1.Introduction

Most semiconductor compounds crystallize in tetrahedral structures, where each atom has as its first neighbors four other atoms bonded by sp³ hybrid orbitals placed at the vertices of a tetrahedron. For the formation of these bonds, an average number of four valence electrons per atom is necessary. However, some derived structures involve the orderly omission of atoms resulting in so-called defect or vacancy semiconductor compounds. In this type of structures, some atom has less than four neighbors, due to the presence of vacancies in the cationic sites ¹. Moreover, magnetic semiconducting compounds and alloys in which manganese is one of the component elements are of interest because of the large magneto-optical effects, which can occur in these materials ². These semimagnetic compounds can be obtained from the tetrahedrally coordinated II-VI semiconductors by replacing a fraction of cations, following the rules of adamantane compounds formation. According to these rules, the cation substitution is performed in such a way that an average number of four valence electrons per atomic site and eight as the value for the ratio valence electrons to anions is maintained. In particular, two possible families of the fourfold defect derivatives of the II-VI binary semiconductors exist; II-III₂-▫-VI₄ and II₂-IV-▫-VI₄, where ▫ represents the cation vacancy which is included to maintain the same number of cations and anions sites ¹.

Materials with composition II₂-IV-▫-VI−4 with II = Mn, Fe, IV = Si, Ge, Sn, VI = S, Se, Te can be useful for applications such as thermoelectrics ^3-5, optoelectronics ⁶, spintronics and magnetic devices ^7-9. The presence of transition metal and chalcogenide elements provide unique interactions between electron spins, from the transition metal, and p-orbital electrons that contribute to modifying the physical properties of these materials, and increase their potential for different applications ¹⁰. It is therefore of great interest to establish its crystal structures and investigate its fundamental properties in order to further the understanding of their physical properties, and increase their potential for different applications.

Regarding to one of the fundamental properties as is their magnetic behavior, Mn derivatives have been reported to have antiferromagnetism behavior with Curie temperatures shown in Table I. The magnetic properties of Mn₂Sn▫Se₄ have not as yet been reported.

These materials generally crystallize in the olivine structure type, as shown in Table I for Mn derivatives, with the VI anions forming a hexagonal close packing, and the cations in tetrahedral (IV) and octahedral (II) coordination.

Particularly for the ternary Mn₂Sn▫Se₄ a poor quality powder diffraction pattern is reported in the Powder Diffraction File PDF-ICDD (039-0879) ¹¹, with cell parameters and space group as unique information. However, a search in the databases Inorganic Crystal Structure Database (ICSD) ¹² and Springer Materials (13), where are reported complete structural studies, showed no entries for this compound.

Therefore, the present work reports the synthesis and structural characterization of the new olivine-type compound Mn₂Sn▫Se₄, included unit cell parameters, atomic coordinates, isotropic temperature factors and geometric parameter values (cation-anion bond and angles), from powder X-ray diffraction data.

2.Experimental

Polycrystalline sample of Mn₂Sn▫Se₄ was synthesized using the melt and annealing technique. Stoichiometric quantities of highly pure Mn, Sn and Se elements, with a nominal purity of at least 99.99% (Sigma-Aldrich), were charged in an evacuated quartz ampoule, previously subject to pyrolysis in order to avoid reaction of the starting materials with quartz. The fusion process, 14 days, was carried out into a furnace (vertical position) heated up to 1050^∘C. Then, the temperature was gradually lowered to 500^∘C. Finally, the furnace was turned off and the ingots were cooled to room temperature. Chemical composition of the resultant ingot was determined at several regions by energy dispersive spectroscopic (EDS) analysis using a JMS-6400 scanning electron microscope (SEM). Three different regions of the ingot were scanned, and the average atomic percentages are: Mn (14.3%), Sn (28.5%) and Se (57.2%), very close to the ideal composition 2 : 1 : 4. The error in standardless analysis was around 5%.

X-ray powder diffraction pattern was collected at room temperature, in a Panalytical X’pert diffractometer using CuKα radiation (λ= 1.5418 Å). A small quantity of the sample was ground mechanically in an agate mortar and pestle and mounted on a flat holder. 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. The Panaytical X’pert Pro analytical software was used to establish the positions of the peaks.

