Coupling of organic cation and inorganic lattice in methylammonium lead halide perovskites: Insights into a pressure-induced isostructural phase transition

Sorb Yesudhas, Randy Burns, Barbara Lavina, Sergey N. Tkachev, Jiuyu Sun, Carsten A. Ullrich, and Suchismita Guha

Phys. Rev. Materials 4, 105403 – Published 16 October 2020

Abstract

Organic-inorganic hybrid metal-halide perovskite (OIHP) materials provide a tunable platform for engineering their optoelectronic properties. Although several high-pressure studies have been conducted from the OIHP family of single crystals and films, the exact nature of the dynamic coupling of the $C H_{3} N H_{3}$ (MA) cation with the octahedral lattice framework and the mechanisms responsible for the structural phase transformation under pressure are not well captured. By combined photoluminescence (PL), synchrotron-based x-ray diffraction, and Raman-scattering studies as a function of pressure from methylammonium lead bromide ( $M A Pb {Br}_{3}$ ), we shed light on an isostructural phase transition due to the coupling of the MA cation and the $Pb {Br}_{6}$ lattice through hydrogen bonding. The sharp discontinuities at $\sim$ 1 GPa and $\sim$ 3 GPa in the PL peak positions correlate with the structural changes observed in high-pressure XRD and Raman-scattering studies. The electronic band edge as a function of pressure is calculated within density-functional theory. The PL peak position, intensity and width of the excitonic peak show significant changes at 2 GPa, which corroborate the changes observed in high-pressure Raman-scattering studies. The frequencies of the lattice modes and the C–H/N–H bending and stretching modes of the MA cation show anomalous changes and other nuances at 2 GPa. The suppression of rotational and orientational disorder of the organic moiety is initiated at 2 GPa and the ordering is completed by 3.0 GPa, leading to an order-disorder type cubic II ( $I m \bar{3}$ ) to orthorhombic ( $P n m a$ ) phase transition. Along with the revelation of an isostructural transformation at 2 GPa, this paper highlights the impact of molecular vibrations on the electronic properties of $M A Pb {Br}_{3}$ under pressure.

Received 29 August 2020
Accepted 22 September 2020

DOI:https://doi.org/10.1103/PhysRevMaterials.4.105403

Physics Subject Headings (PhySH)

Pressure effects

Perovskites Synchrotron radiation facilities

Density functional theory Photoluminescence Raman spectroscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Sorb Yesudhas¹, Randy Burns¹, Barbara Lavina², Sergey N. Tkachev³, Jiuyu Sun¹, Carsten A. Ullrich¹, and Suchismita Guha ^1,*

¹Department of Physics and Astronomy, University of Missouri, Columbia, Missouri 65211, USA
²X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60349, USA
³Center for Advanced Radiation Sources, University of Chicago, Chicago, Illinois 60637, USA

^*guhas@missouri.edu

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Vol. 4, Iss. 10 — October 2020

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Images

Figure 1
(a) PL spectrum of $M A Pb {Br}_{3}$ single crystal under ambient conditions with an excitation wavelength of 457 nm. The shaded regions show the fit to the spectrum with two Gaussian peaks. The red dotted line is the two-photon PL spectrum with an excitation wavelength of 800 nm (100 fs). The inset shows the process of two-photon (TP) PL. (b) Temperature dependence of the two PL peaks. The inset shows the PL spectrum and the deconvolution of the two peaks at 160 K.
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Figure 2
(a) PL spectra of an $M A Pb {Br}_{3}$ single crystal at various pressures. (b) PL peak positions of peak 1 and 2 as a function of pressure. The blue arrows depict the discontinuities in the peak positions. (c) PL peak position and intensity of peak 1 between 1.2 GPa and 2.6 GPa. (d) FWHM of peak 1 as a function of pressure.
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Figure 3
The band-gap energy shift obtained from DFT calculations as a function of pressure is shown in blue. The pressure-induced PL energy shift of peak 1 is shown in red. Inset: Total energy of the $P m \bar{3} m$ and $I m \bar{3}$ phases as a function of unit cell volume obtained from theory.
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Figure 4
High-pressure synchrotron XRD of $M A Pb {Br}_{3}$ . (a) The XRD pattern of $M A Pb {Br}_{3}$ for representative pressure values for the first sample (run 1). The $*$ symbol represents additional peaks that may be due to impurities. The thick red circles at 1 GPa denote the appearance of new peaks. Different phases are indicated with different colors. The ${(200)}_{c}$ and ${(101)}_{o}$ peaks denote the first peak of cubic II and orthorhombic phases, respectively. (b) The XRD pattern of $M A Pb {Br}_{3}$ for selected pressure values for another sample (run 2). The thick red circles at 1.5 GPa denote the appearance of new peaks. (c) Pressure dependence of the $2 θ$ position of the first peak of the orthorhombic phase ( ${(101)}_{o}$ ). The vertical dotted line separates the orthorhombic and disordered phases. The full red lines are a guide to the eye. The pressure versus $2 θ$ position of the first peak of the cubic II and orthorhombic phases are shown as an inset. A vertical dotted line separates the cubic II and orthorhombic phases. (d) Pressure dependence of the volume per formula unit (volume/Z, Z = 1, 8, and 4 for phases I, II, and III, respectively) of the cubic I, cubic II, and orthorhombic phases, including error bars. The different phases are separated by vertical dotted lines.
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Figure 5
(a) Raman spectra of $M A Pb {Br}_{3}$ for selected pressures below 500 ${cm}^{- 1}$ . The red arrows denote the emergence of the 50 ${cm}^{- 1}$ mode beyond 3 GPa and the 70 ${cm}^{- 1}$ mode as it starts shifting with pressure. (b) Raman peak positions of the low-frequency modes seen in (a) as a function of pressure. (c) Close up of the 300 ${cm}^{- 1}$ region for different pressure values. (d) Peak position of the $325 cm^{- 1}$ Raman mode as a function of pressure. The inset shows the peak position of the 70 ${cm}^{- 1}$ mode as a function of pressure. The black dashed line is a guide to the eye. The red arrows denote discontinuity and slope change.
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Figure 6
(a)–(c) Raman spectra of $M A Pb {Br}_{3}$ for selected values of pressure in regions 2, 3, and 4, respectively. The red arrows indicate the splitting of phonon modes during cubic II to the orthorhombic phase transition. (d)–(f) Frequencies of the various Raman modes, corresponding to regions 2–4, as a function of pressure. The red symbols denote the frequencies of new Raman modes that emerge with pressure. The red arrows denote discontinuity in the frequencies and the emergence of new modes.
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