SYNTHESIS, X-RAY DIFFRACTION AND

Adv. Mat. Sci. & Technol. Nº7 Art 3 pp 23-38, 2013 ISSN 1316-2012 Depósito Legal pp 96-0071

Recibido 16 07 2013 Aceptado 07 08 2013 Publicado 31 08 2013 © 2013 CIRES

SYNTHESIS, X-RAY DIFFRACTION AND DIFFERENTIAL THERMAL ANALYSIS OF Cu2(Ge1-xSnx)Se3+ ALLOYS (=1 and x=0, 0.25, 0.50, 0.75, 1) R. PEÑA1, P. GRIMA-GALLARDO 1, L. NIEVES1 , G. MARCANO1 , M. QUINTERO1 , E. MORENO1, M.A. RAMOS2, J.A. HENAO3 & J.M. BRICEÑO4 . 1: Centro de Estudios en Semiconductores (C.E.S.). Departamento de Física, 4: Laboratorio de Análisis Químico y Estructural (LAQUEM) Dpto Física Facultad de Ciencias, Universidad de Los Andes, Mérida 5101-Venezuela 2: Laboratorio de Difracción y Fluorescencia de Rayos-X. Instituto Zuliano de Investigaciones Tecnológicas (INZIT), La Cañada de Urdaneta, Estado Zulia, Venezuela. 3: Laboratorio de Difracción de Rayos-X. Escuela de Química. Universidad Industrial de Santander. Bucaramanga, Colombia.

ABSTRACT Polycrystalline samples (weight ~ 1g), belonging to the Cu2(Ge1-xSnx)Se3+ alloy system, with =1, and in the composition range 0≤x≤1, were prepared by the usual melt and anneal method and characterized by X-Ray Diffraction (XRD) and Differential Thermal Analysis (DTA) techniques. Compositions x=0 (Cu2GeSe4) and 0.25 showed two phases, both indexed in an orthorhombic structure analogue to that observed by Parthé et al (1971) for Cu2GeSe3 [1]. Compositions x=0.50 and 0.75 showed two phases with cubic structure, analogue to that reported by Berger et al for Cu2SnSe3 [2]. Composition x=1 (Cu2SnSe4) showed several cubic phases. This behavior is typical of a spinodal system. All the compositions studied show an incongruent melting point and a solid-to-solid phase transition near of the melting point. Key Words: Semiconductor alloys. X-ray diffraction. Differential Thermal Analysis. Phase diagram.

SÍNTESIS, DIFFRACTION RAYOS-X Y ANÁLYSIS TÉRMICO DIFFERENTIAL DE Cu2(Ge1-xSnx)Se3+ ALEACIONES (=1 and x=0, 0.25, 0.50, 0.75, 1) RESUMEN Por el método tradicional de fusión y recocido, fueron preparadas muestras policristalinas (peso ~ 1g), pertenecientes a la familia de aleaciones Cu2(Ge1-xSnx)Se3+ cony en el rango de composiciones 0≤x≤1; los productos se caracterizaron por las técnicas de Difracción de Rayos X (DRX) y Análisis Térmico Diferencial ATD). Las composiciones x=0 y x=0.25 muestran dos fases que indexan ambas en una estructura ortorrómbica análoga a la reportada por Parthé et al para el compuesto Cu2GeSe3 [1]. Las composiciones x=0.50 y 0.75 muestran también dos fases pero de estructura cívica, análoga a la reportada por Berger et al para el compuesto Cu2SnSe3 [2]. La composición x=1 (Cu2SnSe4) muestra varias fases cúbicas. El comportamiento de este sistema es típico de una descomposición espinodal. Todas las composiciones estudiadas muestran un punto de fusión incongruente y una transición sólido-sólido cercana al punto de fusión. Palabras clave: Aleaciones semiconductoras. Difracción de Rayos X (DRX). Análisis Térmico Diferencial (ATD). Diagrama de fase. FICHA PEÑA, R.; P. GRIMA-GALLARDO; L. NIEVES; G. MARCANO; M. QUINTERO; E. MORENO; M.A. RAMOS J.A. HENAO &J.M BRICEÑO, 2013.- SYNTHESIS, X-RAY DIFFRACTION AND DIFFERENTIAL THERMAL ANALYSIS OF Cu2(Ge1-xSn x)Se3+ ALLOYS (=1 and x=0, 0.25, 0.50, 0.75, 1) Adv. Mat. Sci. & Technol 7(2): 23-38, ISSN 1316-2012

