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Boletín de la Sociedad Chilena de Química

versão impressa ISSN 0366-1644

Bol. Soc. Chil. Quím. v.46 n.3 Concepción set. 2001

http://dx.doi.org/10.4067/S0366-16442001000300002 

A FAMILY OF NEW COMPOUNDS DERIVED FROM
CHALCOPYRITE. COMMON PATTERNS IN THEIR ELECTRONIC
AND CRYSTAL STRUCTURES

RAFAEL RAMIREZa, CARLOS MUJICAb, ANTONIO BULJANb, AND
JAIME LLANOS
b *

a Instituto de Ciencias de Materiales, Consejo Superior de Investigaciones Científicas,
Campus Cantoblanco, 28049 Madrid, Spain

bDepartamento de Química, Facultad de Ciencias, Universidad Católica del Norte,
Casilla 1280, Antofagasta, Chile

(Received: October 6, 2000 - Accepted: November 8, 2000)

ABSTRACT

A series of new compounds has been synthesized by insertion of alkali or alkaline earth-metals in chalcopyrite, CuFeS2. Their crystal structures vary from layered arrangements of tetrahedra in a trigonal (LiCuFeS2, NaCuFeS2) or a tetragonal symmetry (KCuFeS2, CsCuFeS2), to three-dimensional cubic framework (Ba6Cu12Fe13S 27). Despite these differences, the synthesized compounds follow a common structural pattern: the sulfur and M atoms (M= Li, Na, K, Cs, or Ba) are ordered in a distorted face-centered-cubic lattice, where all tetrahedral holes surrounded by S atoms are occupied by the transition metal elements. The tetrahedral holes surrounded by both S atoms and at least one M element are empty. The band structures of these compounds are discussed in the framework of an extended Hückel Hamiltonian.

KEYWORK: chalcogenides, semiconductors, electronic structure, crystal structure.

RESUMEN

Una serie de nuevos compuestos han sido sintetizados por inserción de iones de metales alcalinos o alcalinos-térreos en calcopirita. Sus estructuras cristalinas varían desde un ordenamiento bidimensional de tetraedros con simetría trigonal (LiCuFeS2, NaCuFeS2) o con simetría tetragonal (KCuFeS2, CsCuFeS2), hasta un ordenamiento cúbico tridimensional (Ba6Cu 12Fe13S27). A pesar de estas diferencias, los compuestos sintetizados siguen un patrón estructural común: los átomos de azufre y del tipo M (M= Li, Na, K, Cs, or Ba ) están ordenados en una red cúbica de cara centrada distorsionada, donde todos los huecos tetraédricos que están rodeados por átomos de azufre, son ocupados por Cu o Fe . Los huecos tetraédricos que están rodeados por átomos de S y al menos un elemento del tipo M están vacíos. La estructura de bandas de estos compuestos se calcularon por el método de extended Hückel.

Palabras Claves: calcogenuros, semiconductores, estructura electrónica, estructura cristalina.

INTRODUCTION

One of the aims of solid state chemistry is related to the possibility of improving the physical and chemical properties of a given material by suitable modifications of its composition and crystal structure. In this respect, the study of compounds derived from chalcopyrite, CuFeS2, is of interest because these phases can be synthesized in a large variety of crystal structures. Short after the first report of the synthesis of the phase LiCuFeS2, we succeeded in the preparation of NaCuFeS2, KCuFeS2, CsCuFeS2, and Ba6Cu12Fe13S27 (1-5). In these phases, the transition metal atoms (Cu, Fe) occupy sites with tetrahedral

(T) symmetry. These occupied T-sites form layers in MCuFeS2 (M= Li, Na, K, and Cs), and a three-dimensional framework in Ba6Cu12Fe13S27 . We recall the large variety of structures found for other compounds (e.g., silicates (6)) whose building blocks are T-sites.

