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Cationic and Anionic Disorder in CZTSSe Kesterite Compounds: A Chemical Crystallography Study Pierre Bais,† Maria Teresa Caldes,† Michael̈ Paris,*,† Catherine Guillot-Deudon,† Pierre Fertey,‡ Bernadette Domengès,§ and Alain Lafond*,† †

Institut des Matériaux Jean Rouxel (IMN), Université de Nantes, CNRS, 2 rue de la Houssinière, BP 32229, 44322 Nantes Cedex 3, France ‡ Synchrotron SOLEIL, L’Orme des Merisiers Saint-Aubin, BP 48, Gif-sur-Yvette Cedex 91192, France § LAMIPS-CRISMAT-NXP semiconductor-Presto-Engineering Europe Laboratory, CNRS-UMR6508, ENSICAEN, UCN, Presto-Engineering Europe, 2 rue de la Girafe, 14000 Caen, France S Supporting Information *

ABSTRACT: The cationic and anionic disorder in the Cu2ZnSnSe4-Cu2ZnSnS4 (CZTSe-CZTS) system has been investigated through a chemical crystallography approach including X-ray diffraction (in conventional and resonant setup), 119Sn and 77Se NMR spectroscopy, and high-resolution transmission electron microscopy (HRTEM) techniques. Single-crystal XRD analysis demonstrates that the studied compounds behave as a solid solution with the kesterite crystal structure in the whole S/(S + Se) composition range. As previously reported for pure sulfide and pure selenide compounds, the 119Sn NMR spectroscopy study gives clear evidence that the level of Cu/Zn disorder in mixed S/Se compounds depends on the thermal history of the samples (slow cooled or quenched). This conclusion is also supported by the investigation of the 77Se NMR spectra. The resonant singlecrystal XRD technique shows that regardless of the duration of annealing step below the order−disorder critical temperature the ordering is not a long-range phenomenon. Finally, for the very first time, HREM images of pure selenide and mixed S/Se crystals clearly show that these compounds have different microstructures. Indeed, only the mixed S/Se compound exhibits a mosaic-type contrast which could be the sign of short-range anionic order. Calculated images corroborate that HRTEM contrast is highly dependent on the nature of the anion as well as on the local anionic order.

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INTRODUCTION In the past decade, huge attention has been paid to copper zinc tin chalcogenides (CZTS, CZTSe, and CZTSSe) as absorbers for thin-film-based solar cells. In these materials, the formation of [ZnCu+CuZn] antisite defects (the so-called Cu/Zn disorder) has been pointed out to affect their optical and electronic properties. In particular, the electrostatic potential fluctuations due to such a Cu/Zn disorder are likely to limit the open-circuit voltage of CZTSSe-based solar cells1,2 and affect the band gap.3 This feature leads to an order−disorder transition for which the critical temperatures are 533 and 473 K for CZTS4 and CZTSe,3 respectively. In addition to the issue of the Cu/Zn disorder, the question of anionic disorder in the mixed S/Se compounds has been less investigated but is of interest due to its probable effect on the optoelectronic properties of these materials. In kesterite the position of the valence band maximum is mainly defined by the interaction between the Cu(p) and the chalcogen(d) orbitals. Thus, the partial substitution of sulfur for selenium could lead to local distortions (due to differences in atomic radii) and affect the © 2017 American Chemical Society

electronic properties of mixed CZTSSe compounds through local band gap modifications. The main issue in quantitatively measuring the Cu/Zn disorder by conventional X-ray diffraction comes from the very close scattering factors for Cu and Zn, because these elements are neighbors in the periodic table. A few authors have succeeded in determining the site occupancy factors (sof) of Cu and Zn through Rietveld refinements from neutron data as well as resonant X-ray data.5−7 However, in those studies, no systematic indications about the atomic displacement parameters (ADP) were given, whereas those parameters are highly correlated to the corresponding site occupancy factors. Indeed, during the refinement a wrong distribution of atoms on the crystallographic sites could be almost totally compensated by unrealistic ADPs. This point could explain the discrepancies within the published Cu/Zn distributions. Recently, a multiwavelength resonant X-ray powder diffraction approach has Received: July 13, 2017 Published: September 15, 2017 11779

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Table 1. Sample Labels According to the Cationic Stoichiometry (S for Cu2ZnSn(SxSe1−x)4 and A for Cu1.70Zn1.15Sn(SxSe1−x)4), the Anionic Composition x, and the Cooling Mode at the End of the Annealing Step (SC for Slow Cooled, VSC for Very Slow Cooled, and Q for Quenched) x(target) 0

