The chalcopyrite family of compounds, with formula I-III-VI₂ (I= Cu, Ag, III= Al, Ga, In, VI= S, Se, Te) form an extensive group of semiconductor materials with diverse optical and electrical properties ^1-3. From the structural point of view, chalcopyrite crystallizes with tetragonal symmetry in the space group I 4 ¯ 2 d (N^o122) being isostructural with the CuFeS₂ mineral from which they take their name ⁴.

• ¹
  Ternary Chalcopyrite Semiconductors: Growth Electronic Properties, and Applications, 1975
  Ternary Chalcopyrite Semiconductors: Growth Electronic Properties, and Applications
• ³
  Theory of the band-gap anomaly in AB 𝐶 2 chalcopyrite semiconductors

  Phys. Rev. B, 1984
  Theory of the band-gap anomaly in AB 𝐶 2 chalcopyrite semiconductors
• ⁴
  The Crystal Structure of Chalcopyrite CuFeS2

  Z. Kristall, 1932
  The Crystal Structure of Chalcopyrite CuFeS2

The addition of a II-VI binary compound (II= V, Mn, Fe, Co, Ni, Zn, Cd) to chalcopyrite produces alloys of composition (I-III-VI₂)_1-x (II-VI)_x, and changing the composition variable x, it is possible to find the following compounds in this system: I₂-II-III₂-VI₅ (x = 1/3), I-II-III-VI₃ (x = 1/2), and I-II₂-III-VI₄ (x = 2/3), among others. These families of compounds fulfill the rules of formation of adamantane compounds and belong to the normal semiconductor compound families ⁵. According to these rules, the cation substitution is carried out in such a way that an average number of four valence electrons per atomic site is maintained and in turn value of eight for the ratio between valence electrons to anions. Adamantane compounds are binary, ternary, or quaternary normal tetrahedral structure compounds that are closely related to either cubic or hexagonal diamond ⁵. In our laboratories, we have been studying these type of alloys from its synthesis, thermal and magnetic properties as well as their crystal structures ^6-13. Due to the great variety of possible compositions, these materials can be useful for applications such as tunable semiconductors ¹⁴, photovoltaics ¹⁵, non-linear optics ¹⁶, thermoelectrics ¹⁷, and particularly as spintronic device ¹⁸ due to the discovery of room-temperature ferromagnetism and super-paramagnetism in some of these materials ¹⁹.

• ⁵
  Wurzite and Sphalerite Structures

  Intermetallic Compounds, 1995
  5. E. Parthé, Wurzite and Sphalerite Structures, in Intermetallic Compounds, edited by J. H. Westbrook and R. L. Fleischer, Vol. 1 (Wiley, Chichester, 1995).
• ⁵
  Wurzite and Sphalerite Structures

  Intermetallic Compounds, 1995
  5. E. Parthé, Wurzite and Sphalerite Structures, in Intermetallic Compounds, edited by J. H. Westbrook and R. L. Fleischer, Vol. 1 (Wiley, Chichester, 1995).
• ⁶
  A comparative Study of (Cu-III-Se2)x -(FeSe)1-x Alloys (III: Al, Ga, In) (0 ≤ x ≤ 1) by X-Ray Diffraction, Differential Thermal Analysis and Scanning Electron Microscopy

  Phys. Status Solidi A, 2001
  A comparative Study of (Cu-III-Se2)x -(FeSe)1-x Alloys (III: Al, Ga, In) (0 ≤ x ≤ 1) by X-Ray Diffraction, Differential Thermal Analysis and Scanning Electron Microscopy
• ¹³
  Synthesis and Crystal Structure of Three New Quaternary Compounds in the system Cu- Mn-III-Se3 (III = Al, Ga, In)

  Mater. Res, 2018
  Synthesis and Crystal Structure of Three New Quaternary Compounds in the system Cu- Mn-III-Se3 (III = Al, Ga, In)
• ¹⁴
  Earth Abundant Element Cu2Zn(Sn1-xGex)S4 Nanocrystals for Tunable Band Gap Solar Cells: 6.8% Efficient Device Fabrication

  Chem. Mater, 2011
  Earth Abundant Element Cu2Zn(Sn1-xGex)S4 Nanocrystals for Tunable Band Gap Solar Cells: 6.8% Efficient Device Fabrication
• ¹⁵
  Fabrication of 7.2% Efficient CZTSSe Solar Cells Using CZTS Nanocrystals

