Carmen
Abia
ab,
Carlos A.
López
ac,
Javier
Gainza
a,
João Elias F. S.
Rodrigues
ad,
Mateus M.
Ferrer
e,
N. M.
Nemes
af,
Oscar J.
Dura
g,
José L.
Martínez
a,
María T.
Fernández-Díaz
b,
Consuelo
Álvarez-Galván
h,
Gergely
Németh
i,
Katalin
Kamarás
i,
François
Fauth
j and
José A.
Alonso
*a
aInstituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, 28049 Madrid, Spain. E-mail: ja.alonso@icmm.csic.es; calopez@unsl.edu.ar
bInstitut Laue Langevin, BP 156X, F-38042 Grenoble, France
cInstituto de Investigación en Tecnología Química (UNSL-CONICET) and Facultad de Química, Bioquímica y Farmacia, Almirante Brown 1455, (5700) San Luis, Argentina
dEuropean Synchrotron Radiation Facility (ESRF), 71 Avenue des Martyrs, 38000 Grenoble, France
eCCAF, PPGCEM/CDTec, Federal University of Pelotas, 96010-610 Pelotas, Rio Grande do Sul, Brazil
fDepartamento de Física de Materiales, Universidad Complutense de Madrid, E-28040 Madrid, Spain
gDepartamento de Física Aplicada, Universidad de Castilla-La Mancha, Ciudad Real, E-13071, Spain
hInstituto de Catálisis y Petroleoquímica, CSIC, Cantoblanco, 28049 Madrid, Spain
iInstitute for Solid State Physics and Optics, Wigner Research Centre for Physics, P. O. Box 49, Budapest, H-1525, Hungary
jCELLS–ALBA synchrotron Light Facility, Cerdanyola del Valles, Barcelona, E-08290, Spain
First published on 30th March 2022
Rubidium di-lead pentabromide, RbPb2Br5, belongs to a family of layered lead-containing halides, with the common formula APb2X5 (where A = K, Rb, Cs; X = Cl, Br). The optical properties of RbPb2Br5 and rare-earth doped specimens are promising as low-phonon energy materials for tunable middle infrared and long-wavelength infrared laser sources, with suitable stability and resistance to wet conditions. In contrast to CsPb2Br5, the Rb counterpart has been barely studied and deserves further attention. Up to now, there have been no experimental reports on the transport properties such as the electronic conductivity, Seebeck coefficient or thermal transport. We describe here that this material can be prepared by ball milling in a straightforward way, yielding specimens with superior crystallinity. A structural investigation using synchrotron X-ray powder diffraction (SXRD) data combined with neutron powder diffraction (NPD) in a wide temperature range, from 15 to 573 K, was essential to evaluate the thermal evolution and to determine the Debye constants, yielding information on the relative Rb–Br and Pb–Br chemical bonds. In combination with DSC and TG measurements, no phase transitions were observed. Furthermore, an analysis of SXRD and NPD data (XRD-NPD) at room temperature reveals the directions of electron lone pair of Pb2+ ions lead atoms: its stereochemical effect is obvious in the [PbBr8] octahedral distortions. Diffuse reflectance UV-Vis spectroscopy yields an optical gap of 3.36 eV, close to that determined for a single-crystal material. Photoluminescence measurements indicate a lack of overlap between the excitation and emission spectra, due to the considerable Stokes shift, which prohibits self-absorption and thus enables applications in photovoltaics and biomedicine. The experimental information about the chemical bonds and band gap was studied via first-principles calculations. A maximum positive Seebeck coefficient of 3200 μV K−1 is obtained at 560 K, which is one order of magnitude higher than those reported for other halide perovskites.