0.25

0.50

0.75

1

S50SC A50SC

S75SC A75SC

S100SCS, 100VSC

Slow Cooled Cu2ZnSn(S,Se)4 Cu1.70Zn1.15Sn(S,Se)4

S00SC

S25SC A25SC Quenched

Cu2ZnSn(S,Se)4

S25Q

S50Q

been proposed8 to overcome the difficulty in distinguishing Cu and Zn in conventional powder X-ray diffraction experiments and the large correlation between structural parameters in the standard Rietveld refinements. This method has been applied to the pure selenide CZTSe compound.9 Several published results have demonstrated the linear variations of the tetragonal unit cell parameters of CZTSSe versus the anionic composition (x = S/(S + Se)), but there is no definite evidence of the occurrence of a solid solution through a crystallographic study in the Cu2ZnSnS4-Cu2ZnSnSe4 system. In this paper we present results from chemical crystallography investigations of CZTSSe compounds to address the questions about the atomic distribution, i.e. both the cationic and anionic disorder, in the corresponding kesterite crystal structure. First, the solid solution CZTS-CZTSe is investigated through single-crystal analyses. In a second part, knowledge about the Cu/Zn disorder is extended to the mixed S/Se compounds by combining resonant single-crystal X-ray diffraction technique and 77Se NMR spectroscopy. Finally, the anionic disorder S/Se is analyzed by HREM and image simulations.

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radiation as well as resonant single-crystal X-ray diffraction. Synchrotron measurements were performed at the CRISTAL beamline at the SOLEIL facility. The corresponding experimental details are given in the Supporting Information. From the high-resolution powder X-ray diffraction patterns, Rietveld refinements were performed using the Jana2006 program.12 The synchrotron single-crystal X-ray diffraction experiments were done on two crystals: a quenched crystal with a composition of x = 0.50 (S50Q) and a very slow cooled pure sulfide crystal (S100VSC) both at high energy and close to the Cu absorption edge. NMR. The 119Sn and 77Se NMR spectra were acquired with a Bruker Avance III 300 MHz spectrometer with a 4 mm CP-MAS probe. In both cases, we used the CPMG (Carr−Purcell−Meiboom− Gill) approach combined with MAS for sensitivity enhancement,13 as it was shown to be suitable for 119Sn spectra of sulfoselenide samples.14 The acquisition time τa for each echo was set to 0.35 or 0.51 ms for 119 Sn and to 0.50 or 0.65 ms for 77Se. For rotor-synchronization purposes, the MAS frequency was adjusted to 12714 and 12886 Hz (for 119Sn and 77Se, respectively) according to τa, to the refocusing π pulse length and to additional short delays (40 or 41 μs) for reducing the effects of probe ringing. In all cases, up to 320 echoes were acquired and we used a radio frequency field of 80 kHz. To ensure quantitative spectra, recycle times between scans were systematically adjusted. Due to a small amount of CuS, we used delays ranging from 30 to 75 s for 119Sn and from 15 to 60 s for 77Se. 119Sn spectra were referenced to Me4Sn using Ph4Sn as a secondary reference (−121.15 ppm). 77Se spectra were referenced to Me2Se using ZnSe as a secondary reference (−362 ppm).15 In order to obtain an absorption mode only, spectra were constructed by adding the 10 first full echoes of the CPMG acquisitions as previously described14 (only 5 for the 77 Se spectrum of A25). TEM. Samples were prepared by dispersing the powder in ethanol and depositing the solution obtained on a holey-carbon-coated nickel grid. A high-resolution electron microscopy (HREM) study was carried out with a JEOL ARM200 double-corrected electron microscope operating at 80 kV. Calculated images were obtained using the JEMS program.16