23

that the tetragonal phase is only obtained if the samples have been annealing for a long period time [21-22]

INTRODUCTION The Cu2GeSe3 and Cu2SnSe3 are compounds of the AI2B IVC VI3 family with potential applications in solar cells, thermal to electric conversion and electro-optic devices [3-13]. Apart the scientific interest of these compounds for their technological applications, an academic interest is added by the fact that there was some disagreement about the unit cell size and symmetry of Cu2GeSe3 and also a controversy about the nature of the crystallographic structure of Cu2 SnSe3. Moreover, the compound Cu2GeSe3+with =1 (nominally Cu2GeSe4 ) could not be formed whereas for Cu2SnSe3+, the value =1 (nominally Cu2SnSe4) has been reported by several authors [14-17].

With respect to the pseudobinary system Cu2SeSnSe2 only the compound Cu2SnSe3 has been reported [23]. However, controversy exists about the nature of the crystallographic structure of Cu2 SnSe3. It has been indicated that this compound has no tetragonality and crystallizes in a zinc blende structure with fcc lattice [24]. On the other hand it was proposed that up to 723K, Cu2SnSe3 , has orthorhombic symmetry and at this temperature an order-disorder transformation takes place [25]. The structure of this modified phase was found to be of the sphalerite type. The data of the latest investigations showed that this compound could crystallize in a monoclinic structure with a sphalerite superstructure [24]. Some authors indicated the existence of Cu2SnSe4 in the Cu2Se-SnSe2 section [14-17] but the crystal structure and lattice parameter (a=5.6878Å) of this compound is the same of that of Cu2SnSe3. Actually, Cu2SnSe3 and Cu2SnSe4 are considered to be the two terms of the solid solution Cu2SnSe3+ (0≤≤1). There is agreement in the phase diagrams of the quasibinary system Cu2Se-SnSe2 that have been reported by several authors [26-29]; the compound Cu2SnSe3 has a congruent melting point of 968K and the and interactions between Cu2Se-Cu2SnSe3 Cu2SnSe3 -SnSe2 are of the eutectic type, with invariant point coordinates (in % mol SnSe2 and K) of (21, 938) and (84, 861), respectively.

A recent report about the phase relations on the pseudobinary system Cu2Se-GeSe2 have clarify the disagreement about the unit cell and symmetry of Cu2GeSe3 [18]. It was found than there are only two definite compounds in this section: Cu8GeSe6 (at 20 mol % of GeSe2) and Cu2GeSe3 (at 50 mol % of GeSe2). Cu8GeSe6 has two modifications, corresponding to the low and high temperature range, both hexagonal. Cu2GeSe3 shows also two modifications, the high temperature modification is orthorhombic, space group Imm2 (No 44), with lattice parameters a=11.860 Å, b=3.960 Å and c=5.485 Å, whereas the low temperature modifica(No 122). The tion is tetragonal, space group compound Cu2GeSe3 melts incongruently at 1054K and the solid to solid polymorphic transition (tetragonal to orthorhombic) occurred at 893K. The interactions between Cu8GeSe6 -Cu2GeSe3 and Cu2GeSe3-GeSe2 are of the eutectic type, and their respective invariant point coordinates (in % mol GeSe2 and K) are (38, 1033) and (83, 960), respectively.

