Synthesis and Crystal Structure of Three New Quaternary Compounds in the system Cu-Mn-III-Se 3 ( III = Al , Ga , In )

The Cu-III-Se2 (III= Al, Ga, In) ternary chalcopyrite family have been object of a great quantity of work in the last years, because they form an wide group of semiconductor materials with diverse optical and electrical properties1. These materials crystallize with tetragonal symmetry in the space group I4 2d (N°122), and the addition of a Fe-Se binary compound produces alloys of the type (Cu-IIISe2)1-x (FeSe)x 2. Some previous results on the formation, structural characterization, thermal and magnetic properties on these alloys with compositions Cu-Fe-III-Se3 (x = 1⁄2), Cu-Fe2-III-Se4 (x = 2⁄3) and Cu2-Fe-III-Se5 (x = 1⁄3) have been reported3-15. All these phases fulfill the rules of formation of adamantane compounds2 and belong to the normal semiconductor compound families of the third, fourth and fifth-order derivatives of the II-VI binary semiconductors, respectively16. Adamantane compounds are binary, ternary or quaternary normal tetrahedral structure compounds which are closely related to either cubic or hexagonal diamond. The first crystal structure characterization of one I-IIIII-VI3 semiconductor member, CuFeInSe3, indicated a degradation of symmetry from the chalcopyrite structure I42d to a related structure P42c6. In this work, we report a detailed synthesis and structural analysis of three new members of this family; CuMnAlSe3, CuMnGaSe3 and CuMnInSe3, which was performed using X-ray powder diffraction by means of the Rietveld method.


Introduction
The Cu-III-Se 2 (III= Al, Ga, In) ternary chalcopyrite family have been object of a great quantity of work in the last years, because they form an wide group of semiconductor materials with diverse optical and electrical properties 1 .These materials crystallize with tetragonal symmetry in the space group I4 2d (N°122), and the addition of a Fe-Se binary compound produces alloys of the type (Cu-III-Se 2 ) 1-x (FeSe) x 2 .Some previous results on the formation, structural characterization, thermal and magnetic properties on these alloys with compositions Cu-Fe-III-Se 3 (x = ½), Cu-Fe 2 -III-Se 4 (x = ⅔) and Cu 2 -Fe-III-Se 5 (x = ⅓) have been reported [3][4][5][6][7][8][9][10][11][12][13][14][15] .All these phases fulfill the rules of formation of adamantane compounds 2 and belong to the normal semiconductor compound families of the third, fourth and fifth-order derivatives of the II-VI binary semiconductors, respectively 16 .Adamantane compounds are binary, ternary or quaternary normal tetrahedral structure compounds which are closely related to either cubic or hexagonal diamond.
The first crystal structure characterization of one I-II-III-VI 3 semiconductor member, CuFeInSe 3 , indicated a degradation of symmetry from the chalcopyrite structure I42d to a related structure P42c 6 .In this work, we report a detailed synthesis and structural analysis of three new members of this family; CuMnAlSe 3 , CuMnGaSe 3 and CuMnInSe 3 , which was performed using X-ray powder diffraction by means of the Rietveld method.

Synthesis
Ingots of Cu-Mn-III-Se 3 (III= Al, Ga, In) were prepared by the melt and annealing technique.Starting materials (Cu, Mn, Al, Ga, In and Se), with a nominal purity of at least 99.99 % (GoodFellow) in the stoichiometric ratio, were mixed together in an evacuated and sealed quartz tube with inner walls previously carbonized.Polycrystalline ingots of about 1 g were prepared by the usual melting and annealing technique, lowering the temperature from 1500 to 850 K at a rate of 20 K/h, keeping this temperature for 30 days, and finally, cooling to room temperature at a rate of 10 K/h.Previous experience indicates that this procedure usually gives samples showing conditions corresponding to equilibrium near room temperature.

Chemical analysis
Compositional analysis of the ingots was determined at several points by energy dispersive X-ray (EDX) analysis using a Kevex Model Delta-3 system connected to a Hitachi Model S-2500 scanning electron microscope (SEM).In each case, the average chemical composition of the central part of the ingot from which the crystals were cut, gave the atomic percentage in good agreement with the ideal composition 1:1:1:3 The error in standardless analysis was around 5 %.