EXPERIMENTAL SECTION

Syntheses and Chemical Analyses. The compounds, with target compositions Cu2ZnSn(SxSe1−x)4 and Cu1.70Zn1.15Sn(SxSe1−x)4 with x = 0, 0.25, 0.5, 0.75, 1, were prepared by weighing and grinding Cu, Zn, and Sn powders (kept in an argon-filled glovebox) and S and Se in the appropriate ratios. The ground mixture was pressed into pellets, placed in an evacuated fused silica tube, and heated rapidly to 750 °C over 4 days. The samples were carefully reground and reheated at the same temperature for 4 days to enhance the homogeneity and the crystallization of the powders. The batches were then separated into two parts, which underwent different cooling treatments after annealing at 350 °C (above the order−disorder critical temperature) over 48 h. Either the samples were slowly cooled (5 °C/h) to 150 °C, held at this temperature for 48 h, and then slowly cooled to room temperature or they were quenched in air. For clarity, the samples have been labeled as indicated in Table 1. Additionally, a part of the pure sulfide sample (S100SC) underwent a second annealing with a very slow cooling ramp (1 °C/h) from 350 °C down to 150 °C and were kept for 500 h at that temperature (S100VSC sample). This thermal treatment is expected to lead to a sample with very low disorder, as shown in previous studies.1,10,11 For single-crystal diffraction experiments, crystals were picked from the corresponding powders. Energy dispersive X-ray spectroscopy (EDX) analyses were systematically carried out to ensure the purity and to determine the composition of the samples. Diffraction Analyses. Sample purity and homogeneity have been checked by X-ray powder diffraction with the use of a Bruker D8 Advance diffractometer (Cu Kα1, λ = 1.540598 Å). Additionally, a combination of several diffraction techniques was used, such as laboratory single-crystal X-ray diffraction and highresolution powder X-ray diffraction with the help of synchrotron

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RESULTS AND DISCUSSION Stannite or Kesterite Structure. In the literature, two crystal structures have been envisioned for CZTS: namely, stannite (ST) and kesterite (KS) with I4̅2m and I4̅ space groups, respectively.17 In both crystal structures the cationic planes are stacked along the c axis of the tetragonal unit cell alternating with the chalcogen planes; the sequence is (Zn− Sn)/(Cu−Cu)/(Zn−Sn) and (Cu−Sn)/(Cu−Zn)/(Cu−Sn) in ST and KS, respectively. The difference between these two structures comes therefore from the occupancy of the 2a(000) Wyckoff position: Zn in ST and Cu in KS. Although the calculated energy difference between these two structures is quite low,18,19 accurate structural investigations have definitely shown that the stannite structure is never observed for pure sulfide and pure selenide CZTS compounds (Cu2ZnSnS4 and Cu2ZnSnSe4).20−22 Additionally, the Cu/Zn disorder was 11780

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Table 2. Main Crystallographic Data and Refinement Results of the ST and D-KS Models for Stoichiometric Single Crystalsa x(target) 0b sample label x(EDX) x(PDRX)c a (Å) c (Å) no. of hkl obs (I > 2σ(I))/all no. of refined params GOF(obs) R/Rw(obs) R/Rw(all) Ueq (Å2) Zn_2a Cu_4d Sn_2b Fourier difference map (e/Å3) GOF(obs) R/Rw(obs) R/Rw(all) Ueq (Å2) Cu_2a Cu/Zn_4d Sn_2b site occupancy factors S (x) Fourier difference map (e/Å3)

S00SC

0.25

0.50

S75SC 0.81 0.740 5.4797 10.9366 344/397 15

5.4334(2) 10.8382(6) 389/392 14

1.16 2.83/7.32 4.09/7.54

1.66 3.38/8.14 3.42/8.16

0.0282(7) 0.0196(6) 0.0196(2) 0.0191(4) 0.0116(3) 0.01177(14) 0.0124(3) 0.0071(3) 0.00661(10) 1.07/−1.26 0.81/−1.13 0.59/−0.48 Disordered-Kesterite Model (Space Group I4̅2m) 1.88 1.11 0.95 3.33/10.50 2.55/6.82 2.45/5.79 3.97/10.77 3.90/7.41 3.30/6.03

0.0164(3) 0.0099(2) 0.0070(2) 1.28/−1.00

0.02217(3) 0.01678(3) 0.0121(2) 1.31−2.37

0.89 2.40/5.60 3.59/5.85

1.47 3.07/7.24 3.11/7.26

0.0230(7) 0.0196(5) 0.0123(3)

0.0126(2) 0.01095(13) 0.00648(10) 0.773(6) 0.55/−0.54

0.0192(2) 0.0178(1) 0.0119(1)

5.69647(5) 11.3394(2) 391/440 14 1.97 3.44/10.97 4.06/11.24

0.78/−1.10

S25SC S50SC 0.27 0.53 0.253 0.496 5.6244 5.5560 11.2230 11.0792 355/429 359/413 15 15 Stannite Model (Space Group I4̅2m) 1.31 1.08 2.83/8.05 2.67/6.57 4.23/8.58 3.57/6.79