EXPERIMENTAL PROCEDURE Preparation of the samples. The samples were synthesized using the melt and annealing technique. Stoichiometric quantities of Cu, Ge, Sn and Se elements with purity of 99.99% were charged in an evacuated quartz ampoule, which was previously subjected to pyrolysis in order to avoid reaction of the starting materials with quartz. Then, the ampoule was sealed under vacuum (~10-4 Torr) and the fusion process was carried out inside a furnace (vertical

The previous confusion with the crystal structure and unit cell of Cu2GeSe3 was probably partially produced by the fact that Cu2GeSe3 is very sensitive to Ge concentration: a slight deficiency lowers the cell symmetry to a monoclinic (nominal composition Cu2Ge0.85Se3 , lattice parameters a=5.512 Å, b=5.598 Å, c=5.486 Å, =89.7o), while an excess raises it to a cubic (nominal composition Cu2Ge1.55Se3, lattice parameter a=5.569 Å) [19-20]. In addition, it is seem 24

position) heated up to 1500K at a rate of 20o/h, with a stop of 48 h at 490K (melting temperature of Se) in order to maximize the formation of binary species at low temperature and minimize the presence of unreacted Se at high temperatures. The ampoule was shaken using a mechanical system during all the heating process in order to help the complete mixing of all the elements. The maximum temperature (1500K) was keeping for other 48 hours with the mechanical shaken system on. Then, the mechanical shaken system was turning off and the temperature was gradually down, at the same rate of 20o/h, until 800K. The ampoule was kept at this temperature for a period of 30 days. Finally, the sample was cooled to room temperature at a rate of 10o/h.

DTA-7 with aluminum and gold used as reference materials. The charge was of powdered alloy of approximately 100-mg weight. Both heating and cooling runs were carried out on each sample, the average rates of these runs being approximately 10 K/min. The error in determining these temperatures is of about ±10 K. The temperature values of the thermal transitions were obtained using the criteria of the interception of the base line with the beginning of the corresponding peak.

The obtained ingots were gray color and homogeneous; moreover, the absence of the characteristic Se smell when the capsules were opened were an indication that the whole Se had been reacted.

In Figure 1, the experimental patterns obtained by XRD are displayed. At first sight, in Figure 1a (left), we observe, for all compositions, a single phase with very close lattice parameters; however, when the strongest peak is amplified (Figure 1b, right), we can observe that:

ANALYSIS AND DISCUSSION 3.1 XRD measurements.

XRD measurements. A small amount of each compound was gently ground in an agate mortar and sieved to a grain size of less than 38 μm. Each sample was mounted on a zero-background specimen holder for the respective measurement. Xray powder diffraction patterns of the samples were recorded using a D8 FOCUS BRUKER Rigaku D/MAX IIIB diffractometer operating in BraggBrentano geometry equipped with an X-ray tube (CuKα radiation: λ = 1.5406 Å, 40 kV and 40 mA) using a nickel filter and an one dimensional LynxEye detector. A fixed antiscatter slit of 8 mm, receiving slit of 1 mm, soller slits of 2.5° and a detector slit of 3 mm were used. The scan range was from 2 to 70° (2θ) with a step size of 0.02° (2θ) and a counting time of 0.4 s/step.

a) The strongest peak of Cu2GeSe4, Cu2(Ge0.75Sn0.25)Se4 , Cu2(Ge0.5Sn0.5)Se4 and Cu2(Ge0.25Sn0.75)Se4 are double. b) The strongest peak of Cu2 SnSe4 is triple.

This behavior is typical of a spinodal system. Effectively, the diffraction pattern of Cu2GeSe4 and Cu2(Ge0.75Sn0.25)Se4 can be fully indexed with two orthorhombic phases (named as 1 and 2 in the text), with the same structure and very close lattice parameters; the diffraction patterns of Cu2(Ge0.5Sn0.5)Se4 and Cu2(Ge0.25Sn0.75)Se4 can be fully indexed with two cubic phases (named 1 and 2 in the text); and the diffraction pattern of Cu2SnSe4 can be indexed with three cubic phases (1, 2 and 3 ). The respective lattice parameters were obtained indexing with the computer program DICVOL04 [30].

