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 CH3NH3 (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 (MAPbBr3), we shed light on an isostructural phase transition due to the coupling of the MA cation and the PbBr6 lattice through hydrogen bonding. The sharp discontinuities at 1 GPa and 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 (Im¯3) to orthorhombic (Pnma) 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 MAPbBr3 under pressure.

Coupling of organic cation and inorganic lattice in methylammonium lead halide perovskites: Insights into a pressure-induced isostructural phase transition, Phys. Rev. Materials 4, 105403, DOI abstract

(a) PL spectrum of MAPbBr3 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.