1b

0.75

0.0163(5) 0.0129(2) 0.0076(2) 0.245(5) 0.70/−0.76

0.0165(2) 0.01330(13) 0.00671(9) 0.497(5) 0.64/−0.51

S100SC

1.34/−2.14

a

For the mixed S/Se compounds, data were collected at 100 K. bFrom already published data28,29 collected at room temperature. cDetermined from the single-crystal unit cell parameters and the calibrated curves obtained from powder X-ray diffraction.14

shown to lead to a higher crystal symmetry described in the I4̅2m space group where Cu and Zn at the z = 1/4 and z = 3/4 planes are randomly distributed on the 4d Wyckoff position (0,1/2,1/4); the corresponding structure is named disordered kesterite (D-KS). Because they correspond to the same space group, the ST structure was often proposed instead of the D-KS structure for such CZTS-related compounds. The question of the actual crystal structure for mixed S/Se compounds has not been yet addressed. On the other hand, the literature gives a great deal of powder X-ray diffraction (PXRD) investigations demonstrating the linear variation of the tetragonal unit cell parameters when S is substituted for Se in CZTSSe.14,23−26 Thus, the existence of a solid solution in the CZTSe-CZTS system is highly expected. However, no clear information about the actual crystal structure of the studied samples is given in these papers. In this section, the stannite and kesterite structure models are compared for both stoichiometric and copper-poor zinc-rich compounds through a laboratory single-crystal X-ray diffraction investigation. As presented in a following section, under these conditions it is not possible to distinguish the ordered-kesterite and disordered-kesterite structures; therefore, the ST structure is compared to the D-KS structure, both in the I4̅2m space group. Because the I4̅2m space group is a noncentrosymmetric and nonenantiomorphic space group, the absolute chirality27 determination is not an issue. During the data collection and data processing, attention was paid to always keeping the same absolute directions of the crystal axes leading to the anion position close to (0.76,0.24,0.13) as reported by Hall.17

The diffraction patterns of the stoichiometric single crystals were correctly indexed in tetragonal unit cells. As expected, the unit cell parameters decrease when x = S/(S + Se) increases (see Table 2). Although the scattering factors of Cu and Zn in conventional X-ray diffraction experiments (here the wavelength was Mo Kα, 0.71073 Å) are very close, it is possible to distinguish Cu and Zn on the 2a site. Since the residual factors and the goodness of fit parameters are significantly lower for DKS than for ST (Table 2), the disordered-kesterite structure appears to be the best solution for all of the studied crystals. In addition, the atomic displacement parameters of atoms located on 2a and 4d sites are much more homogeneous for the D-KS structure. Thus, we can conclude that, regardless of the anionic composition, CZTSSe compounds definitely adopt the kesterite structure. Additionally, in the whole composition range, the anionic distribution on the 8i site (x,x,z) of the I4̅2m space group is fully random at the XRD scale. This is in perfect agreement with the 119 Sn NMR investigation already published.14 Therefore, we can definitely conclude that the CZTSSe series behaves as an isostructural solid solution from x = 0 to x = 1. Atomic positions and atomic displacement parameters of the S/Se atoms as well as cation−anion distances and bond valence sums are gathered in Table S1 in the Supporting Information. It is worth noting that, despite the shrinkage of the unit cell with increase in sulfur content, the cation bond valence sums remain almost the same. Let us now present the results for the Cu-poor compounds. The EDX compositions of samples with targeted compositions Cu1.70Zn1.15Sn(SxSe1−x)4 (x = 0.25, 0.50, 0.75; samples A25SC, A50SC, and A75SC, respectively) indicate that these samples 11781

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agreement with targeted and EDX values. In addition, the distribution of intensities with n agrees with a binomial distribution, which corroborates the expected random distribution of chalcogen atoms. According to EDX measurements, the proportion of copper vacancies versus the tin content is 10%. In the case of isolated vacancies, each of them is expected to affect four tin atoms. However, the fraction of Sn atoms experiencing copper vacancies as second neighbors is only 18%. Such a value, half the value expected for isolated vacancies, demonstrates the propensity for A-type defects to aggregate as in pure sulfide11 or selenide29 compounds. The 119Sn NMR spectra of samples with x = S/(S + Se) targeted to 0.25 and 0.5 are given in Figure S1 in the Supporting Information. Investigation of the Cu/Zn Disorder. These last few years, NMR was shown to be a powerful technique for structural investigations of CZTS-related compounds, especially for the study of Cu/Zn disorder. In contrast to the 119Sn, 65Cu, and 67Zn NMR spectra of CZTSSe compounds, the related 77 Se NMR spectra, which represent the anionic point of view,

