Evidence of a new ordered vacancy crystal structure in the compound Cu₃In₇Te₁₂

ABSTRACT

The crystal structure of the ordered vacancy compound (OVC) Cu₃In₇Te₁₂ is analyzed using powder X-ray diffraction data. It is found that this OVC crystallizes with a chalcopyrite-related structure, in the tetragonal space group P4 2c (Nº 112), with unit cell parameters and volume a = 6.1720(2) Å, c = 12.3597(8) Å, and V = 470.83(4) ų. The Rietveld refinement of 28 instrumental and structural parameters led to R_p = 9.27 %, R_wp = 10.30 %, R_exp = 6.95% and S = 1.48, for 4501 step intensities and 130 independent reflections, respectively. This compound is isostructural with Cu₃In₇Se₁₂, and has a defect adamantane structure.

Keywords
semiconductors; ordered vacancy compounds; crystal structure; X-ray diffraction; Rietveld

1. INTRODUCTION

The ternary compound CuInTe₂, belonging to the I-III-VI₂ family of chalcopyrite semiconductors, crystallize in the tetragonal space group I4 2d with unit cell parameters a = 6.1944(20) and c = 12.4157(40) Å [¹]. This material has been found to be a suitable material for thermoelectric (TE) applications [²-⁵]. Hence, it is also expected that ordered vacancy compounds (OVC's) of the Cu-In-Te system [⁶], which can be described as normal tetrahedral structures with a certain fixed number of unoccupied structure sites [⁷] and can be generated through the formula Cu_.n-3In_n+1Te_2n, such as CuIn₅Te₈ (n = 4), CuIn₃Te₅ (n = 5), Cu₃In₇Te₁₂ (n = 6), Cu₅In₉Te₁₆ (n = 8), and Cu3In5Te9 (n = 9) [⁸], could also be employed for TE applications [⁹]. This is because most of them have low carrier concentrations [¹⁰], a tetragonal distortion parameter η (η = c/2a) close to unity [⁶], and also an energy gap less than about 1.2 eV [¹¹], which are some of the conditions required to achieve promising materials for TE applications [²]. As regard to the crystal structure of these Cu-In-Te OVC's, although it has been suggested that most of them crystallize in tetragonal chalcopyrite-related structures [⁴,¹⁰,¹²-¹⁴], their actual crystal structures and corresponding space groups have not yet been established, as evidenced by a search in the Inorganic Crystal Structure Database (ICSD) [¹⁵].

Hence, in the present work, the crystal structure of Cu₃In7Te₁₂ (or Cu₃In7[]₂Te₁₂, where [] represents the cationic vacancy) is established from the Rietveld refinement analysis of powder X-ray diffraction data.

2. MATERIALS AND METHODS

Polycrystalline samples of Cu₃In₇Te₁₂ used in this study were prepared from the melt by the vertical Bridgman-Stockbarger technique in a multiple zone furnace. Stoichiometric mixture of highly pure components of Cu, In and Te (99.999 %) were introduced in a quartz ampoule sealed under vacuum (~ 10^-3 Pa). Initially, the ampoule was heated from room temperature to 1170 K at a rate of 20 K/h. The molten mixture was then heated to 1370 K at 10 K/h and kept at this temperature for 12 h. To assure a homogeneous mixing of the melt, the ampoule was carefully agitated periodically. It was later cooled at 10 K/h to 1090 K, and at 5 K/h to 800 K. In order to guarantee the equilibrium condition of the synthetized material, the ingot was annealed at this temperature for 120 h. The furnace was then turned off and the ingot cooled down to the room temperature.

For the X-ray diffraction analysis, a small quantity of the sample, cut from the ingot, was ground mechanically in an agate mortar and pestle. The resulting fine powder was mounted on a flat zerobackground holder covered with a thin layer of petroleum jelly. The powder X-ray diffraction data was collected at 293(1) K, in θ/θ reflection mode using a Siemens D5005 diffractometer equipped with an X-ray tube (CuKα radiation: λ= 1.5418 Å; 40kV, 30mA). A fixed aperture and divergence slit of 1 mm, a 1 mm monochromator slit, and a 0.1 mm detector slit, were used. The specimen was scanned from 10°-100° 2θ, with a step size of 0.02° and counting time of 10s. Quartz was used as an external standard. The Bruker AXS analytical software was used to establish the positions of the peaks.

