. This is a new compound of the II 2 -IV-VI 4 family of semiconductors with an olivine-type structure. The crystal structure knowledge of Fe 2 SnSe 4 allows further investigation of this material about their structure-property relationship. This ternary semiconductor compound can be considered as a potential candidate for device thermoelectric applications.

Acknowledgments
This work was supported by FONACIT (Grant LAB-97000821).

References
1. Vincent H, Bertaut EF, Baur WH, Shannon RD. Polyhedral deformations in olivine-type compounds and the crystal structure