Mechano-Chemical Synthesis, Structural Features and Optical Gap of Hybrid CH3NH3CdBr3 Perovskite

Hybrid methyl-ammonium (MA:CH3NH3+) lead halide MAPbX3 (X = halogen) perovskites exhibit an attractive optoelectronic performance that can be applied to the next generation of solar cells. To extend the field of interest of these hybrid materials, we describe the synthesis by a solvent-free ball-milling procedure, yielding a well crystallized, pure and moisture stable specimen of the Cd tribromide counterpart, MACdBr3, which contains chains of face-sharing CdBr6 octahedra in a framework defined in the Cmc21 (No 36) space group. The details of the structural arrangement at 295 K have been investigated by high angular resolution synchrotron x-ray diffraction (SXRD), including the orientation of the organic MA units, which are roughly aligned along the c direction, given the acentric nature of the space group. UV-vis spectra unveil a gap of 4.6 eV, which could be useful for ultraviolet detectors.


Introduction
Hybrid organic-inorganic halide perovskites are suitable as light absorbers in solar cells [1][2][3][4], with an efficiency of power conversion (PCE) of about 23%, similar to siliconbased devices. In particular, methyl-ammonium lead iodide CH 3 NH 3 PbI 3 (CH 3 NH 3 + : MA) is the prototype of light harvester in hetero-junction solar cells [5][6][7]. Hybrid perovskites MAPbX 3 (X = I, Br and Cl) also exhibit properties such as ambipolar charge mobility, low exciton binding energy and tolerance to structural defects [8][9][10][11][12][13][14]. Other divalent elements instead of Pb 2+ have been evaluated; experimental [15] and theoretical [16,17] studies have been carried out for other divalent cations, such as those of Group 12 (Zn, Cd and Hg) that also are closed-shell divalent cations. However, structural and electronic studies on these materials are scarce. In particular, the Cd-containing phase MACdBr 3 is little studied with only structural and theoretical analysis [17][18][19]. The addition of Cd 2+ is paradigmatic since it is responsible for interesting phenomena; the electroluminescence is not inhibited, while a composite with polystyrene is effective in preventing the degradation of cadmium halide due to humidity [20].
The crystal structure of MACdBr 3 differs from those of MAPbX 3 (X = I, Br and I), most of which can be described in the cubic arystotype perovskite structure, defined in the space group Pm3m. Given the smaller size of Cd 2+ (0.95 Å) vs. Pb 2+ (1.19 Å), the tolerance factor of the hypothetical perovskite would be higher than unity, and therefore, instead of the classical framework of corner-sharing octahedra, this Cd compound presents chains of face-sharing CdBr 6 octahedra in a quasi 1D arrangement. It was described by Hassen et al. [19] from a single crystal prepared by solution chemistry. Additionally, Kallel et al. [18] described a phase transition at 170 K from the above described structure to a complex orthorhombic unit cell. Regarding the absorption properties, theoretical calculations in an unrealistic cubic structure yields a gap of 1.3 eV [17]. The experimental optical gap has not been reported in the literature.
In this paper, we describe the preparation of MACdBr 3 by an alternative mechanosynthesis procedure, with green credentials since the utilization of organic solvents is not required. A sample with an excellent crystallinity was obtained by ball-milling under N 2 atmosphere, characterized by laboratory x-ray diffraction (XRD), and the crystal structure was refined from synchrotron x-ray diffraction (SXRD) data. We propose the H positions, unveiling the H-bond interactions between the CH 3 NH 3 + units and CdBr 6 octahedral chains, where the particular distribution of ADP parameters is a result of N-H···Br interactions. The diffuse reflectance spectra of the sample were measured for the first time, observing a band gap of~4.6 eV, shifted to the UV region with respect to the lead-containing MAPbBr 3 counterpart.