DTA measurements. Phase transition temperatures were obtained from differential thermal analysis (DTA) measurements, in the temperature range between 300 and 1500K, using a Perkin-Elmer 25

Figure 1. Figure 1a (left): Experimental diffraction patterns of the Cu2(Ge1-xSnx)Se3+ alloy system; Figure 1b (right): Amplification around the strongest peak. Table I. Indexation of the 1 phase of Cu2GeSe4 2obs(o) dobs(Å)

(I/I0)obs

hkl

2cal(o) dcal(Å)

2(o)

27.799 45.878

3.20665 1.97639

100.0 12.7

101 002

27.795 45.887

3.20706 0.004 1.97602 -0.009

46.336 54.492

1.95791 1.68257

32.0 13.0

211 112

46.333 54.499

1.95803 0.003 1.68238 -0.007

55.290 66.895

1.66016 1.39756

6.8 2.1

301 022

55.290 66.904

1.66015 0.000 1.39739 -0.009

68.305 73.914 74.598

1.37211 1.28124 1.27117

1.2 1.2 3.0

400 103 312

68.309 73.932 74.602

1.37204 -0.004 1.28096 -0.018 1.27112 -0.004

85.243 86.228

1.13756 1.12707

2.8 1.6

213 402

85.261 86.234

1.13736 -0.018 1.12700 -0.006

91.530 92.193

1.07511 1.06910

1.0 0.7

123 303

91.549 92.202

1.07493 -0.019 1.06902 -0.009

93.486

1.05769

0.6

501

93.496

1.05760 -0.010

Figures of merit [30]: M (14) =129.8; F (14) =55.2 (0.0040, 64) Lattice parameters: a= (5.4882±0.0002) Å; b= (3.9528±0.0001) Å; c= (11.856±0.001) Å

Table II. Indexation of the 2 phase of Cu2GeSe4 26

2obs(o) dobs(Å)

(I/I0)obs

hkl

2cal(o) dcal(Å)

2(o) 0.010

27.873

3.19830

100.0

101

27.863

3.19938

46.010 46.458 54.636

1.97102 1.95306 1.67847

15.2 27.5 13.1

002 211 112

46.001 46.451 54.637

1.97140 0.009 1.95334 0.007 1.67844 -0.001

55.440 67.085

1.65602 1.39407

5.8 2.0

301 022

55.439 67.079

1.65605 1.39418

68.497 74.137

1.36873 1.27793

0.9 1.8

400 103

68.504 74.136

1.36861 -0.007 1.27795 0.001

74.815 85.505

1.26802 1.13474

2.7 2.6

312 213

74.813 85.511

1.26806 0.002 1.13468 -0.006

86.493 91.816 92.490

1.12430 1.07251 1.06645

1.3 1.0 0.6

402 123 303

86.498 91.824 92.488

1.12425 -0.005 1.07244 -0.008 1.06646 0.002

0.001 0.006

Figures of merit [30]: M (13) =127.4; F (13) =45.2 (0.0046, 62) Lattice parameters: a= (5.4745±0.0004) Å; b= (3.9439±0.0002) Å; c= (11.828±0.002) Å

Figure 2. Indexation of the sample Cu2GeSe4. The diffraction pattern has been divided in two parts for clarity. The labels correspond to the hkl-Miller indices and the respective phases (1 or 2). Table III. Indexation of the 1 phase of Cu2(Ge0.75 Sn0.25)Se4. 27

2obs(o)

dobs(Å)

(I/I0)obs

hkl

2cal(o)

dcal(Å)

2(o)

27.866

3.19908

100.0

101

27.898

3.19552

-0.032

45.937

1.97399

23.3

002

45.916

1.97483

0.021

46.211

1.96292

32.4

211

46.220

1.96257

-0.009

54.443

1.68398

19.4

112

54.464

1.68336

-0.022

54.951

1.66958

8.4

301

54.953

1.66953

-0.002

67.672

1.38340

7.4

400

67.673

1.38338

-0.001

73.766

1.28345

2.4

103

73.747

1.28373

0.019

74.188

1.27719

3.5

312

74.239

1.27643

-0.051

84.893

1.14135

4.3

213

84.912

1.14114

-0.019

85.536

1.13441

3.7

402

85.567

1.13407

-0.032

Figures of merit [30]: M (12) =46.1; F (14) =18.4 (0.0126, 52) Lattice parameters: a= (5.5528±0.0011) Å; b= (3.9714±0.0006) Å; c= (11.889±0.001) Å