A recognized strategy to improve the stability in CsPbI3 concerns a partial or complete substitution of I by Br, which leads to a more stable orthorhombic CsPbBr3 perovskite.4–12 Bromide also acts by changing the bandgap according to its content. Such a perovskite shows a high and constant photoresponse, being also more stable than CsPbI3 to ambient atmosphere and humidity. However, CsPbBr3 presents a higher bandgap (2.3 eV), that would hinder its application in solar cell devices, although allowing its use in optoelectronic systems.13
Very recently, interesting alternative strategies have been developed to achieve higher photoefficiencies in these halide perovskites, which include structural ordering and different topologies for the octahedral arrangements in the case of Cs3Bi2Br914–19 and CsPb2Br5.20–24 From a topological view, the former has 0D dimensionality (i.e. no connection between octahedra) and the latter a 2D one, instead of the classical 3D arrangement of CsPbBr3 with corner-sharing [PbBr6] units. Cs3Bi2Br9 exhibits good stability, while CsPb2Br5 has added interest due to its layered structure for thin film fabrication and application in photovoltaic devices.24–26
Rubidium di-lead pentabromide, RbPb2Br5, belongs to this fascinating family of layered lead-containing halides, with the common formula APb2X5 (where A = K, Rb, Cs; X = Cl, Br). The crystal growth and emission properties of RbPb2Br5 were described in the nineties,27 and the electronic structure and optical properties were estimated more recently,28 finding this halide and rare-earth doped specimens to be promising as low-phonon energy materials for tunable middle infrared (mid-IR) and long-wavelength infrared (long-wave-IR) laser sources for remote sensing and other applications.29,30 In contrast to CsPb2Br5, the Rb counterpart has been barely studied and it deserves further attention. Despite the mentioned antecedents, up to now there are no experimental reports on the transport properties such as the electronic conductivity, Seebeck coefficient or thermal transport.
In this work, we have succeeded in the synthesis of a well-crystallized RbPb2Br5 specimen using a solvent-free mechano-chemical method, enabling an accurate structural study from synchrotron X-ray diffraction (SXRD) and neutron powder diffraction (NPD) in a wide temperature range. RbPb2Br5 is tetragonal, defined in the space group I4/mcm (#140),31 and does not experience any phase transition below the melting point.32 In this paper, we derive the Debye temperatures from the atomic displacement factors, yielding interesting trends for the relative strength of Rb–Br vs. Pb–Br chemical bonds, complemented by DFT-based calculations. An optical gap of 3.36 eV is determined from UV-Vis spectra. The characterization of this ball-milled specimen is complemented by DSC measurements and scanning microscopy. Transport properties reveal a Seebeck coefficient of 13000 μV K−1 at 440 K.
The resulting powder obtained from ball milling was pressed into a pellet by hand using a cold press, while the sample was kept at room temperature throughout the whole process. The resistivity and thermal conductivity were measured in the pressed pellet, which was cut into a bar shape using a diamond saw to measure the Seebeck coefficient. The Seebeck coefficient was obtained by simultaneously measuring drop voltages across the sample and a constantan reference wire with an electrometer (Keithley 6517B) and nanovoltmeter (Keithley 2182A) under vacuum (10−3 mbar). Electrical conductivity measurements were carried out by using a HP 4284 LCR analyzer in the 20 Hz–1 MHz frequency range, with an applied ac voltage of 0.5 V. Electrodes were deposited on the pellet's flat surfaces by applying silver paste. The total thermal conductivity was calculated from the thermal diffusivity (α) using a Linseis LFA 1000 equipment, using the laser-flash technique. Both faces of the sample were covered with a thin graphite layer to ensure black body emissivity. The thermal conductivity (κ) was determined using κ = αcpd, where cp is the specific heat and d is the sample density.
Fig. 1a and b illustrate the DSC curves (in the heating and cooling runs) and the TG curve, respectively. No significant events are identified in the calorimetric curve in the 130–520 K temperature range, while the weight low above 600 °C indicated full decomposition of the sample, by Br loss.
Fig. 1c and d illustrates some FE-SEM images, giving insight into the microstructure of this product, synthesized by ball milling. An overall view with low magnification (18500×) shows irregular-shaped clusters of particles of different sizes (Fig. 1c). However, in a large magnification view (100000×), Fig. 1d unveils that they are indeed formed by tiny nano-particles of uneven form, with a typical size of 140–170 nm, which are grown during the ball milling process. EDX analysis coupled to the FE-SEM images yields an atomic composition close to 1:1:3 for the Rb:Pb:I ratio. A typical EDX spectrum is included in Fig. S2 in the ESI;† other SEM images are included in Fig. S3 (ESI†).