One interest of the phases derived by insertion of alkali atoms in chalcopyrite is their possible use as cathode in secondary batteries (1). From this point of view a convenient structural property is the low-dimensionality as realized by layers of alkali metal atoms intercalated between two-dimensional sandwiches formed by S and (Cu, Fe) atoms. The total or partial removal of alkali atoms is possible by a redox process, where the Fe2+ ions are oxidized to Fe3+ (7). Other structural aspect that can influence the physical properties of these materials is the distribution of (Cu, Fe) atoms in the T-sites of the lattice. The X-ray investigations reveal absence of long-range order in the (Cu, Fe) distributions (1-3,7). We have found by classical Monte Carlo simulations that at the synthesis temperature of these compounds (about 1200 K) there appears short-range order in the T-atom distribution due to the tendency of Fe to occupy nearest-neighbors T-sites (8). Therefore, it can be expected the presence of magnetic phases in these compounds as a consequence of the coupling between neighboring Fe atoms.

In this contribution, we present a structural analysis of the phases MCuFeS2 (M= Li, Na, K, and Cs), Ba6Cu12Fe13S27 , and of the parent compound, CuFeS2. For each compound, we have compared the actual structure with an ideal one, derived from a supercell of a simple face-centered-cubic (fcc) lattice. Furthermore, we have investigated the relation between the electronic and crystal structures of these compounds by means of extended Hückel (EH) band-structure calculation (9). This paper is organized as follows. The structural principle common to the synthesized compounds is discussed in Section 2. Section 3 gives the results of the band-structure calculations with a brief discussion on the electrical and magnetic properties expected for these compounds. The conclusions are reserved for Section 4.

STRUCTURAL ANALYSIS

The crystal structures of the new compounds derived from chalcopyrite can be rationalized on the basis of very simple rules. Each phase is closely related to an ideal structure derived from the cubic closest-packed system. The ideal structures can be derived by the following rules: i) the S atoms and the alkali or earth alkaline metals form together a distorted fcc packing; ii) those T-sites surrounded by four S atoms are occupied by transition metal atoms and those T-sites where at least one vertex of the tetrahedra is an alkali or earth alkaline metal are empty.

To show the relation between the actual and ideal structures, we compare in Tabs. 1-6 the fractional coordinates (x, y, z) of the atoms in the asymmetric unit of each phase, referred to the crystallographic basis vectors (a, b, c), with those fractional coordinates derived for the ideal structure. We present also the transformation matrix M, that relates the crystallographic basis vectors (a, b, c) of the actual structure, to the cubic basis vectors (a', b', c') of the ideal one:

(a,b,c) = (a',b',c')M.

{1}

The c/a ratio obtained from X-ray diffraction experiments and that one derived for the ideal structure are also given in the tables. A particular important structural property is the occupancy of T-sites by the transition metal atoms. This is shown in Figs. 1 for CuFeS2, Fig. 2 for LiCuFeS2 and NaCuFeS2, Fig. 3 for KCuFeS2 and CsCuFeS2, and Fig. 4 for Ba6Cu12Fe13S27 .

Fig. 1.T-sites in the chalcopyrite structure. Filled circles represent T-sites occupied either by Fe or by Cu atoms, and open circles represent vacant sites. The crystallographic unit cell of CuFeS2 corresponds to two fcc unit cells on top of each other along the c' axis.

Fig. 2. Occupancy of T-sites in the phases LiCuFeS2 and NaCuFeS2. Filled circles represent T-sites occupied either by Fe or by Cu atoms and open circles represent vacant sites. The representation corresponds to a 2 x 2 x 2 supercell of a fcc lattice. The crystallographic unit cell of MCuFeS2 (M = Li or Na) has trigonal symmetry with the c axis oriented along the [111] direction of the displayed cubic cell.

Fig. 3.T-sites in the phases KCuFeS2 and CsCuFeS2. Filled circles represent T-sites occupied either by Fe or by Cu atoms and open circles represent vacant sites. The representation corresponds to a 1 x 1 x 3 supercell of the idealized fcc lattice. The crystallographic unit cell of MCuFeS2 (M = K or Cs) has tetragonal symmetry and coincides with the fcc supercell apart from a shift in the cell origin.