3. RESULTS AND DISCUSSION

Figure 1 shows the resulting powder X-ray diffractogram for Cu₃In₇Te₁₂. The 20 first peak positions were indexed using the program Dicvol04 [¹⁶], which gave a unique solution in a tetragonal cell. Lack of systematic absences (hkl: h+k+l), indicates a primitive type lattice. In addition, the condition hhl: l =2n+1 suggests the extension symbol P4 2c. A revision of the diffraction lines, taking into account the sample composition, unit cell parameters, and lattice-type, suggests that this material is isostructural with Cu₃In₇Se₁₂. This is the first compound with the I3-III7-[]2-VI12 formula which has been reported to crystallize in a tetragonal structure with space group P4 2c (Nº 112) [¹⁷]. The resulting X-ray powder diffraction data for Cu₃In₇Te₁₂ will be submitted to the Powder Diffraction File of the International Centre for Diffraction Data (ICDD) [¹⁸].

The Rietveld refinement [¹⁹] of the whole diffraction pattern was carried out using the Fullprof program [²⁰, ²¹], with the unit cell parameters found in the indexing. For the refinement of Cu₃In₇Te₁₂, the atomic coordinates of the compound Cu₃In₇Se₁₂ [¹⁷], were used as initial model, with the cation distribution shown in Table III. The angular dependence of the peak full width at half maximum (FWHM), was described by the Cagliotti’s formula [²²]. Peak shapes were described by the parameterized Thompson-Cox-Hastings pseudo-Voigt profile function [²³]. 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 are summarized in Table 1. Figure 1 shows the observed, calculated, and difference profile for the final cycle of refinement. Atomic coordinates, isotropic temperature factor, bond distances, and angles are shown in Table 2. This Table also shows the Bond Valence Sum (BVS) [²⁴,²⁵] results for Cu₃In₇Te₁₂, indicating that the oxidation state for each ion is in good agreement with the expected formal oxidation state of Cu¹⁺, In³⁺ and Te^2-ions.

Figure 1
Rietveld refinement plot for Cu3In7Te12. The lower trace is the difference curve between observed and calculated patterns. The Bragg reflections are indicated by vertical bars.

Table 1
Rietveld refinement results for Cu₃In₇Te₁₂.

molecular formula	Cu3In7Te12	wavelength (CuKα)	1.5418 Å
molecular weight (g/mol)	1683.7	data range 2Θ (°)	10-100
a (Å)	6.1720(2)	step size 2Θ (°)	0.02
c	12.3597(8)	counting time (s)	10
c/a	2.00	step intensities	4501
V (Å3)	470.83(4)	independent reflections	130
Z	0.667	Rp (%)	9.27
crystal system	tetragonal	Rwp (%)	10.30
space group	P 4 2c (N° 112)	Rexp (%)	6.95
dcalc (g/cm-3)	5.99	RB (%)	6.10
temperature (K)	298(1)	S	1.48

• R_exp = 100 [(N-P+C) / Σw(yobs²)]^1/2
• R_p = 100 Σ|y_obs - y_calcl / Σ|y_obs
• R_wp = 100 [Σ_wj_yobs - y_calc|² / Σ_w|y_obs|²]^1/2
• R_b = 100 Σ_k|I_k -Ic_k| / Σ_k |I_k|
• S = [R_wp / R_exp]
• N-P+C is the number of degrees of freedom

Table 2
Atomic coordinates, isotropic temperature factors, bond distances (Å) and angles (°) for Cu₃In₇Te₁2, derived from the Rietveld refinement. Bond valence sum (BVS) are also showed.

Atom	Ox.	BVS	Site	x	y	z	foc	B (A2)
Cu	+1	1.3	2e	0	0	0	1	0.4(3)
□			2a	0	0	¼	1
In1			2b	½	0	¼	1	0.4(3)
In2	+3	3.3	2d	0	½	¼	1	0.4(3)
In3			2f	½	½	0	½	0.4(3)
Te	-2	2.4	8n	0.223(1)	0.261(1)	0.118(1)	1	0.4(3)
Cu-Sei	2.572(9)		In1-Teiii	2.860(9)	In2-Teii	2.595(9)	In3-Teiv	2.688(9)

• Symmetry codes:
• ⁽ⁱ⁾
  y, -x, -z;
• ⁽ⁱⁱ⁾
  1-x, 1-y, z;
• ⁽ⁱⁱⁱ⁾
  x, -y, 0.5-z;
• ^(iv)
  -x, y, 0.5-z.
• Bond valence sum (BVS): V_i = Σ_j exp [(R_o - R_ij / b], b = 0.37 Å, r_o (Cu-Te) = 2.27 Å, (In-Te) = 2.69 Å

From refinement by the Rietveld method by using powder X-ray diffraction data, it has been found that this material crystallizes in the tetragonal space group P4 2c, with a chalcopyrite-related structure. This consists of a three-dimensional arrangement of distorted CuTe₄ and InTe₄ tetrahedra connected by common faces. In this structure, each Te atom is coordinated by four cations (one Cu and three In) located at the corners of a slightly distorted tetrahedron. In the same way, each cation is tetrahedrally bonded to four anions. This array is expected for adamantane compounds [⁷].