are slightly less Cu poor than targeted. Single crystals suitable for X-ray analysis have been successfully picked from the prepared powders, and diffraction data have been collected at room temperature. The same crystal structure investigation as for the stoichiometric compounds has been performed. It demonstrates that, regardless of the actual anionic composition, all of the studied off-stoichiometric compounds adopt the kesterite crystal structure (and not the stannite structure). It is noteworthy that, for the three studied compounds, the copper vacancies are located exclusively on the 2a site, the 4d(Cu/Zn) and 2b(Sn) sites remaining with full occupancies. This is in perfect agreement with the already published results on the selenium-free CZTS.28 Hence, as shown for the stoichiometric compounds, the Cu1.70Zn1.15Sn(SxSe1−x)4 compounds belong to a random solid solution, at least on the long-range scale investigated by the X-ray diffraction technique. To complete this XRD crystal structure study, 119Sn NMR investigation has been done on these off-stoichiometric samples. Figure 1 shows the 119Sn MAS spectrum of the

Figure 2. 77Se MAS spectra (reconstructed from CPMG) for stoichiometric Cu2ZnSn(SxSe1−x)4 samples for x = 0.25, 0.50, 0.75. Asterisks denote spinning side bands. Figure 1. 119Sn MAS spectrum (reconstructed from CPMG) of a Cupoor CZTSSe sample with targeted x = 0.75 exhibiting four Tn environments (n = 1−4). The proportion of Sn atoms experiencing vacancies as second neighbors is 18%.

have never been reported to date. Figures 2 and 3 show the 77 Se MAS spectra of the stoichiometric (S) and Cu-poor (A) series (slow cooled) as a function of x. In all cases, the spectra consist of a single line whose isotropic chemical shift (δiso) depends on the cationic and anionic compositions. At given x,

A25SC sample. The four main lines at −141, −230, −314, and −395 ppm correspond to tin atoms belonging to different [SnSnSe4−n] tetrahedra (hereafter labeled Tn for n = 1−4). It was shown that the asymmetric broadening of the 119Sn NMR lines is a signature of the occurrence of Cu/Zn disorder.10 In contrast, the line shape of this off-stoichiometric compound is quite symmetrical, demonstrating the low level of disorder in accordance with the presence of Cu vacancies ([VCu + ZnCu] Atype defect complex).11 Owing to the good spectral resolution, three low signals at −103, −190, and −272 ppm can be observed. These kinds of signals, left-shifted by ca. +40 ppm, were already observed in pure sulfur Cu-poor CZTS phases and were assigned to Sn atoms experiencing a Cu vacancy as a second neighbor.11 Hence, the −103, −190, and −272 ppm lines are attributed to such Sn atoms belonging to [SnSnSe4−n] tetrahedra with n = 4, 3, 2, respectively. According to the approach previously described,14 the x = S/(S + Se) values can be accurately determined from the relative intensities of the Tn lines. The intensities reported in Figure 1 give x = 0.74 in

Figure 3. 77Se MAS spectra (reconstructed from CPMG) of Cu-poor targeted compositions Cu1.70Zn1.15Sn(SxSe1−x)4 samples for x = 0.25, 0.50, 0.75. Asterisks denote spinning side bands. 11782