3.Results and Discussion

X-ray powder diffractogram of Mn₂SnSe₄ shows a single phase (Fig. 1). The 20 first measured reflections were completely indexed using the program DICVOL04 ²⁵, which gave a unique solution in an orthorhombic cell with unit cell parameters a= 12.895(1) Å, b= 7.860(1) Å, c= 6.478(1) Å. Systematic absence of analysis indicates 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, crystallizing in the orthorhombic space group Pnma (N^∘ 62). The crystal structure refinement carried out by the Rietveld method ²⁶ was performed using the FULLPROF program ²⁷, with the unit cell parameters obtained in the indexed. The atomic coordinates of the isomorphic compound Mn2SnTe4 ²⁰ were used as starting parameters for the refinement. The instrumental and structural variables adjusted during the refinement were; zero shift, scale factor, asymmetry parameter, three pseudo-Voigt parameters of the peak-shape function, three unit cell parameters, eleven positional parameters and one overall isotropic temperature factor. The background was described by the automatic interpolation of 67 points throughout the whole pattern. The refinement converged to the final profile agreement factors summarized in Table II. Figure 1 shows the observed, calculated and difference profile for the final cycle of Rietveld refinement. Atomic coordinates, isotropic temperature factor, bond and angle distances are shown in Table III. Figure 2 show the unit cell diagram of Mn₂SnSe₄.

FIGURE 1 Observed, calculated, and difference plot of the final Rietveld refinement of Mn2Sn¤Se4. The Bragg reflections are indicated by vertical bars. 

FIGURE 2 Unit cell diagram of the new olivine-type compound Mn₂SnSe₄ (Pnma) showing the MnSe₆ octaedra and SnSe4 tetrahedra sharing faces in the crystal structure. 

Mn₂SnSe₄ crystallize in an olivine-type structure which consists of a three-dimensional arrangement of distorted MnSe₆ octahedra and SnSe₄ tetrahedra connected by common faces. The olivine structure can be described as a hexagonal close packing of Se⁻² anions with the Mn⁺² cations occupying half of the octahedral sites and the Sn⁺⁴ cations occupying an eighth of the tetrahedral sites. The Mn₁Se₆ octahedra are located at a center of symmetry and form infinite edge-shared chains parallel to [010]. In alternating positions to the left and right of the chains and situated half way between two Mn₁Se₆ octahedra, the Mn₂Se₆ octahedra are straddling the mirror planes perpendicular to [010]. The selenide ion common to the two octahedra Mn−1Se₆ and the Mn₂Se₆ octahedron forms one of the apices of an occupied SnSe₄ tetrahedron; the other 3 apices are located in a horizontal plane and are provided by 3 selenide ions of the chain below or above. Each Mn₁Se₆ octahedron shares: 2 edges with 2 Mn₁Se₆ octahedra, 2 edges with 2 Mn₂Se₆ octahedra, 2 edges with 2 SnSe₄ tetrahedra; while each Mn₂Se₆ octahe- dron shares: 2 edges with 2 Mn₁Se₆ octahedra, 1 edge with 1 SnSe4 tetrahedron. Figure 2 shows how the octahedra and tetrahedra share faces.

The interatomic distances are shorter than the sum of the respective ionic radii for structures tetrahedrally bonded ²⁸. The Mn-Se [mean value 2.72(2) Å] and Sn-Se [mean value 2.53(2) Å] bond distances, compare well with the same distances found in related adamantane compounds such as Cu₂SnSe₃²⁹, CuMn₂InSe₄³⁰, MnIn−2Se₄³¹, Cu₂MnSnSe₄³² and the systems CuGa _(1−X) Mn _(X) Se₂³³ and CuMn-III-Se₃ (III= Al, Ga, In) ³⁴. All of these phases were found in the Inorganic Crystal Structure Database (ICSD) ¹².

4.Conclusions

The ternary compound Mn₂SnSe₄ was synthesized and its crystal structure was determined using X-ray powder diffraction. This material crystallizes in the orthorhombic space group Pnma with an olivine-type structure and corresponds with a new compound of the II-III₂--VI₄ family with this crystalline arrangement.