Table IV. Indexation of the 2 phase of Cu2(Ge0.75 Sn0.25)Se4. 2obs(o)

dobs (Å)

(I/I0)obs

hkl

2cal(o)

dcal (Å)

2(o)

27.866

3.19908

100.0

101

27.852

3.20069

0.014

46.044

1.96964

14.3

002

46.030

1.97023

0.014

46.346

1.95754

17.7

211

46.304

1.95921

0.042

54.554

1.68080

15.1

112

54.503

1.68226

0.051

55.103

1.66536

6.1

301

55.112

1.66510

-0.009

66.953

1.39649

6.0

022

66.946

1.39663

0.007

74.000

1.27996

2.9

103

74.021

1.27965

-0.021

74.411

1.27391

2.8

312

74.389

1.27423

0.022

85.065

1.13948

3.5

213

85.075

1.13938

-0.010

85.820

1.13139

2.2

402

85.832

1.13125

-0.013

Figures of merit [30]: M (10) =35.1; F (10) =10.9 (0.0177, 52) Lattice parameters: a= (5.524±0.002) Å; b= (3.955±0.002) Å; c= (11.893±0.001) Å

28

Figure 3. Indexation of the sample Cu2Ge0.75Sn0.25Se4. The diffraction pattern has been divided in two parts for clarity. The labels correspond to the hkl-Miller indices and the respective phases (1 or 2).

Table V. Indexation of the  phase of Cu2(Ge0.5Sn0.5)Se4. 2obs(o)

dobs (Å)

(I/I0)obs

hkl

2cal(o)

dcal (Å)

2(o)

27.656

3.22288

100.0

111

27.494

3.24153

0.162

45.723

1.98271

31.1

220

45.667

1.98503

0.056

54.181

1.69149

18.8

311

54.134

1.69284

0.047

66.512

1.40469

4.7

400

66.569

1.40363

-0.057

73.424

1.28858

5.1

331

73.458

1.28805

-0.035

84.437

1.14634

3.5

422

84.464

1.14606

-0.026

90.768

1.08214

2.0

511

90.943

1.08051

-0.175

Figures of merit [30]: M (7) =16.5; F (7) =3.1 (0.0797, 28) Lattice parameter: a= (5.615±0.003) Å.

29

Table VI. Indexation of the  phase of Cu2(Ge0.5Sn0.5)Se4. 2obs(o) dobs (Å) (I/I0)obs h k l 2cal(o)

dcal (Å)

2(o)

27.671

3.22122

100.0

111

27.571

3.23261

0.099

45.848

1.97761

46.7

220

45.800

1.97956

0.048

54.331

1.68716

18.1

311

54.296

1.68818

0.035

66.881

1.39782

3.6

400

66.776

1.39976

0.105

73.629

1.28550

6.8

331

73.694

1.28451

-0.066

84.661

1.14389

4.4

422

84.751

1.14290

-0.090

91.036

1.07965

2.2

511

91.265

1.07754

-0.229

Figures of merit [30]: M (7) =12.4; F (7) =2.6 (0.0962, 28) Lattice parameter: a= (5.599±0.003) Å.

Figure 3. Indexation of the sample Cu2Ge0.5Sn0.5Se4. The diffraction pattern has been divided in two parts for clarity. The labels correspond to the hkl-Miller indices and the respective phases (1 or 2). 30

Table VII. Indexation of the  phase of Cu2(Ge0.25Sn0.75)Se4. 2obs(o)

dobs (Å)

(I/I0)obs

hkl

2cal(o)

dcal (Å)

2(o)

27.517

3.23885

100.0

111

27.504

3.24031

0.013

45.531

1.99065

44.1

220

45.520

1.99110

0.011

53.905

1.69951

23.3

311

53.907

1.69944

-0.002

66.184

1.41085

4.7

400

66.211

1.41033

-0.028

73.038

1.29443

6.1

331

73.022

1.29468

0.016

83.886

1.15247

4.5

422

83.883

1.15250

0.003

90.270

1.08681

2.1

511

90.267

1.08684

0.003

Figures of merit [30]: M (7) =107.4; F (7) =21.5 (0.0108, 30) Lattice parameter: a= (5.6587±0.0006) Å.