  J. Am. Chem. Soc, 2010
  Fabrication of 7.2% Efficient CZTSSe Solar Cells Using CZTS Nanocrystals
• ¹⁶
  Electronic, optical and lattice dynamic properties of the novel diamond-like semiconductors Li2CdGeS4 and Li2CdSnS4

  J. Phys. Condens. Matter, 2011
  Electronic, optical and lattice dynamic properties of the novel diamond-like semiconductors Li2CdGeS4 and Li2CdSnS4
• ¹⁷
  Thermoelectric transport properties of CuFeInTe3

  J. Alloys Compd, 2015
  Thermoelectric transport properties of CuFeInTe3
• ¹⁸
  New Materials for Spintronics

  MRS Bull, 2003
  18. S. A. Chambers and Y. K. Yoo, New Materials for Spintronics, MRS Bull. 28 (2003) 706, https://doi.org/10.1557/mrs2003.210.
• ¹⁹
  Superparamagnetism in CuFeInTe3 and CuFeGaTe3 alloys

  Phys. Status Solidi A, 2012
  Superparamagnetism in CuFeInTe3 and CuFeGaTe3 alloys

In particular, the ternary chalcopyrite semiconductor CuInSe₂ is one of the most studied materials due to its high optical absorption coefficient ( α ∼ 104 cm^-1 at 1 eV), which is essential for thin films photovoltaic applications. It crystallizes in an ordered structure, and melts congruently at 1259 K with an order-disorder thermal transition at 1083 K ²⁰. The addition to the ternary CuInSe₂ of a metal transition element, as in the FeSe binary compound, produces alloys of the type (CuInSe₂)_1-x (FeSe)_x. For this system, a phase diagram was proposed based on XRD and DTA measurements. At 600 K, two single-phase fields, chalcopyrite and semi-ordered phase, separated by a relatively narrow two-phase field were observed ¹². These results suggest a phase sequence process as a function of composition (x) could be from the ordered chalcopyrite structure x = 0, to x = 1/3 and 1/2 as intermediate disordered phases, before a reordering of the cationic sublattice occurs at values of x = 2/3. Table Ishow the crystallographic parameter comparison for the four compositions of the system (CuInSe₂)_1-x (FeSe)_x. These results suggest that composition x = 1/3, Cu₂FeIn₂Se₅, could crystallize with a disordered structure in its cationic network, however until now its crystalline structure had not been established.

• ²⁰
  Crystal data for CuInSe2

  J. Appl. Cryst, 1973
  Crystal data for CuInSe2
• ¹²
  Phase Diagram of (CuInSe2)1-x (FeSe)x alloys

  J. Alloys Compd, 2015
  Phase Diagram of (CuInSe2)1-x (FeSe)x alloys

For this reason, to derive a model that explains well all the X-ray diffraction peaks observed in the powder pattern of this compound, a detailed structural analysis of the Cu₂FeIn₂Se₅alloy using powder X-ray diffraction was performed. The structure of the quaternary Cu₂FeIn₂Se₅completes the phase transition produced in the (CuInSe₂)_1-x (FeSe)_xsystem between the values x = 0 to x = 2/3.

Starting materials (Cu, Al, Ta, and Se) with nominal purity of 99.99 wt. % in the stoichiometric ratio were mixed in an evacuated (10^-4 Torr) and sealed quartz tube with the inner walls previously carbonized to prevent the chemical reaction of the elements with quartz Polycrystalline ingots of about 1 g were prepared by the melting and annealing technique. The quartz ampoule is heated until 493 K (melting point of Se), keeping this temperature for 48 h and shaking all the time using an electromechanical motor. This procedure guarantees the formation of binary species at low temperatures avoiding the existence of Se free gas at high temperature, which could produce explosions or Se deficiency in the ingot. Then the temperature was slowly increased until 1423 K, with the mechanical shaker always connected for better mixing of the components. After 24 h, the cooling cycle begins until the anneal temperature (800 K) with the mechanical shaker is disconnected. The ampoule is keeping at the annealing temperature for 1 month to assure the thermal equilibrium. Then the furnace is switching off. This preparation method has proven to give good results ^6,12.