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Inorganic Chemistry the δiso value is slightly lower for the Cu-poor series than for the stoichiometric series and, for both series, δiso decreases when x increases. A final distinction between the two series lies in the shape of the line, showing a left-hand asymmetry for stoichiometric compounds (partially hidden by the overlap with the first spinning sideband). The 119Sn NMR spectra have proven that the S series exhibits a significant level of Cu/Zn disorder (even after slow cooling aiming at decreasing such a disorder).14 In contrast, the A series shows only a low level of disorder (see above and Figure S1 in the Supporting Information). Therefore, as for 119Sn MAS NMR, we propose to associate this asymmetric broadening with the occurrence of Cu/Zn disorder. This interpretation is further supported by the higher broadening observed for a quenched stoichiometric sample (x = 0.5) in comparison to its slow-cooled analogue (Figure S2 in the Supporting Information). Moreover, this interpretation also gives meaning to the opposite asymmetry observed for 77Se versus 119Sn lines (left hand versus right hand). Indeed, the 119Sn line width corresponds to a distribution of δiso which reflects itself a distribution of electronic environment of tin nuclei. As Sn and Se are directly bound, their electronic environments vary in the opposite way and lead to reverse distributions. Thus, we demonstrated that both 77Se and 119Sn NMR give the same information regarding the Cu/Zn disorder. Although not extremely useful for CZTSSe compounds because of the higher sensitivity of 119 Sn over 77Se, this fact can dramatically change for other compounds for which Sn would be replaced by atoms (e.g., Ge) having NMR active nuclei that are less sensitive, are less abundant, and/or have a spin higher than 1/2 (which could subsequently cause severe additional broadening due to potential electrical field gradient distribution). In such a case, 77 Se NMR would be a much more practical way to assess disorder. Finally, to conclude this very first description of 77Se NMR of CZTSSe compounds, we wish to point out that, up to now, we were unfortunately unable to detect a signal stemming from selenium with a copper vacancy as a first neighbor. It is likely that this line could be too large and too weak to be easily observable. Knowing that CZTSSe compounds do not adopt the stannite structure, we now study the Cu/Zn distribution on the z = 1/4 and z = 3/4 atomic planes in the kesterite structure through resonant single-crystal diffraction techniques. As presented in the previous sections, although Cu and Zn have very close scattering factors, it is possible to distinguish the stannite (Zn on 2a site) and kesterite (Cu on 2a site) structures with the help of conventional single-crystal X-ray diffraction experiments. However, the occupation of Cu and Zn on the 2c and 2d sites in the I4̅ space group cannot be determined from these data. Indeed, there is strictly no difference between solutions for which Cu and Zn are permuted on 2c and 2d (see Table S2 in the Supporting Information). Let us interpret this behavior through the calculation of the contribution of the atoms located on the 2c and 2d sites in the I4̅ space group to the structure factors. The value of such structure factors, F(hkl), depends on the parity of the l index:

kesterite structure derives from the cubic ZnS (sphalerite) structure (with doubling of the c unit cell parameter), the l = 2n + 1 reflections remain of very low intensities for the whole structure (contribution of all atoms). Consequently, the permutation of Cu and Zn on the 2c and 2d sites has strictly no effect on the reflection intensities when l = 2n while low reflections with l = 2n + 1 are barely modified. In a previous paper,30 we have shown that single-crystal X-ray resonant diffraction is sensitive enough to investigate the Cu/ Zn disorder in the pure sulfide CZTS material. The same study has been done on a crystal picked from the S50Q sample (anionic composition of x = 0.50, quenched). In the Cu Kα resonant condition setup (λ = 1.38144 Å), the reachable (sin θ)/λ range is quite narrow ( 2). Therefore, a second data set was collected at high energy (18 keV), far from the Cu K edge. Several Cu/Zn distributions have been envisioned in the space group I4̅. In addition to the ordered and disordered kesterite structural models, we considered a third model in which Cu and Zn are inverted (see Table 3). The residual factors, R/Rw(all), converge to Table 3. Single-Crystal Structure Refinement Results for Different Cation Distributions in I4̅ Space Group against Data Collected both at High Energy (18 keV) and under Resonant Conditions at Cu K Edge for S50Q Samplea crystal structure

sof 2c GOF (all) R/Rw(all) Ueq (Å2) 2c 2d Fourier difference (e/ Å3) a

ordered kesterite

inverted kesterite

disordered kesterite

100% Cu 0% Zn 2.31 3.33/8.23

0% Cu 100% Zn 2.33 3.32/8.31

50% Cu 50% Zn 1.89 2.77/6.71

0.0269(4) 0.0204(3) 1.05/-1.07

0.0208(3) 0.0262(4) 1.50/-0.73

0.0236(3) 0.0227(3) 0.78/-0.63

The numbers of reflections are 689 (18 keV) and 214 (9 keV).

3.33/8.23, 3.32/8.31, and 2.77/6.71 for ordered-kesterite, inverted-kesterite, and disordered-kesterite structures, respectively (for 903 reflections and 22 refined parameters). The refinement of the sof of Cu and Zn atoms located on 2c and 2d sites lead to values not significantly different than 0.5 corresponding to the full Cu/Zn disorder. It is worth noting that, using only the data set at the Cu edge, the three tested models give roughly the same R/Rw factors because during the refinement the ADPs of Cu and Zn atoms located on 2c and 2d sites are adjusted to compensate a wrong atomic distribution. In that case, the obtained ADPs are totally not realistic for the two ordered models (for example, Ueq(2c) = 0.004(1) Å2 and Ueq(2d) = 0.041(1) Å2 for ordered kesterite). Therefore, we can clearly conclude that the best solution corresponds to the random distribution of Cu and Zn on the z = 1/4 and z = 3/4 atomic planes, which is in full agreement with the thermal process used for this sample (quenched after annealing above the order−disorder transition temperature). In that case, the crystal structure has to be described in the I4̅2m space group. The final structure refinement, using only the data collected at high energy, leads to a quite good solution with R/Rw = 1.83/