Fig. 4. T-sites in Ba6Cu12Fe13S27 . Filled circles represent T-sites occupied either by Fe or by Cu atoms and open circles represent vacant sites. The representation corresponds to a 2 x 2 x 2 supercell of the idealized fcc lattice and coincides with the crystallographic unit cell with its cell origin shifted by (1/2, 1/2,1/2).

The structural data corresponding to CuFeS2 are presented in Tab. 1 (10). The S atoms in the ideal structure form a fcc lattice. The tetragonal unit cell of CuFeS2 has the c axis two times larger than that of the ideal fcc packing, as can be noted by the value of the (3,3) element of the diagonal matrix M. Deviations from the ideal arrangement are found only in the x coordinate of the S atom and in the c/a ratio. The spatial arrangement of occupied T-sites is displayed in Fig. 1. Note that the T-sites (both occupied and vacant) form a simple cubic lattice in the ideal structure. In CuFeS2, rule ii is not fulfilled as all T-sites are surrounded by four S atoms, but only one-half of them are occupied by Cu or Fe. In this phase, the (Cu, Fe) distribution displays long-range order.

The phases LiCuFeS2 and NaCuFeS2 are isostructural and crystallize in the CaAl2Si2 structure-type (11). The T-sites occupied by (Cu, Fe) atoms are shown in Fig. 2 within a 2 x 2 x 2 supercell of the ideal fcc lattice. The occupancy of T-sites follows rule ii mentioned above. According to the X-ray structural analysis, the (Cu,Fe) distribution does not present long-range order (1,2). However, from results obtained by finite temperature Monte Carlo simulations, we expect the existence of short-range order due to the tendency of the Fe atoms to occupy nearest-neighbors T-sites (8). Deviations with respect to the ideal structure are found only in the values of the z coordinate of the S and the T-atoms (see Tabs. 2-3). These deviations are larger for the phase containing Li than for the phase containing Na, as a consequence of the larger difference in atomic size between Li and S, with respect to Na and S. In the ideal structure the alkali and S atoms are arranged as closest-packed layers, perpendicular to the [111] direction of the cubic cell displayed in Fig. 2, alternating one layer of alkali atoms with two layers of S atoms. Therefore, the alkali and S atoms form a distorted fcc lattice (rule i). The trigonal distortion can be quantified by the c/a ratio. In the Li phase, this ratio is 32 % smaller than that of the ideal structure, while in the Na phase this deviation is reduced to 27 %.

The crystallographic data of the isostructural phases KCuFeS2 and CsCuFeS2 are summarized in Tabs. 4-5. Their crystal structures corresponds to the ThCr2Si2 structure-type (12). The T-sites occupied by the (Cu, Fe) atoms form layers as displayed in Fig. 3. Again we find that the vacant T-sites are those that have at least one nearest neighbor that is not at S atom (rule ii). There appears an alternation of one alkali layer and two S layers in a direction perpendicular to the [001] direction of the idealized fcc structure (i.e., c' axis in Fig. 3). Topologically, the alkali and S atoms form a fcc lattice (rule i). The comparison between the actual and the ideal structure shows differences in the z coordinate of the S atoms and in the c/a ratio. The tetragonal distortion measured by the difference between the actual and ideal c/a ratio, is of 18 % in the phase containing K, and of 15 % in the phase containing Cs.

Table I. Comparison of the atomic coordinates found for CuFeS2 with those parameters derived from an ideal fcc structure with partial occupancy of T-sites. The fractional coordinates (x, y, z) are referred to the crystallographic basis vectors (a, b, c). The transformation matrix M is defined as (a, b, c) = (a', b', c') M, where (a', b', c') are the fcc basis vectors.

Table II. Comparison of the atomic coordinates found for parameters derived from an ideal fcc structure with partial according to rule ii in the text. See caption of Tab. I.

 

 

Table III. Comparison of the atomic coordinates found for NaCuFeS2 with those parameters derived from an ideal fcc structure with partial occupancy of T-sites according to rule ii in the text. See caption of Tab. 1.