The Cu-Te and In-Te bond distances for Cu₃In₇Te₁₂ here obtained, are in good agreement with those reported in the ICSD database [¹⁵] for other adamantane structure compounds such as CuInTe₂ [²⁶], AgIn₅Te₈ [²⁷], CuTa₂InTe₄ [²⁸], Cu₃NbTe₄ [²⁹], AgInTe₂ [¹] and CuCO₂InTe₄ and CuNi₂InTe₄ [³⁰].

Figure 2
Unit cell diagram for the ordered vacancy compound Cu₃In₇Te₁₂.

4. CONCLUSIONS

In conclusion, it is established that the ordered vacancy compound Cu₃In₇Te₁₂ (or Cu₃In₇[]₂Te₁₂) crystallizes with a chalcopyrite-related structure in the tetragonal space group P4 2c, and represents a new semiconductor with formula I₃-III₇-[]₂-VI₁₂.

ACKNOWLEDGMENTS

This work was supported by CDCHTA-ULA and FONACIT (Grant LAB-97000821).

BIBLIOGRAPHY

• ¹
  KNIGHT, K. S., “The crystal structures of CuInSe2 and CuInTe2”, Materials Research Bulletin, v. 27, n. 2, pp. 161-167, Feb. 1992.
• ²
  ZHANG, J., LIU, R., CHENG, N., et al, “High‐performance pseudocubic thermoelectric materials from non‐cubic chalcopyrite compounds”, Advanced Materials, v. 26, n. 23, pp. 3848-3853, Jun. 2014.
• ³
  ZHOU, G., WANG, D., “High thermoelectric performance from optimization of hole-doped CuInTe2”. Physical Chemistry Chemical Physics, v. 18, n. 11, pp. 5925-5931, Nov. 2016.
• ⁴
  PLIRDPRING, T., KUROSAKI, K., KOSUGA, A., et al, “High‐temperature thermoelectric properties of Cu2In4Te7”, Physica Status Solidi rapid research letters, v. 6, n. 4, pp. 154-156, April 2012.
• ⁵
  PLIRDPRING, T., KUROSAKI, K., KOSUGA, A., et al, “High-temperature thermoelectric properties of Cu3In5Te9 with defect-chalcopyrite structure”, Advances Science Letters, v. 19, n. 1, pp. 183-185, Jan. 2013.
• ⁶
  RINCÓN, C., WASIM, S. M, MARÍN, G., “Scattering of the charge carriers by ordered arrays of defect pairs in ternary chalcopyrite semiconductors”, Applied Physics Letters, v. 80, n. 6, pp. 998-1000, Feb. 2002.
• ⁷
  RINCÓN, C., WASIM, S. M, MARÍN, G., et al, “Effect of ordered arrays of native defects on the crystal structure of In-and Ga-rich Cu-ternaries”, Applied Physics Letters, v. 83, n. 7, pp. 1328-1330, Aug. 2003.
• ⁸
  PARTHÉ, E. “Intermetallic compounds, principles and applications”, Chichester, UK: John Wiley & Sons, 1995.
• ⁹
  ZHANG S. B., WEI S-H., ZUNGER A., “Stabilization of Ternary Compounds via Ordered Arrays of Defect Pairs”, Physical Review Letters v. 80, n. 21, pp. 4059-4062, May 1997.
• ¹⁰
  KOSUGA, A., HIGHASINE, R., PLIRDPRING, T., et al, “Effects of the defects on the thermoelectric properties of Cu-In-Te chalcopyrite-related compounds”, Japanese Journal of Applied Physics, v. 51, n. 12R, pp. 121803, Nov. 2012.
• ¹¹
  RINCÓN, C., WASIM, S. M., MARÍN, G., et al, “Effect of ordered arrays of native defects on the crystal structure of In-and Ga-rich Cu-ternaries”, Applied Physics Letters, v. 83, n. 7, pp. 1328-1330, Aug. 2003.
• ¹²
  WASIM, S. M, RINCÓN, C., MARÍN, G., et al, “On the band gap anomaly in I-III-VI2, I-III3-VI5, and I-III5-VI8 families of Cu ternaries”, Applied Physics Letters, v. 77, n. 1, pp. 94-96, Jul. 2000.
• ¹³
  PARLAK, M., ERCELEBI, C., GUNAL I., et al, “Crystal Data, Electrical Resisitivity and Mobility in Cu3In5Se9 and Cu3In5Te9 Single Crystals” Crystal Research and Technology, v. 32, n. 3, pp. 395-400, Jan 1997.
• ¹⁴