The synchrotron pattern collected at room temperature confirms the tetragonal symmetry in the I4/mcm space group (#140). The Rb1+ and Pb2+ ions are located in 4a (0,0,1/4) and 8h (x,x + 0.5,0) Wyckoff sites; Br1 and Br2, the two types of bromine, are located at 4c (0,0,0) and at 16l (x,y,z) respectively. The quality of the final refinement using this model is illustrated in Fig. 1e, and the main crystallographic results are listed in Table 1. Two views of the crystal structure are shown in Fig. 2a and b. The unit cell parameters refined at room temperature, of a = 8.44479(4) Å, c = 14.59265(7) Å, V = 1040.667(8) Å3, are comparable to those reported, a = 8.4455(1) Å, c = 14.5916(3) Å, V = 1040.77 Å3.40 The structure consists of layers of [PbBr8] polyhedra sharing edges and triangular faces, allowing for short Pb–Pb distances of 4.009(1) Å at RT across these faces. The layers are connected by Rb1+ ions, in ten-fold coordination with eight shorter (3.605(1) Å) and two longer (3.648 Å) Rb–Br bond lengths. In fact, the [PbBr8] polyhedra are strongly distorted, as derived from the stereochemical effect of the Pb2+ lone electron pair.
Atom | x | y | z | U iso | Occ | |
---|---|---|---|---|---|---|
R p: 7.66%; Rwp: 9.29%; Rexp: 5.76%; χ2:2.72; RBragg: 5.15%. System: tetragonal, space group: I4/mcm, Z = 4. Unit-cell parameters: a = 8.44479(4) Å, c = 14.59265(7) Å and V = 1040.67(1) Å3. | ||||||
Rb | 4a | 0.16783 (9) | 0.66783 (9) | 0 | 0.0290 (3) | 1 |
Pb | 8h | 0 | 0 | 0.25 | 0.0333 (8) | 1 |
Br1 | 4c | 0 | 0 | 0 | 0.0261 (8) | 1 |
Br2 | 16l | 0.6557 (1) | 0.1557 (1) | 0.1351 (1) | 0.0301 (4) | 1 |
The crystal structure in the 15–396 K interval was additionally investigated from NPD data collected in the D20 diffractometer (ILL). No structural changes were observed, other than the expected thermal contraction of the unit-cell parameters upon cooling down the sample (Fig. 2d–f). The volume has a thermal expansion with temperature of 0.1198(12) Å3 K−1, while the a and c unit cell parameters increase as 2.93(2) × 10−4 Å K−1 and 6.85(8) × 10−4 Å K−1, respectively. The moderate absorption of neutrons by the heavy Rb and Pb atoms was suitable to study the thermal variation of the displacement factors in all the temperature range. The quality of the NPD Rietveld fit is illustrated for the 15 K pattern, included in Fig. 1f.
An analysis of SXRD and NPD data (XRD-NPD) was useful to obtain additional information about the electron lone pair of Pb2+ ions. This procedure uses the structural information refined from NPD data to perform difference Fourier syntheses using SXRD data, which contain information on the electron density in the crystal, as was reported earlier.41 The results are plotted in Fig. 2c, where the difference electron density is shown as a yellow isosurface. This picture reveals a non-negligible density up and down Pb2+ ions along the c-axis. We assume that the 6s2 electrons are delocalized between both c-axis directions. This delocalized character of the Pb2+ electron lobes does not yield a net polarization, in agreement with the centrosymmetric nature of the crystal structure.
Topological analysis of the main Bond Critical Points (BCPs) of the theoretical model for RbPb2Br5 was performed in order to evaluate the above experimental estimates. The BCP parameters are listed in Table 2.
