High-Resolution In-Situ Synchrotron X-Ray Studies of Inorganic Perovskite CsPbBr3 : New Symmetry Assignments and Structural Phase Transitions

Abstract

Perovskite photovoltaic ABX₃ systems are being studied due to their high energy-conversion efficiencies with current emphasis placed on pure inorganic systems. In this work, synchrotron single-crystal diffraction measurements combined with second harmonic generation measurements reveal the absence of inversion symmetry below room temperature in CsPbBr₃ . Local structural analysis by pair distribution function and X-ray absorption fine structure methods are performed to ascertain the local ordering, atomic pair correlations, and phase evolution in a broad range of temperatures. The currently accepted space group assignments for CsPbBr₃ are found to be incorrect in a manner that profoundly impacts physical properties. New assignments are obtained for the bulk structure: $Im\overline{3}$ (above ≈410 K), P2₁ /m (between ≈300 K and ≈410 K), and the polar group Pm (below ≈300 K), respectively. The newly observed structural distortions exist in the bulk structure consistent with the expectation of previous photoluminescence and Raman measurements. High-pressure measurements reveal multiple low-pressure phases, one of which exists as a metastable phase at ambient pressure. This work should help guide research in the perovskite photovoltaic community to better control the structure under operational conditions and further improve transport and optical properties.

Keywords: CsPbBr3; all-inorganic perovskites; in-situ single-crystal diffraction; local structure; phase transitions; space groups.

Figure 1

a) Contour plot of the temperature dependence of the Raman spectra. b) Low energy region of Raman spectra fitted by a sum of Lorentzian functions. The black open circles give the experimental data, the solid red line represents the sum of the fitting functions, and the dashed lines show the individual fitting components. A double‐peak structure is shown, which is shaded in red and blue, for the modes at 73 and 79 cm⁻¹, respectively. The ratio of fitted peak area and peak width are shown in panel (c), indicating a phase transitions onset near 170 and 300 K.

Figure 2

a) The temperature‐dependent structure of CsPbBr₃ from single‐crystal synchrotron X‐ray diffraction measurements. Above 410 K, the space group is cubic $Im\overline{3}$. Between 410 and 300 K, the structure is monoclinic P2₁/m. The inset shows the Z fractional coordinate of Cs4 and the Y fractional coordinate of Br5 as a function of temperature. An isostructural phase transition is observed at 350 K. Below 300 K, the space group is Pm. The Pm and previously reported Pnma unit cells are given as the solid and dotted lines, respectively. b) The quality‐of‐fit parameter, R ₁, of Pm, P2₁/m, and I‐3m structures. The solid squares indicate the R1 parameters which incorporate a pseudomerohedry twin law matrix (1 0 0, 0 −1 0, 0 0 −1), while the open squares are the R1 parameters of racemic twinning for Pm structure and no twinning for P2₁/m structure. The temperature‐dependent pseudomerohedry twin domain fraction is shown in (c). The single‐crystal diffraction derived lattice parameters as a function of temperature are given in (d) and (e).

Figure 3

a) Single‐crystal X‐ray diffraction reciprocal lattice image of the (hk 0) plane at 450 K. The (hkl) grid corresponds to the previously reported $Pm\overline{3}m$ space group with lattice‐constant a _p ≈ 5.87 Å. The insets show the 3D intensity of some selected reflections with an asymmetric diffuse scattering background. Diffraction spots with half‐integer h and k values are observed, indicating the correct lattice constant should be doubled. The temperature‐dependent intensity of (h −2 0) and (h −2.5 0) reflections are given in panels (b) and (c), respectively, which are the selected regions shown in panel (a).

Figure 4

a) Diagram of the RA‐SHG setup. The fundamental beam is focused on the ab plane of the crystal with a fixed small incident angle θ. The scattering plane rotates about the surface normal by an angle ϕ. b) The spectrum of the incident fundamental light, centered at 800 nm with a FWHM of 41 nm. The shaded area at 400 ± 20 nm represents the bandpass region of the filter used to only collect SHG signals. c) The RA‐SHG patterns in S _in − S _out and S _in‐P _out channels under 190 K (blue) and 290 K (red). ϕ = 0 represents the direction of a natural edge of the sample. SHG barely shows any difference between data sets at these two temperatures. Inset: the picture of a measured CsPbBr₃ sample. The red bar marks its natural edge.

Figure 5

Results for local pair distribution function measurements. a) The goodness‐of‐fit parameter, Rw, versus temperature for different models, P2₁2₁2₁, Pna2₁, Pnma, P4/mbm, $Pm\overline{3}m$, I4/m, and $Im\overline{3}$. Atomic displacement parameters as a function of temperature for the Br, Cs, and Pb sites derived from the Pna2₁ model are shown in (b) and (c). In panels (b) and (c), solid symbols and open symbols are given for two independent data sets collected. The ADP parameters reveal clear structural changes at ≈170 K and ≈410 K. Broad regions of structural change are seen in ADPs between 265 and 345 K for the Br sites and between 170 and 300 K for the Cs sites (in panes (b) and (c)). d) PDF derived temperature‐dependent lattice parameters between 10 and 500 K with an abrupt change near 350 K.

Figure 6

High‐pressure structural changes at room temperature. a) High‐pressure Raman spectra for pressures between 0.6 and 15 GPa. The fitted Raman peak positions are shown in (b), indicating three phase transitions. The inset in (b) shows the sample in the diamond anvil cell with spot from a 646 nm laser. c) Representative high‐pressure powder X‐ray diffraction pattern measured at 0.6 GPa. The inset shows the sample in the diamond anvil cell. d) The 2D intensity plot of the pressure‐dependent X‐ray diffraction patterns indicates transitions at ≈1, 2, 6, and 13 GPa. The pressure‐dependent structural phases are labeled phase I to phase V.

Figure 7

a) Expanded region of the 2D X‐ray diffraction map between 0.6 and 3 GPa showing a new phase that covers the region between 1 and 2 GPa. b) The pressure‐dependent Raman peak positions with dot‐dashed lines giving predicted positions at standard temperature and pressure (STP) from linear extrapolation of the phase II peaks, indicated with * symbols. In the top panel of (c), the Raman spectrum of the original sample is shown while the spectrum of a sample after compression between glass slides is given in the bottom panel. The appearance of additional peaks is indicated by the * symbols. Compression between the glass slides brings the samples into phase II and a part of the sample is maintained in this phase after release of pressure (metastable phase II).