• ⁶
  A comparative Study of (Cu-III-Se2)x -(FeSe)1-x Alloys (III: Al, Ga, In) (0 ≤ x ≤ 1) by X-Ray Diffraction, Differential Thermal Analysis and Scanning Electron Microscopy

  Phys. Status Solidi A, 2001
  A comparative Study of (Cu-III-Se2)x -(FeSe)1-x Alloys (III: Al, Ga, In) (0 ≤ x ≤ 1) by X-Ray Diffraction, Differential Thermal Analysis and Scanning Electron Microscopy
• ¹²
  Phase Diagram of (CuInSe2)1-x (FeSe)x alloys

  J. Alloys Compd, 2015
  Phase Diagram of (CuInSe2)1-x (FeSe)x alloys

Stoichiometric relations of the samples were investigated by scanning electron microscopy (SEM) technique, using a Hitachi S2500 equipment. The microchemical composition was found by an energy-dispersive X-ray spectrometer (EDS) coupled with a computer-based multichannel analyzer (MCA, Delta III analysis, and Quantex software, Kevex). For the EDS analysis, K α lines were used. The accelerating voltage was 15 kV. The samples were tilted 35 degrees. A standardless EDS analysis was made with a relative error of ±5-10% and detection limits of the order of 0.3 wt %, where the k-ratios are based on theoretical standards. Table IIshows the experimental stoichiometry of the sample Cu₂FeIn₂Se₅.

Differential Thermal Analysis (DTA) measurements were carried out in a fully automatic Perkin-Elmer apparatus, which consists of a Khantal resistance furnace (T_max = 1650 K) equipped with Pt/Pt-Rh thermocouples and an informatics system for the automatic acquisition data. The internal standard used was a high purity (99.99 wt. %) piece of gold. The temperature runs have been performed from ambient temperature to 1400-1500 K, which is the recommended operative limit. The heating rate is controlled electronically to 20 Kh^-1; the cooling rate was given by the natural cooling of the furnace after switching off. From the thermogram, transition temperatures are manually obtained from the Δ T vs. T graph with the criteria that the transition occurs at the intersection of the baseline with the slope of the thermal transition peak, as usual. The maximum error committed in the determination of transition temperatures by this method is estimated to be ±10 K.

The X-ray powder diffraction data were collected at room temperature, in a θ/ θ reflection mode using a Siemens D5005 diffractometer equipped with an X-ray tube (CuK α 1 radiation: λ = 1.54056 Å; 40 kV, 30 mA) and a diffracted beam graphite monochromator. A 1 mm aperture slit, a 1 mm divergence slit, a 0.1 mm monochromator slit, and a 0.6 mm detector slit were used. The specimen was scanned in the 2θ range of 10-110^o, the scan step was 0.02^o, and the time of counting in every step was 10 s. Quartz was used as an external standard. The instrument analytical software was used to establish the positions of the peaks.

In Fig. 1, the thermogram for sample Cu₂FeIn₂Se₅is displayed.

In the heating cycle, it can be observed two thermal transitions at 1017 and 964 K. The shape of the peak is typical of an incongruent melting point where the solid phase transits to a solid + liquid region at 964 K and then to a liquid phase (melting) at 1017 K. However, in the cooling cycle, up to five thermal transitions are observed. The fact that only two thermal transitions are observed in the heating cycle and five in the cooling is probably due to the difference between the heating and cooling rates in competition with the velocity of the thermal transitions. The heating rate is electronically fixed at 10 K/min, whereas the cooling rate is variable, given by the natural cooling of the furnace after switching off. Transitions solid-to-liquid (and viceversa) are faster than solid-to-solid and involve higher energies (variation in the enthalpy, Δ H ), for these reasons, solid-to-solid transitions are better observed in the cooling cycle.

The high-temperature transition at 1366 K in the cooling, coincides with the melting point of FeSe reported as 1348 K ²² suggesting that, at this temperature, the liquid phase undergoes to a liquid + FeSe region. The liquid + FeSe region is wide, from 1366 K to 1109 K (257 degrees). At 1109 K, the liquid phase solidifies, possibly in the disordered sphalerite β- phase accompanied by FeSe-phase. At 1013 K, the semi-ordered α ' - phase coexists with the β ' - phase and FeSe, at 953 K, the region is a´ + β and finally, at 923 K, the region is only a´. In Fig. 2, a schematic representation of the successive phase transitions is given.