F(hkl)2c + 2d = ±2 × (f (Cu) + f (Zn)) if l = 2n

F(hkl)2c + 2d = ±2i × (f (Cu) − f (Zn)) if l = 2n + 1

Because f(Cu) and f(Zn) are very close in conventional X-ray diffraction experiments, these contributions are large when l = 2n and very low when l = 2n + 1. It is noteworthy that, as the 11783

DOI: 10.1021/acs.inorgchem.7b01791 Inorg. Chem. 2017, 56, 11779−11786

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Inorganic Chemistry 4.33 for 400 reflections and 15 refined parameters (see Table S3 in the Supporting Information. The possible existence of a long-range Cu/Zn order has been investigated through resonant diffraction on a crystal picked from the S100VSC sample (very slow cooled sample). The refinement procedure used for the S50Q sample (see above) was also applied in the case of the S100VSC crystal: i.e., the comparison of the ordered and disordered kesterite structural models (in I4̅ space group) using the data collected both at high energy and close to the Cu K edge. The residual factors, R/Rw(all), converge to 2.71/6.05, 2.73/6.15, and 2.43/5.25 (for 1043 reflections and 21 refined parameters) for orderedkesterite, inverted-kesterite, and disordered-kesterite structures, respectively. Here too, the refinement of the sof of Cu and Zn atoms located on 2c and 2d sites confirms that the best solution corresponds to the full Cu/Zn disorder. Finally, the structure refinement has been done with the disordered-kesterite structure in the I42̅ m space group with the data set collected at high energy, leading to a quite good solution: R/Rw(all) = 1.78/4.10 for 460 reflections and 14 refined parameters (see Table S3 in the Supporting Information). From 119Sn NMR investigation, the very slow cooling procedure was shown to clearly decrease the level of Cu/Zn disorder.10 Since this decrease is not observed by single-crystal X-ray diffraction, one can conclude that it is not a long-range phenomenon. This result contrasts with already published results in which high order level (at a long-range scale) has been observed on powdered samples by the use of neutron diffraction.7,31 The reason for this discrepancy may come from the difference in the thermal treatments. Local Anionic Disorder. Once cationic and anionic disorders were characterized by single-crystal X-ray diffraction and NMR, we attempted to study their influence on the microstructure of Cu2ZnSn(SxSe1−x)4 compounds. It is likely that the partial substitution of sulfur for selenium could induce local structural distortions, leading to an increase in strains. For that purpose, XRPD line broadening analysis and highresolution electron microscopy (HREM) were performed to evaluate strains on the CZTSSe series. In order to minimize reflection broadening due to instrumental effects, synchrotron data were used. The Rietveld XRPD plot for S25Q is shown in Figure S3 in the Supporting Information. All observed reflections can be indexed in the space group I4̅2m (i.e., D-KS), attesting to the absence of a long-range anionic order in this mixed S/Se compound. However, reflections are significantly broader than those of S00SC (Figure 4). The [100] filtered HRTEM images of two crystals from S00SC and S25Q samples are presented in Figure 5. The experimental images (Figure S4 in the Supporting Information) were filtered to improve the contrast. The microstructures of these compounds appear quite different. Therefore, while the S25Q sample exhibits a mosaic-type contrast (built from domains of different contrast), the S00SC sample shows a homogeneous contrast over nearly the whole studied area. The mosaic-type contrast could be interpreted as the sign of a shortrange anionic order. As mentioned previously, a 119Sn NMR study of CZTSSe compounds showed that five types of [SnSnSe4−n] tetrahedra coexist in the samples. Accordingly, in order to understand the experimental HRTEM contrast of the S25Q crystal, image simulations were performed using crystal structures obtained from the single-crystal investigation presented above (x = 0,

Figure 4. Comparison of the (112) diffraction peaks for S00SC (x = 0) and S25Q (x = 0.25) samples from high-resolution synchrotron data (2θ−2θBragg). The line broadening for the mixed S/Se compound is correlated to a decrease in the crystallite size and/or an increase in the microstrains.