Materials and Methods
MACdBr 3 was prepared in polycrystalline form by ball milling (mechano-chemical synthesis) starting from stoichiometric proportions of CdBr 2 and MABr. The mixture of bromides (totalling 1.5 g), was set into a zirconia-lined jar together with 30 zirconia balls (5 mm diameter) and sealed in a N 2 -filled glove box. The mechanically activated reaction was carried out in a Retsch (Haan, Germany) PM100 mill for 4 h at 400 rpm. Laboratory XRD was used for assessing phase purity; the XRD patterns were recorded in a Bruker (Germany) D5 diffractometer with K α Cu (λ = 1.5418 Å) radiation; the 2θ range was 4 • up to 90 • with increments of 0.05 • .
The crystal structure of MACdBr 3 was investigated at RT (295 K) by synchrotron X-ray powder diffraction (SXRD) using the MSPD station at the ALBA facility, Barcelona (Spain). Radiation with 38 keV energy, λ = 0.32511 Å, was selected in the high angular resolution mode (MAD set-up) [21]. The sample was measured in a glass capillary of 0.7 mm diameter, which rotated during data acquisition. The structural refinement by the Rietveld method [22] was carried out with the Fullprof software [23]. The full refinement of the profiles included the zero-point error; scale factor; background coefficients; unit-cell parameters; pseudo-Voigt shape parameters; atomic coordinates; anisotropic displacements for the metal and halogen atoms and isotropic for C and N from the methyl-ammonium groups.
Field-effect scanning electron microscopy (FE-SEM) pictures were collected on an FEI-Nova microscope, with an acceleration potential of 5 kV. The optical diffuse-reflectance spectrum was recorded at room temperature in a UV-vis spectrophotometer Varian Cary 5000. A photodetector device was fabricated by drop-casting the phase solution in dimetilformamide onto Au/Cr pre-patterned electrodes with a gap of 10 µm, and drying in a hot plate at 100 • C. The illumination power was 20 µW.

Initial Characterization: FE-SEM
The hybrid CH 3 NH 3 CdBr 3 compound was obtained as a white polycrystalline material. High-resolution FE-SEM images were obtained to get an insight into the microstructure of this product obtained by ball milling (Figure 1). Figure 1a illustrates an overall view with low magnification (800×), showing irregular-shaped clusters of particles of different sizes. Figure 1b,c unveil the presence of microcrystals of uneven form, with flat facets in the micrometer-size range (e.g., the crystal in Figure 1c has a width of 13 µm), which are grown during the ball milling process. Figure 1d illustrates the homogeneity of the crystals, with no particular microstructural features, in a large magnification view (24,000×). EDX analysis coupled to the FE-SEM images yields an atomic composition close to 1:3 for the Cd/Br ratio. A typical EDX spectrum is included in Figure S1 in the Supplementary Information; other SEM images are included in Figure S2. facets in the micrometer-size range (e.g., the crystal in Figure 1c has a width of 13 µ m), which are grown during the ball milling process. Figure 1d illustrates the homogeneity of the crystals, with no particular microstructural features, in a large magnification view (24000×). EDX analysis coupled to the FE-SEM images yields an atomic composition close to 1:3 for the Cd/Br ratio. A typical EDX spectrum is included in Figure S1 in the Supplementary Information; other SEM images are included in Figure S2.

Structural Characterization
High angular resolution SXRD data allowed us a precise structural characterization. This is essential to accurately define the symmetry and crystallographic features. The pattern can be indexed in an orthorhombic unit cell with a = 7.91722(5) Å , b = 13.7108(1) Å , c = 6.89374(2) Å , and the crystal structure can be defined in the acentric Cmc21 (No 36) space group, confirming the work by Ben Hassen et al. from single-crystal x-ray diffraction [19].
Cd 2+ cations are allocated at 4a (0,y,z) Wyckoff sites, while Br1 and Br2 atoms are placed at 8b (x,y,z) and 4a sites, respectively. C and N atoms are also located at 4a positions. Figure 2 illustrates the quality of the fit from SXRD data, and Figure 3 displays two views of the crystal structure, including displacement ellipsoids for Cd and Br atoms. Figure 3b highlights the face-sharing of CdBr6 octahedra along the c axis. Table 1 contains the main crystallographic parameters after the Rietveld refinement.