Bond | Distance (Å) | ρ (×10−3) | ∇2ρ (×10−2) | G (×10−3) | V (×10−3) | H (×10−3) | |V|/G |
---|---|---|---|---|---|---|---|
Rb–Br | 3.68 | 6.20 | 2.22 | 4.36 | −3.17 | 1.19 | 0.73 |
Pb–Br | 2.94 | 36.0 | 7.28 | 21.3 | −24.4 | −3.11 | 1.14 |
Pb–Br | 3.16 | 22.1 | 4.94 | 12.5 | −12.7 | −0.20 | 1.02 |
Pb–Br | 3.32 | 18.1 | 3.75 | 9.49 | −9.60 | −0.10 | 1.01 |
According to the parameters in Table 2, all the bonds present low values of ρ and positive values of ∇2ρ, which indicate a preponderant ionic character. On the other hand, the parameters between Rb–Br and Pb–Br bonds have some crucial differences. The H parameter of BCPs attributed to Rb–Br is positive, being negative for those attributed to the Pb–Br bonds. In addition, the |V|/G ratio of Rb–Br is less than 1, while in the Pb–Br BCPs it is greater than 1. These parameters of Pb–Br BCPs indicate that such bonds fall into a transient class showing a relevant covalent contribution.46 Therefore, the theoretical data exhibited a good agreement with the results obtained by Debye analysis, where the Rb–Br bond has a greater ionic character than the Pb–Br bond.
The Laplacian of the electronic density isolines at the (100) and (001) planes are depicted in Fig. 4. The bond Rb–Br along the (100) plane with isolines close to Rb atoms are very isolated, probably due to their predominant ionic character. The bond Pb–Br presented along the (001) plane has a distance of 3.16 Å. It is interesting to highlight the difference of the isoline profiles between Rb–Br and Pb–Br bonds. The isolines of Pb along the (001) plane show a certain entanglement degree with the Br isolines, which is due to the contribution of covalence of such bonds. It is important to make it clear that the assignment of covalence is a relative comparison with the Rb–Br bond, since both bonds have an ionic character.
Fig. 4 (100) and (001) planes and their respective Laplacians of electronic density (isolines) of the RbPb2Br5 halide model. |
The Seebeck coefficient strongly varies with temperature, adopting a negative value immediately above room temperature (e.g. −13000 μV K−1 at 440 K) and then changing its sign and reaching S = 3,200 μV K−1 at 560 K, and then becoming negative again at 620 K. The maximum positive value is about one order of magnitude higher than the Seebeck coefficients reported for other halide perovskites like CsSnBr3 and hybrid perovskites like MAPbBr3.47,48 Even these high Seebeck coefficient values are not enough to achieve a great power factor (S2σ), which remains low due to the high resistivity.
The thermal conductivity κ (Fig. 6a) is, on the other hand, lower than that reported for other halide and hybrid perovskites,47,48 and remains always below 0.26 W m−1 K at all the measured temperatures, from 300 K up to 573 K. Such a value has intrinsic origin based on low Debye temperatures for Rb, Pb, and Br. We can combine these values in a zT figure of merit (Fig. 6b), defined as zT = S2σT/κ, that yields zT = 1 × 10−6 at 570 K, which is low compared with the state-of-the-art thermoelectric materials, but is comparable to other reported values for halide perovskites, like Bi-doped MAPbBr3, which shows a zT of 1.8 × 10−6 at 293 K.48
Fig. 6 (a) Thermal conductivity of RbPb2Br5. (b) Thermoelectric figure of merit for the RbPb2Br5 halide, with a guiding curve to show its tendency. |
Fig. 7 (a) Kubelka–Munk (KM) transformed diffuse reflectance spectrum of RbPb2Br5 halide. (b) Photoluminescence by emission and excitation. |
Afterwards, the emission spectrum was determined with 350 nm (3.54 eV, slightly above the band gap) excitation wavelength, obtaining maximum intensity at 600 nm (2.06 eV). The excitation spectrum was then measured by setting the detection monochromator to 600 nm and tuning the excitation wavelength between 300 and 500 nm. The excitation spectrum (red curve in Fig. 7b) follows the absorption determined by diffuse reflectance (Fig. 7a). The lack of overlap between the excitation and emission spectra (black curve in Fig. 7b), due to the considerable Stokes shift, prohibits self-absorption and thus enables applications in photovoltaics and biomedicine. A similar large Stokes shift was reported in RbSn2Br5.50
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2tc00653g |
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