0.25, 0.50, 0.75, 1). Obviously, for pure sulfide and selenide compounds a single type of tin environment was considered: T4 and T0, respectively. For mixed S/Se samples although several Tn types could be considered, only one type was retained for each composition: T1, T2, and T3 for x = 0.25, 0.50, 0.75, respectively. However, for each Tn environment two models of local anionic order were considered (Figures S6-S9 in the Supporting Information). Calculated images corroborate that the HRTEM contrast is highly dependent on the nature of the anion as well as on the local anionic order. The best agreement between calculated and experimental images for S25Q corresponds to the anionic environment T1 (Figure 6). However, the contrast of the whole image cannot be explained either with a single type of local anionic order or with only one type of Tn, in contrast to what is observed in the case for S00SC (Figure 6). It should be noted that the use of the disordered-kesterite structure as a model for image calculations leads to the same results. This means that HRTEM contrast is not sensitive to Cu/Zn disorder. In conclusion, the HRTEM study indicates that S/Se anions are not fully randomly distributed and so several local anionic orders take place in the compound S25Q. This order locally modifies lattice dimensions, giving rise to strains. However, due to the difficulty in a quantitative understanding of the HRTEM contrast, a scanning transmission electron microscopy (STEM) study, in high-angle annular dark-field (HAADF) as well as in high-resolution bright-field (BF) imaging modes, of the CZTSSe compounds is underway. This approach should exalt the contrast difference between anions in order to better visualize their spatial distribution and to map strains.

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CONCLUDING REMARKS Beyond the definitive demonstration from single-crystal diffraction that the mixed sulfoselenide compounds form an isostructural solid solution with the kesterite type structure, this study has opened up the investigations of Cu/Zn and S/Se distributions to considerations about the length scale that occurs. In accordance with published results, the level of Cu/Zn disorder was shown to decrease with the duration of the cooling ramp following the annealing step. Because it appears 11784

DOI: 10.1021/acs.inorgchem.7b01791 Inorg. Chem. 2017, 56, 11779−11786

Article

Inorganic Chemistry

Figure 5. [100] filtered HREM images of two crystals from S00SC and S25Q samples. Fourier transform images are shown as insets. Enlarged images are available in Figure S5 in the Supporting Information.

with a clear deviation from the full S/Se disorder at a few-unitcell scale.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01791. Additional experimental details, results of single-crystal X-ray diffraction analyses, 119Sn and 77Se NMR spectra for A25SC, A50SC, A75SC, S50SC, and S50Q, Rietveld plot for the high-resolution powder X-ray diffraction pattern of S25Q, and experimental, filtered, and calculated HRTEM images (PDF) Accession Codes

Figure 6. Enlarged areas of [100] filtered images of two crystals of the S25Q and S00SC samples. Corresponding calculated images (defocus 25 nm and thickness 4 nm) are inserted.

CCDC 1560389−1560391 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

to be more ordered by NMR and totally disordered by resonant X-ray diffraction, the very slow cooled sample has pointed out the need to associate a length scale with the concept of order in CZTSSe materials. Ordering at the local scale remains undetected by X-ray diffraction if it occurs within domains whose sizes are lower than the sizes needed for coherent diffraction. Moreover, these domain sizes seem to depend on the thermal history of the sample, since a recent neutron powder diffraction investigation of pure sulfide CZTS has demonstrated that full Cu/Zn order can be reached with a 2 week annealing step below the critical temperature7 (in contrast to our condition starting above the critical temperature). Similarly, discussing the ordering of the anionic distribution depends on the length scale involved. Both X-ray diffraction (long-range) and 119Sn NMR (a few angstroms and summed on the overall sample) argue for a fully random distribution of S and Se, since no superstructures are observed in the diffraction patterns and since the [SnSnSe4−n] tetrahedra population is fully described by the binomial law. However, the HRTEM images show a mosaic-type contrast which was convincingly associated

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AUTHOR INFORMATION

Corresponding Authors

*M.P.: tel, +33 240 37 39 01; fax, +33 240 37 39 99; e-mail, [email protected]. *A.L.: tel, +33 240 37 39 44; fax, +33 240 37 39 99; e-mail, [email protected]. ORCID

Michaël Paris: 0000-0002-8671-0630 Alain Lafond: 0000-0001-5981-5480 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Region Pays de la Loire and the Synchrotron Soleil for financing the Ph.D. thesis of P.B. 11785

DOI: 10.1021/acs.inorgchem.7b01791 Inorg. Chem. 2017, 56, 11779−11786

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DOI: 10.1021/acs.inorgchem.7b01791 Inorg. Chem. 2017, 56, 11779−11786