Structural Characterization
High angular resolution SXRD data allowed us a precise structural characterization. This is essential to accurately define the symmetry and crystallographic features. The pattern can be indexed in an orthorhombic unit cell with a = 7.91722(5) Å, b = 13.7108(1) Å, c = 6.89374(2) Å, and the crystal structure can be defined in the acentric Cmc2 1 (No 36) space group, confirming the work by Ben Hassen et al. from single-crystal x-ray diffraction [19].
Cd 2+ cations are allocated at 4a (0,y,z) Wyckoff sites, while Br1 and Br2 atoms are placed at 8b (x,y,z) and 4a sites, respectively. C and N atoms are also located at 4a positions. Figure 2 illustrates the quality of the fit from SXRD data, and Figure 3 displays two views of the crystal structure, including displacement ellipsoids for Cd and Br atoms. Figure 3b highlights the face-sharing of CdBr 6 octahedra along the c axis. Table 1 contains the main crystallographic parameters after the Rietveld refinement. In comparison with previous results [19], a subtle increase in the unit-cell parameters is observed in the present sample. This volume expansion can be associated with the presence of structural defects (vacancies), as it was observed previously in both hybrid and all-inorganic halide systems [24,25]. Additional refinements considering atomic vacancies did not lead to an improvement of the discrepancy factors. Considering that the volume cell increase is small (≈0.4%) it is possible to infer that the vacancy level is also low. These structural differences also are observed in the Cd-Br distances, as illustrated in Table 2. The distance increment mainly affects the Cd-Br2 bonds, which allows supposing that this bromine site is more plausible to present vacancies.   In comparison with previous results [19], a subtle increase in the unit-cell parameters is observed in the present sample. This volume expansion can be associated with the presence of structural defects (vacancies), as it was observed previously in both hybrid and all-inorganic halide systems [24,25]. Additional refinements considering atomic vacancies did not lead to an improvement of the discrepancy factors. Considering that the volume cell increase is small (≈0.4%) it is possible to infer that the vacancy level is also low. These structural differences also are observed in the Cd-Br distances, as illustrated in Table 2. The distance increment mainly affects the Cd-Br2 bonds, which allows supposing that this bromine site is more plausible to present vacancies.  In comparison with previous results [19], a subtle increase in the unit-cell parameters is observed in the present sample. This volume expansion can be associated with the presence of structural defects (vacancies), as it was observed previously in both hybrid and all-inorganic halide systems [24,25]. Additional refinements considering atomic vacancies did not lead to an improvement of the discrepancy factors. Considering that the volume cell increase is small (≈0.4%) it is possible to infer that the vacancy level is also low. These structural differences also are observed in the Cd-Br distances, as illustrated in Table 2. The distance increment mainly affects the Cd-Br2 bonds, which allows supposing that this bromine site is more plausible to present vacancies.  With the collected data (both angular resolution and high Q range), it is possible to reasonably refine anisotropic displacement parameters (ADP). The displacement factors of Br atoms are quite anisotropic, as shown in Figure 3, and they display a flattened shape (oblate type) with the disks perpendicular to the Cd-Br chemical bonds. In this configuration, quite standard in perovskites, the thermal vibrations are allowed in the perpendicular plane to the covalent Cd-Br bonding. The anisotropy of Br1 is much superior, with r.m.s. (root mean square) displacements of 0.11 Å parallel to the chemical bond and 0.30 Å and 0.22 Å perpendicular to it (for Br2, r.m.s.'s are 0.23 Å, 0.25 Å and 0.26 Å, respectively). As also shown in Figure 3, CH 3 NH 3 + groups are in the space between the chains, roughly aligned along c axis. H atoms could not be localized from SXRD data. However, the MA position and the r.m.s. displacements suggest that the H-bond interaction of MA is stronger with Br2 atoms. To stand out this, the H atoms were added considering: (a) the expected distances and angles for the MA molecule and (b) the closeness with bromine atoms, with the aim to visualize the probable H-bond interactions. Figure 4 shows the environments of a MA unit, and the dashed lines highlight the H···Br distances less than 3 Å. This figure reveals that the disk-like ADP bromine is perpendicular to the H···Br directions, hence, it is possible to speculate that the particular distribution of ADP parameters is a result of N-H···Br interactions. Finally, a highly polar character is expected for this compound, since all the CH 3 NH 3 + units lie along the same direction, giving the acentric nature of the space group.