Table IV.Comparison of the atomic coordinates found for KCuFeS2 with those parameters derived from an ideal fcc structure with partial occupancy of T-sites ac-cording to rule ii in the text. See caption of Tab. 1.

Table V. Comparison of the atomic coordinates found for CsCuFeS2 with those parameters derived from an ideal fcc structure with partial occupancy of T-sites according to rule ii in the text. See caption of Tab. 1.

Table VI. Comparison of the atomic coordinates found for Ba~Cu12Fe13S27 with those parameters derived from an ideal fcc structure with partial occupancy of T-sites according to rule ii in the text. See caption of Tab. 1.

The most interesting example of the close structural relationship between the derivatives of chalcopyrite is provided by the phase Ba6Cu12Fe13S27 (5). The structure-type of this compound corresponds to djerfisherite (13). We find that the Ba and S atoms form a distorted fcc lattice, according to rule i. The comparison between the ideal and experimental fractional coordinates in Tab. 6 reveals that only the Ba atoms at position 6e and S atoms at 8g deviate appreciably from the ideal structure. In the latter, the six Ba atoms at 6e and the eight S atoms at 8g are arranged as the atoms of the fcc cell, with the Ba atoms located at the center of the faces and the S atoms located at the vertices of the cube. In the actual structure, the Ba atoms are displaced along the axes of the cubic cell outwards from the cube center. The ideal fractional coordinates (1/4, 0, 0) for Ba is shifted to (0.3, 0, 0) in the actual structure. The S (8g) atoms are displaced along the cube diagonals inwards to the cube center, so that the ideal coordinates (1/4,1/4,1/4) change to (0.22, 0.22, 0.22). The occupancy of T-sites by (Cu, Fe) atoms is displayed in Fig. 4. It is interesting to note that this complex structure with 56 atoms in the unit cell follows the same structural pattern as the other derivatives of chalcopyrite and the occupancy of T-sites is again according to rule ii. This common pattern found in five derivatives of chalcopyrite leads us to expect the possibility of finding new compounds following the same structural rules by changing the chemical composition. A structural property of the phase Ba6Cu12Fe13S27 is that two octahedral holes per crystallographic unit cell are occupied. There appears a S atom in the octahedral hole (la) surrounded by the six Ba atoms in the unit cell, and a Fe atom in the octahedral hole (1b) surrounded by six S atoms.

The fraction of occupied T-sites in each of the studied structures is 1/2 for CuFeS2, LiCuFeS2, and NaCuFeS2, 3/8 for Ba6Cu12Fe13S27 , and 1/3 for KCuFeS2, and CsCuFeS2, while the fraction of octahedral holes occupied in Ba6Cu12Fe13S27 is 1/16.