• ²²
  The fese (ironselenium) system

  J. Ph. Equilibria, 1991
  22. H. Okamoto, The fese (ironselenium) system, J. Ph. Equilibria 12 (1991) 383, https://doi.org/10.1007/BF02649932.

Figure 3 shows the resulting X-ray powder pattern for the Cu₂FeIn₂Se₅’ compound. When the 2θ positions of the 20 first peaks in the diffraction pattern are introduced into the auto-indexing program Dicvol04 ²³, a tetragonal cell of dimensions a = 5.780(1) Å, c = 11.610(2) Å is obtained. These parameters are similar in magnitude to the parent’s chalcopyrite structure CuInSe₂²¹ and P-chalcopyrite structure CuFeInSe₃¹⁰. The systematic absence condition in the general reflections of the type hkl indicating a P-type cell, and the hhl:l = 2n and 00l:l = 2n conditions suggests the extension symbol P 4 ¯ 2 c . To find the atomic positions to adjust the diffraction pattern was employed a similar analysis to that used in the structural determination of the quaternary alloy CuFeInSe₃, which crystallize in the same space group ¹⁰. It should be noted that this analysis was carried out starting from the prototype of the P-chalcopyrite structure, which was the structure of the Cu-poor Cu-In-Se compound β -Cu_0.39In_1.2Se₂²⁴.

• ²³
  Powder patter indexing with the dichotomy method

  J. Appl. Cryst, 2004
  Powder patter indexing with the dichotomy method
• ²¹
  The crystal structures of CuInSe2 and CuInTe2

  Mater. Res. Bull, 1992
  The crystal structures of CuInSe2 and CuInTe2
• ¹⁰
  Crystal structure of CuFeInSe3 from X-ray powder diffraction data

  Phys. Status Solidi A, 2007
  Crystal structure of CuFeInSe3 from X-ray powder diffraction data
• ¹⁰
  Crystal structure of CuFeInSe3 from X-ray powder diffraction data

  Phys. Status Solidi A, 2007
  Crystal structure of CuFeInSe3 from X-ray powder diffraction data
• ²⁴
  Crystal structures of two quenched Cu-In-Se phases

  Cryst. Res. Technol, 1988
  Crystal structures of two quenched Cu-In-Se phases

Table IIIshows the 6 better models used in the cation distribution analysis on the available Wyckoff positions. In this Table the Rietveld refinement ²⁵ results are shown. Many other tests were performed where the Cu⁺ cations were moved from the origin (2e), and Wyckoff positions (2a) and (2c) were used for the cations distribution, but only with poor results. The final model was confirmed by checking the chemical sense of the structure in terms of its distances and bond angles.

• ²⁵
  A profile refinement method for nuclear and magnetic structures

  J. Appl. Cryst, 1969
  A profile refinement method for nuclear and magnetic structures

The program Fullprof ²⁶ was employed for the Rietveld refinement analyzes. In each case, the angular dependence was described by the usual constrain imposed by the Cagliotti’s formula ²⁷, and the peak shapes were described by the Thompson-Cox-Hastings pseudo-Voigt profile function ²⁸. The background was described by the automatic interpolation of 67 points throughout the whole pattern. One overall isotropic temperature factor was refined to describe the thermal motion of the atoms. Model 3 showed the best fit and the Rietveld refinement results are shown in Table IV. Figure 3 shows the Rietveld refinement plot for the quaternary compound Cu₂FeIn₂Se₅. Table V shows the atomic coordinates, isotropic temperature factor, bond distances, and angles for the new compound.

• ²⁶
  Fullprof ver. 7.3, Laboratoire Léon Brillouin, 2020
  Fullprof ver. 7.3, Laboratoire Léon Brillouin
• ²⁷
  Choice of collimators for a crystal spectrometer for neutron diffraction

  Nucl. Instrum, 1958
  Choice of collimators for a crystal spectrometer for neutron diffraction
• ²⁸
  Rietveld refinement of Debye-Scherrer synchrotron X-ray data from Al2O3

  J. Appl. Cryst, 1987
  Rietveld refinement of Debye-Scherrer synchrotron X-ray data from Al2O3

Cu₂FeIn₂Se₅is a normal adamantane structure compound ⁵, and consists of a three-dimensional arrangement of distorted CuSe₄, FeSe₄ and InSe₄ tetrahedral connected by common faces (Fig. 4b). In this compound, as in the related CuFeInSe₃, occurs a degradation of symmetry from the chalcopyrite structure I 4 ¯ 2 d to a related structure P 4 ¯ 2 c ¹⁰. In this adamantane model, each cation is tetrahedrally bonded to four anions and at the same time, each Se anion is coordinated by four cations [one Cu1, one Fe, one In1, and one M cation (either Cu2 or In2)] located at the corners of a lightly distorted tetrahedron.