X-ray powder diffraction
For the X-ray analysis, small quantities of the samples were ground mechanically in an agate mortar and pestle.The resulting fine powders, sieved to 106µ, were mounted on a flat zero-background holder covered with a thin layer of petroleum jelly.The X-ray powder diffraction data were collected at 293(1) K, in θ/θ reflection mode using a Siemens D5005 diffractometer equipped with an X-ray tube (CuKα radiation: λ= 1.54056 Å; 40kV, 30mA) using a secondary beam graphite monochromator.A fixed aperture and divergence slit of 1 mm, a 1 mm monochromator slit, and a 0.1 mm detector slit were used.The specimens were scanned from 10°-100° 2θ, with a step size of 0.02° and counting time of 40s.Quartz was used as an external standard.

Results and Discussion
The X-ray diffractograms of three alloys Cu-Mn-III-Se 3 (III= Al, Ga, In) showed single phases.The powder patterns were indexed using the program Dicvol04 17 , and tetragonal cells with similar magnitudes to the parent chalcopyrite structures, CuAlSe 2 18 , CuGaSe 2 19 , CuInSe 2 20 were founds.Systematic absences are consistent with a P-type Bravais lattice.A detailed pattern examination taking in account the sample composition, cell parameters and lattice-type, suggested that all compounds are isostructural with previously reported CuFeInSe 3 6 which crystallizes in the space group P42c.The Rietveld refinements 21 of the structures were carried out using the Fullprof program 22 .The atomic coordinates of CuFeInSe 3 6 were used as starting model for each refinement.The angular dependence of the peak full width at half maximum (FWHM) was described by the Caglioti's formula 23 .Peak shapes were described by the parameterized Thompson-Cox-Hastings pseudo-Voigt profile function 24 .The background variation was described by a polynomial with six coefficients.The thermal motion of the atoms was described by one overall isotropic temperature factor.The results of the Rietveld refinement for the three alloys are summarizes in Table 1.Fig. 1 shows the observed, calculated and difference profile for the final cycle of Rietveld refinements.Atomic coordinates, isotropic temperature factor, bond distances and angles for each compound are shown in Tables 2, 3 and 4.
CuMnAlSe 3 , CuMnGaSe 3 and CuMnInSe 3 are normal adamantane-structure compounds 2 , where occurs a degradation of symmetry from the chalcopyrite structure I42d to a related

Atom
Ox.     structure P42c.This situation can be clearly seen in Fig. 2 where a comparison is made between the chalcopyrite Cu-III-Se 2 I42d structure and the P42c structure of Cu-Mn-III-Se 3 (III= Al, Ga, In).Therefore, in the quaternary structures, the introduction of an additional cation (Mn) produces an effect of "dilution" of this cation in the chalcopyrite structure leaving the cell volume almost unchanged 6 .Table 5 show a comparison between the unit cell parameters and the bond distances for the three phases of both families of compounds.The structure of the tetragonal phases Cu-Mn-III-Se 3 can be described as derivative of the sphalerite structure 2 .In this structure the Se atoms form a close-packed arrangement where each anion is coordinated by four cations located at the corners of a slightly distorted tetrahedron.All cations are similarly coordinated by four anions.Fig. 3 shows a polyhedral view of the crystal structure with four types of atoms-centered tetrahedra MS 4 [CuS 4 , MnS 4 , IIIS 4 and (CuInMn)S 4 ] where all polyhedra are oriented in the same direction and are connected by the corners.
An important structural characteristic of the compounds under study is the parameter of tetragonal lattice distortion, which is determined as a deviation of the ratio η = c/2a (a and c are unit-cell parameters) from unity 25 .The Table 5 contains the a, c, and η values for the Cu-Mn-III-Se 3 compounds.One can see that η is close to unity for all compositions, which is indicative of small lattice distortions in the samples synthesized.
The Cu-Se, Mn-Se and III (Al,Ga,In)-Se bond distances in all compounds (Tables 2, 3 and 4) are in good agreement with those observed in the parent chalcopyrite structures (Table 5) and other quaternary adamantane structure compounds such as CuFe(Al,Ga,In)Se 3 6,12 , CuNi(Ga,In) Se

Conclusions
The crystal structure of the semiconductor compounds CuMnAlSe 3 , CuMnGaSe 3 and CuMnInSe 3 were determined using X-ray powder diffraction data.All compounds crystallize in the tetragonal space group P42c, with a sphalerite derivative structure, and are isomorphic with CuFeInSe 3 .

Figure 2 .
Figure 2. Unit cell diagram for the chalcopyrite Cu-III-Se 2 compared to the Cu-Mn-III-Se 3 compounds.

Figure 3 .
Figure 3. Polyhedral view of the Cu-Mn-III-Se 3 structures showing tetrahedral units.

Table 5 .
Comparative table of unit cell parameters and bond distances for the Cu-III-Se2 chalcopyrite compounds and the related Cu-Mn-III-Se3 alloys (III= Al Ga, In).([*] = this work).