BAND STRUCTURE RESULTS

The band structures of the derivatives of chalcopyrite have been studied by means of extended Hückel calculations (9). A qualitative picture of their electronic structure may help us to rationalize some physical properties expected for these compounds. The calculated density of states (DOS) of the phases CuFeS2, NaCuFeS2, CsCuFeS2, and Ba6Cu12Fe13S27 are collected in Figs. 5-8 within an energy window between -9 and -17 eV. Technical details concerning the calculations are given in the appendix. The results for LiCuFeS2 and KCuFeS2 are not displayed as they are almost indistinguishable form their isostructural compounds. In this energy region, the energy bands correspond to admixtures of S 3p, Fe 3d, and Cu 3d orbitals. The S 3s bands are found at lower energies (around -21 eV), outside the displayed window. The projected DOS distributions for Fe and Cu atom orbitals (AO's) are shown in the figures. The 3d Cu AO's lie at lower energies than the 3d Fe orbitals. Formally, the transition metal atoms in the derivatives of chalcopyrite appear as Cu1+ ions with d10 configuration, and as Fe2+ ions with d6 configuration. However, in chalcopyrite, there are Fe3+ ions with d5 configuration. According to the valence rules, in Ba6Cu12Fe13S27 there appear 4 Fe3+ ions and 9 Fe2+ ions per unit cell (5). The counting of electrons with a and b spin was performed considering that the Fe ions have high-spin configuration, which is the one expected for a crystal field with tetrahedral symmetry (14). The Fermi energy corresponding to the a and b electrons is indicated in the DOS curves. For the a electrons, the Fermi level coincides with the top of the Fe valence 3d bands in all studies phases. The calculated energy gap separating these states from the unoccupied antibonding states, which are mainly formed by Cu 3d and 5 AO's, is 6 eV for LiCuFeS2, 5.7 eV for NaCuFeS2, 5.5 eV for KCuFeS2, 4.9 eV for CsCuFeS2, 3 eV for Ba6Cu12Fe13S27 , and 5.8 eV for CuFeS2. In all these phases, the Fermi level associated to the minority b-spin electrons appears at energies with finite density of states, where the corresponding one-electron states are mainly of Fe 3d character (see Figs. 5-8). Furthermore, the temperature dependence of the electrical conductivity of Ba6Cu12Fe13S27 , indicates also semiconducting behavior with an energy gap of 0.2 eV. obtained by transport measurement (15). For KCuFeS2 and CsCuFeS2 the energy gaps derived from electrical conductivity data are 0.2 and 0.1 eV, respectively (16). The discrepancy between the theoretical and experimental electrical properties can be attributed to a positional disorder of the transition metal atoms in the crystal structure of the above mentioned phases, which reduces the effective long-range overlap of orbitals necessary for metallic conduction.

Fig. 5. DOS curve obtained for CuFeS2. The total DOS (continuous line) and the projected DOS curves for the Cu AO's (broken line) and Fe AO's (dotted line) are displayed. The Fermi level has been calculated for electrons with a and b spin, assuming a high spin configuration of the tetrahedral Fe ions.

Fig. 6. DOS curve obtained for NaCuFeS2. See caption of Fig. 5.

Fig. 7. DOS curve obtained for CsCuFeS2. See caption of Fig. 5.

Fig. 8. DOS curve obtained for Ba6Cu8Fe17S27 . See caption of Fig. 5.

The net charge associated to spin polarized electrons in the case of NaCuFeS2 is 0.02 e at Na, 0.53 e at S, 2.61 e at Fe, and 0.33 e at Cu. The largest spin polarization is found for Fe. The other derivatives of chalcopyrite display comparable spin densities, while in the case of chalcopyrite, with presence of formally Fe+3ions, the corresponding values are 0.58 e at S, 3.33 e at Fe, and 0.50 e at Cu. Note that this spin density at Fe is about 0.8 e larger than in the case of NaCuFeS2. The large contribution of Fe AO's in the DOS distributions of the derivatives of chalcopyrite at energies around the Fermi level of the b-spin electrons implies that in the case of redox reactions the charge transfer will involve mainly 3d electrons of Fe. This fact agrees with the apparent oxidation of Fe found by X-ray photoelectron spectroscopy experiments upon deintercalation of the alkali metal in LiCuFeS2 (7).

The high-spin configuration of Fe ions, together with the tendency of the Fe ions to occupy nearest-neighbors T-sites, suggest the possibility of magnetic ordering in these phases as a function of temperature (8). It is known that chalcopyrite displays antiferromagnetic ordering at temperatures below 825 K (17). This magnetic behavior has been also reported in several compounds whose building blocks are Fe ions tetrahedrally coordinated to 8 atoms as in KFeS2 and Na3FeS3 (18). Therefore, we expect to find antiferromagnetic properties in the synthesized derivatives of chalcopyrite, i.e.Ba6Cu12Fe13S 27, preliminary studies on this compound showed a maximum around 50K in the susceptibility plot, which is consistent with the presence of an antiferromagnetic type of coupling. In order to test for the possibility of clustering or metal-glass, the magnetization of this phase under field-cool (FC) and zero-field-cool (ZFC) conditions was measured. The fact that both curves diverge at temperatures well above the maximum of the ZFC curve indicates a system in which the iron atoms form cluster, each with a net, not quantized magnetic moment. This clustering behavior is similar to, but different from, a spin-glass system, where the quantized magnetic moment of each particular iron contributes individually to the spin-glass behavior. (19). Also the temperature dependent magnetic susceptibility of KCuFeS2 and CsCuFeS2 show that both phases have a spin-glass behavior (16).