• ⁵
  Wurzite and Sphalerite Structures

  Intermetallic Compounds, 1995
  5. E. Parthé, Wurzite and Sphalerite Structures, in Intermetallic Compounds, edited by J. H. Westbrook and R. L. Fleischer, Vol. 1 (Wiley, Chichester, 1995).
• ¹⁰
  Crystal structure of CuFeInSe3 from X-ray powder diffraction data

  Phys. Status Solidi A, 2007
  Crystal structure of CuFeInSe3 from X-ray powder diffraction data

The tetrahedra containing the Cu1 atoms [mean Se...Se distance 3.970(6) Å] are lightly smaller than those containing the M (Cu2 or In2) [means Se...Se distance 4.108(6) Å], Fe atoms [mean Se...Se distance 4.012(6) Å], and In1 atoms [mean Se...Se distance 4.294(6) Å] respectively.

The interatomic distances are shorter than the sum of the respective ionic radii for structures tetrahedrally bonded ²⁹. The Cu-Se [2.431(5) Å], Fe-Se [2.458(5) Å] and In-Se [2.630(5) Å], bond distances compare well to those observed in some other adamantane structure compounds such as CuInSe2 (2.432-2.591Å) ²¹, Cu₂SnSe₃ (2.415 Å) ³⁰, CuFeInSe₃ (2.421-2.520 Å) ₍₁₀₎, CuFe₂InSe₄ (2.417-2.50 Å) ¹¹, CuMn₂InSe4 (2.447-2.594 Å) ³¹, CuMnInSe₃ (2.428 -2.614 Å) ¹³, CuVInSe₃ (2.518−2.530 Å) ³² and Cu₃In₇Se₁₂ (2.419−2.523 Å) ³³.

• ²⁹
  Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides

  Acta Cryst. A, 1976
  Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides
• ²¹
  The crystal structures of CuInSe2 and CuInTe2

  Mater. Res. Bull, 1992
  The crystal structures of CuInSe2 and CuInTe2
• ³⁰
  Crystal structure refinement of the semiconducting compound Cu2SnSe3 from X-ray powder diffraction data

  Mater. Res. Bull, 2003
  Crystal structure refinement of the semiconducting compound Cu2SnSe3 from X-ray powder diffraction data
• ¹¹
  Crystal structure of CuFe2InSe4 from X-ray powder diffraction

  J. Alloys Compd, 2008
  Crystal structure of CuFe2InSe4 from X-ray powder diffraction
• ³¹
  Crystal structure of the new diamond-like semiconductor CuMn2InSe4

  Bull. Mater. Sci., 2016
  Crystal structure of the new diamond-like semiconductor CuMn2InSe4
• ¹³
  Synthesis and Crystal Structure of Three New Quaternary Compounds in the system Cu- Mn-III-Se3 (III = Al, Ga, In)

  Mater. Res, 2018
  Synthesis and Crystal Structure of Three New Quaternary Compounds in the system Cu- Mn-III-Se3 (III = Al, Ga, In)
• ³²
  Preparation, differential thermal analysis and crystal structure of the new quaternary compound CuVInSe3

  Rev. Mex. Fis, 2018
  Preparation, differential thermal analysis and crystal structure of the new quaternary compound CuVInSe3
• ³³
  Crystal structure of the ternary semiconductor Cu2In14/4*4/3 Se8 determined by X-ray powder diffraction data

  Powder Diffr., 2018
  Crystal structure of the ternary semiconductor Cu2In14/4*4/3 Se8 determined by X-ray powder diffraction data