Optical Gap by UV-Vis Spectra and Photocurrent Properties
The absorption capacity of MACdBr3 powder prepared by mechano-chemistry investigated by diffuse reflectance UV/vis spectroscopy. Figure 5a shows the UV-v sorption spectrum, where there are two relative maximum absorption regions plac the ranges 300-330 and 700-740 nm. The slope disruption at 300-330 nm correspon an exciton absorption peak that reveals the UV light activity of the sample; this ex peak is due to the formation of an electron-hole pair that is favoured in nanostruc samples, containing nanoplates, as observed here (Figure 1) [26,27]. The onset at ~75 shows the absorption by minor MABr impurities [28]. Figure 5b illustrates the optical absorption coefficient related to the Kubelka-M function (F(R) = (1 − R) 2 /2R), being R the reflectance of each sample, vs. wavelength i The band gap has been calculated by extrapolating the linear region to the abscissa value obtained for MACdBr3 (~4.6 eV; ~270 nm) is shifted to the UV region in compa to the corresponding lead composition (MAPbBr3), which presents a band gap aroun eV [29]. This shift will hamper its use in photovoltaic devices but could make this ma useful for optical applications with short-wavelength UV radiation.
The optoelectronic properties of this phase were evaluated from photocurrent m urements. Figure 5c,d show the I vs. V curves at different wavelengths and an ima the used device, respectively. Figure 5c plots a typical ohmic behavior and display a ligible effect of illumination at different energies within the visible light range. This r confirms that this system is not an appropriate material for solar cells technology. H ever, as was mentioned above, it may find applications in the ultraviolet spectra ran

Optical Gap by UV-Vis Spectra and Photocurrent Properties
The absorption capacity of MACdBr 3 powder prepared by mechano-chemistry was investigated by diffuse reflectance UV/vis spectroscopy. Figure 5a shows the UV-vis absorption spectrum, where there are two relative maximum absorption regions placed in the ranges 300-330 and 700-740 nm. The slope disruption at 300-330 nm corresponds to an exciton absorption peak that reveals the UV light activity of the sample; this exciton peak is due to the formation of an electron-hole pair that is favoured in nanostructured samples, containing nanoplates, as observed here (Figure 1) [26,27]. The onset at~750 nm shows the absorption by minor MABr impurities [28].

Conclusions
In contrast with the well-known MAPbBr3 hybrid perovskite, structurally consisting of a 3D framework of corner-sharing PbBr6 octahedra, the Cd counterpart contains infinite chains of face-sharing CdBr6 octahedra, with the CH3NH3 + units lying in between. The acentric nature of the Cmc21 space group implied that the organic units are all aligned along the same direction, conferring a polar character to this compound. Starting from the obtained atomic positions and anisotropic displacement factors for Cd, Br, C and N, it was possible propose the H positions, unveiling the H-bond interactions between the CH3NH3 +  The band gap has been calculated by extrapolating the linear region to the abscissa. The value obtained for MACdBr 3 (~4.6 eV;~270 nm) is shifted to the UV region in comparison to the corresponding lead composition (MAPbBr 3 ), which presents a band gap around 2.2 eV [29]. This shift will hamper its use in photovoltaic devices but could make this material useful for optical applications with short-wavelength UV radiation.
The optoelectronic properties of this phase were evaluated from photocurrent measurements. Figure 5c,d show the I vs. V curves at different wavelengths and an image of the used device, respectively. Figure 5c plots a typical ohmic behavior and display a negligible effect of illumination at different energies within the visible light range. This result confirms that this system is not an appropriate material for solar cells technology. However, as was mentioned above, it may find applications in the ultraviolet spectra range.

Conclusions
In contrast with the well-known MAPbBr 3 hybrid perovskite, structurally consisting of a 3D framework of corner-sharing PbBr 6 octahedra, the Cd counterpart contains infinite chains of face-sharing CdBr 6 octahedra, with the CH 3 NH 3 + units lying in between. The acentric nature of the Cmc2 1 space group implied that the organic units are all aligned along the same direction, conferring a polar character to this compound. Starting from the obtained atomic positions and anisotropic displacement factors for Cd, Br, C and N, it was possible propose the H positions, unveiling the H-bond interactions between the CH 3 NH 3 + units and CdBr 6 octahedral chains. The different structural arrangement is a consequence of the much smaller size of Cd 2+ vs. Pb 2+ , implying a tolerance factor greater than unity for the hypothetical perovskite. The large band gap and photocurrent results disable this material for solar cell applications, but it may find interest as a UV detector.
Supplementary Materials: The following are available online at https://www.mdpi.com/article/ 10.3390/ma14206039/s1, Figure S1: EDX spectrum and quantitative results for MACdBr 3 obtained with 18 kv of acceleration potential. H, N and C could not be observed in the presence of heavy Cd and Br atoms. The theoretical Cd/Br ratio of 1:3 is close to that found of 26.32%/73.68%, Figure S2