CONCLUSIONS

In this work, we presented some derivatives of chalcopyrite as a family of compounds, which share a common structural pattern. It will be interesting to find out if new synthetic derivatives of chalcopyrite follow the same structural scheme. The S atoms and the cations inserted in chalcopyrite are ordered topologically as a fcc packing of atoms, with the transition metal ions located at those T-sites that are surrounded exclusively by S atoms. LiCuFeS2 and NaCuFeS2 crystallize in the CaAl2Si2 structure-type; KCuFeS2 and CsCuFeS2 crystallize in the ThCr2Si2 structure-type; while Ba6Cu12Fe13S27 displays a djerfisherite structure.

Parallel to the common pattern in their crystal structures, we find that the electronic structures of these compounds are closely related. Both the electrical and magnetic properties are determined by the presence of Cu and Fe atoms in the T-sites of the structure.

ACKNOWLEDGMENTS

This work was supported by FONDECYT (Contracts No. 1941129, 1960372, 3980044) and by the Programa de Intercambio Científico (CSIC, CONICYT-Fundación Andes).

APPENDIX

The atomic parameters used in the EH calculations are given in Tab. 7. The lattice sums necessary for the evaluation of the elements of the Hamiltonian matrix were carried out up to a cut-off radius of at least 15 Å. The k-points in the calculation of the DOS curves were selected according to Ref. (21). For CuFeS2, we employed a k-grid of 16 k-points in the Brillouin zone (BZ). For NaCuFeS2, we performed the calculation in a (2 x 1 x 1) supercell of the crystallographic unit cell. The transition metal atoms were distributed at the T-sites with the Fe atoms located at (2/3,1/3, -z) and (1/3,2/3, z), and the Cu atoms located at (5/3,1/3, -z) and (4/3,2/3, z), with z = 0.6201. The k-grid in the case of NaCuFeS2 included 196 points in one-half of the BZ. For CsCuFeS2, we used a (÷2 x ÷2 x 1) supercell with a (Cu, Fe) distribution given by Fe at (1/2,0,1/4), (1,1/2,1/4), (1/2,0,3/4), and (1,1/2,3/4). The number of k-points in the grid was 320 in one-half of the BZ. In the case of Ba6Cu12Fe13S27 the T-site distribution was chosen with the Cu atoms at (x, x, z), (-x-, -x, -z), (z, x, x), (x, z, x), (-x, z, x), (z, x, x), (x, x, z), (-x, x, z), (x, z, -x), (-z, x, x), (z, x, x), and (-x, z, x) with x=3.787 Å and z = 1.432 Å. The number of k-points was of 8 in one-half of the BZ.

Table VII. Atomic parameters of the EH calculations. The 3d orbitals are given as a linear combination of two Slater type functions, and each is followed in parentheses by the expansion coefficient. A modified Wolfsberg-Helmholz formula was used to calculate Huv (20).


Atom

Orbital

Hii

x1

x2

c1

c2


Ba

6s

-6.62

1.214

     
 

6p

-3.92

1.214

     

S

3s

-20.00

1.817

     
 

3p

-13.30

1.817

     

Fe

4s

-9.10

1.900

     
 

4p

-5.32

1.900

     
 

3d

-12.60

5.350

2.000

0.5505

0.6260

Cu

4s

-11.40

2.200

     
 

4p

-6.06

2.200

     
 

3d

-14.00

5.950

2.300

0.5933

0.5744

Na

3s

-5.10

0.733

     
 

3p

-3.00

0.733

     

Cs

3s

-3.88

1.060

     
 

3p

-2.49

1.06

     

 

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