The chemical structural model was checked by the analysis of the interatomic distances using the BVS formula based on the bond-strength examination ^34,35. The atomic valence of an atom is assumed to be distributed between the bonds that it forms. BVS of atom 𝑖, denoted 𝑉 𝑖 , is then V i = ∑ j S j = ∑ j e x p [ R o - R i j b ] , where S_j is the valence of one bond, and the sum is over all neighbors j. The constant b =0.37 was empirically determined ³⁴. R_o represents the length of a bond of a unit valence, and R_ij is the experimentally determined distance between atoms i and j. The values for the reference distance R_o for Cu-Se, Fe-Se, and In-Se are 2.02, 2.28 and 2.47 Å, respectively ³⁵. Table VI shows the BVS results for Cu₂FeIn₂Se₅, indicating that the oxidation state for each ion is in good agreement with the expected formal oxidation state of Cu⁺, Fe²⁺, In³⁺, and Se^2- ions.

• ³⁴
  Bond-valence parameters obtained from a systematic analysis of the Inorganic Crystal Structure Database

  Acta Cryst. B, 1985
  Bond-valence parameters obtained from a systematic analysis of the Inorganic Crystal Structure Database
• ³⁵
  Bond-valence parameters for solids

  Acta Cryst. B, 1991
  Bond-valence parameters for solids
• ³⁴
  Bond-valence parameters obtained from a systematic analysis of the Inorganic Crystal Structure Database

  Acta Cryst. B, 1985
  Bond-valence parameters obtained from a systematic analysis of the Inorganic Crystal Structure Database
• ³⁵
  Bond-valence parameters for solids

  Acta Cryst. B, 1991
  Bond-valence parameters for solids

Figure 4 shows the crystal structure evolution of (CuInSe₂)_1-x (FeSe)_xalloys, which confirms the phase diagram proposed for this system ¹². Starting from the chalcopyrite structure (Fig. 4a) CuInSe₂ with space group I 4 ¯ 2 d , when introducing a transition metal (Fe) into the chalcopyrite matrix, a first effect is the disorder of the cationic network. This effect is observed in the P-chalcopyrite structures with (Fig. 4b) x = 1/3 Cu₂FeIn₂Se₅(this work) and (Fig. 4c) x = 1/2 CuFeInSe₃, both crystallize in space group P 4 ¯ 2 c , where a cationic disorder resulting from the occupation of several cations in the same Wyckoff site is observed. By increasing the amount of the transition metal to x = 2/3, the cationic network is reordered in a tetragonal space group I 4 ¯ 2 m CuFe₂InSe₄ (Fig. 4d), which crystallize with a stannite-type structure.

• ¹²
  Phase Diagram of (CuInSe2)1-x (FeSe)x alloys

  J. Alloys Compd, 2015
  Phase Diagram of (CuInSe2)1-x (FeSe)x alloys

From the magnetic point of view, these materials -due to their cationic ordering- are diamagnetic, ferromagnetic, and ferromagnetic, respectively ^6,12.

• ⁶
  A comparative Study of (Cu-III-Se2)x -(FeSe)1-x Alloys (III: Al, Ga, In) (0 ≤ x ≤ 1) by X-Ray Diffraction, Differential Thermal Analysis and Scanning Electron Microscopy

  Phys. Status Solidi A, 2001
  A comparative Study of (Cu-III-Se2)x -(FeSe)1-x Alloys (III: Al, Ga, In) (0 ≤ x ≤ 1) by X-Ray Diffraction, Differential Thermal Analysis and Scanning Electron Microscopy
• ¹²
  Phase Diagram of (CuInSe2)1-x (FeSe)x alloys

  J. Alloys Compd, 2015
  Phase Diagram of (CuInSe2)1-x (FeSe)x alloys

A new quaternary chalcogenide, belonging to the system (CuInSe₂)_1-x (FeSe)_xwith x = 1/3, has been synthesized and structurally characterized. The DTA indicates that this compound melts at 1017 K. The crystal structure solution of the semiconductor alloy Cu₂FeIn₂Se₅was resolved in the space group P 4 ¯ 2 c by the evaluation of different models derived from the CuFeInSe₃ structure against the powder X-ray diffraction data, using the Rietveld method. This compound crystallizes in a P-chalcopyrite structure and is the first structural report on a member of the I₂-II-III₂-VI₅ semiconductor composition. Its structure completes the phase transition produced in the (CuInSe₂)_1-x (FeSe)_xsystem between the values x = 0 to x = 2/3.