  GUEDEZ, E., MOGOLLÓN, L., MARCANO, et al, “Structural characterization and optical absorption spectrum of Cu3In5Te9”, Materials Letters, v. 186, pp. 155-157, Jan. 2017.
• ¹⁵
  DÍAZ, R., BISSON, L., AGULLÓ-RUEDA F., et al, “Effect of composition gradient on CuIn3Te5 single-crystal properties and micro-Raman and infrared spectroscopies”, Applied Physics A, v. 81, pp. 433-438, April 2005.
• ¹⁶
  ICSD-Inorganic Crystal Structure Database, Gemlin Institute, Kalrsruhe, Germany, 2016.
• ¹⁷
  BOULTIF, A., LÖUER, D., “Powder pattern indexing with the dichotomy method”, Journal of Applied Crystallography, v. 37, n. 5, pp. 724-731, Oct. 2004.
• ¹⁸
  DELGADO, G. E., MANFREDY, L., LÓPEZ-RIVERA, S. A., “Crystal structure of the ternary semiconductor Cu2In14/3[]4/3Se8 determined by X-ray powder diffraction data”, Powder Diffraction, v. 33, n.3, pp. 237-241, Oct. 2018./p>
• ¹⁹
  PDF-Powder Diffraction File (set 1-65), International Centre for Diffraction Data, Newtown Square, PA, USA, 2013.
• ²⁰
  RIETVELD, H. M., “A profile refinement method for nuclear and magnetic structures”, Journal of Applied Crystallography, v. 2, n. 2, pp. 65-71, Feb. 1969.
• ²¹
  RODRÍGUEZ-CARVAJAL, J., “Recent advances in magnetic structure determination by neutron powder diffraction”, Physica B, v. 192, n. 1-2, pp. 55-69, Oct. 1993.
• ²²
  RODRÍGUEZ-CARVAJAL, J., “Fullprof”, version 5.8, LLB, CEA-CNRS, France, 2016.
• ²³
  CAGLIOTTI, G., PAOLETTI, A., RICCI, F. P., “Choice of collimators for a crystal spectrometer for neutron diffraction”. Nuclear Instruments, v. 3, n. 4, 223-228, Oct. 1958.
• ²⁴
  THOMPSON, P., COX, D. E., HASTINGS, J. B., “Rietveld refinement of Debye–Scherrer synchrotron X-ray data from Al2O3”, Journal of Applied Crystallography, v. 20, n. 2, pp. 79-83, Apr. 1987.
• ²⁵
  BROWN, I. D., ALTERMATT, D., “Bond-valence parameters obtained from a systematic analysis of the inorganic crystal structure database”. Acta Crystallographica B, v. 41, n. 4, pp. 244-247, Aug. 1985.
• ²⁶
  BRESE, N. E., O’KEEFFE, M., “Bond-valence parameters for solids”, Acta Crystallographica B, v. 47, n.2, pp. 192-197, Apr. 1991.
• ²⁷
  MORA, A. J., DELGADO, G. E., PINEDA, C., et al, “Synthesis and structural study of the AgIn5Te8 compound by X‐ray powder diffraction”, Physica Status Solidi A, v. 201, n. 7, pp. 1477-1483, May. 2004.
• ²⁸
  DELGADO, G. E., MORA, A. J., GRIMA-GALLARDO, P., et al, “Crystal structure of the quaternary compound CuTa2InTe4 from X-ray powder diffraction”, Physica B, v. 403, n. 18, pp. 3228-3230, Sep. 2008.
• ²⁹
  DELGADO, G. E., MORA, A. J., GRIMA-GALLARDO, P., et al, “Synthesis and characterization of the ternary chalcogenide compound Cu3NbTe4”, Chalcogenide Letters, v. 6, n. 8, pp. 335-338, Aug. 2009
• ³⁰
  DELGADO, G. E., MORA, A. J., PINEDA, C., et al, “X-ray powder diffraction data and rietveld refinement of the ternary semiconductor chalcogenides AgInSe2 and AgInTe2”, Revista Latinoamericana de Metalurgia y Materiales, v. 35, n. 1, pp. 110-117, Mar. 2015.
• ³¹
  DELGADO, G. E., GRIMA-GALLARDO, P., NIEVES, L., et al, “Structural characterization of two new quaternary chalcogenides: CuCo2InTe4 and CuNi2InTe4”, Materials Research, v. 19, n. 6, pp. 1423-1428, Dec. 2016.

Publication Dates

• Publication in this collection
  20 May 2019
• Date of issue
  2019

History

• Received
  05 Mar 2017
• Accepted
  29 Sept 2017