Table VIII. Indexation of the  phase of Cu2(Ge0.25Sn0.75)Se4. 2obs(o)

dobs (Å)

(I/I0)obs

hkl

2cal(o)

dcal (Å)

2(o)

27.517

3.23887

100.0

111

27.481

3.24303

0.036

45.600

1.98779

36.5

220

45.620

1.98698

-0.020

54.043

1.69548

15.7

311

54.069

1.69472

-0.026

66.475

1.40537

4.4

400

66.475

1.40537

0.000

73.324

1.29008

4.0

331

73.348

1.28973

-0.024

84.355

1.14725

3.4

422

84.321

1.14763

0.034

90.772

1.08210

1.8

511

90.780

1.08203

-0.008

Figures of merit [30]: M (7) =58.4; F (7) =11.1 (0.0210, 30) Lattice parameter: a= (5.6241±0.0009) Å.

31

Figure 4. Indexation of the sample Cu2Ge0.25Sn0.75Se4. The diffraction pattern has been divided in two parts for clarity. The labels correspond to the hkl-Miller indices and the respective phases (1 or 2). Table IX. Indexation of the  phase of Cu2SnSe4 . 2obs(o) dobs (Å) (I/I0)obs

hkl 2cal(o)

dcal (Å)

2(o)

27.101

3.28761

100.0

111 27.114

3.28611 -0.013

45.020

2.01204

41.7

220 45.013

2.01233

53.367

1.71535

18.6

311 53.341

1.71612 0.026

65.524

1.42345

3.5

400 65.551

1.42293 -0.027

72.312

1.30562

4.7

331 72.303

1.30577 0.010

83.057

1.16185

3.4

422 83.060

1.16182 -0.003

89.371

1.09539

2.0

511 89.374

1.09537 -0.002

Figures of merit [30]: M (7) =92.4; F (7) =18.8 (0.0124, 30) Lattice parameter: a= (5.6917±0.0006) Å.

32

0.007

Table X. Indexation of the  phase of Cu2SnSe4. 2obs(o)

dobs (Å)

(I/I0)obs

hkl

2cal(o)

dcal (Å)

2(o)

27.160

3.28065

100

111

27.184

3.27778

-0.024

45.149

2.00658

28.4

220

45.134

2.00722

0.015

53.493

1.71161

12.5

311

53.488

1.71177

0.005

65.695

1.42015

2.8

400

65.739

1.41932

-0.043

72.520

1.30239

3.1

331

72.516

1.30246

0.004

83.343

1.15859

2.2

422

83.318

1.15887

0.024

Figures of merit [30]: M (6) =67.6; F (6) =12.9 (0.0194, 24) Lattice parameter: a= (5.6773±0.0009) Å.

Table XI. Indexation of the  phase of Cu2SnSe4 . 2obs(o)

dobs (Å)

(I/I0)obs

hkl

2cal(o)

dcal (Å)

2(o)

27.025

3.29671

100

111

27.050

3.29374

-0.025

44.882

2.01790

50.9

220

44.907

2.01684

-0.025

53.264

1.71842

30.6

311

53.214

1.71993

0.051

65.417

1.42553

4.7

400

65.389

1.42607

0.028

72.130

1.30847

5.8

331

72.119

1.30864

0.010

82.800

1.16481

3.9

422

82.839

1.16435

-0.039

Figures of merit [30]: M (6) =43.7; F (6) =8.4 (0.0296, 24) Lattice parameter: a= (5.704±0.001) Å.

33

Figure 4. Indexation of the sample Cu2SnSe4 . The diffraction pattern has been divided in two parts for clarity. The labels correspond to the hkl-Miller indices and the respective phases (1 , 2 or 3 ). Finally, The cell parameters of 1 and 2 himself as 1, 2 and 3 phases are very close as it was be expected for a spinodal decomposition.

In Figure 5 we show the variation of the lattice parameters with composition for x=0, 0.25, 0.5, 0.75 and 1. It can be seen that the a lattice parameter follows a linear behavior in the compositions range 0≤x