Decoupled molecular and inorganic framework dynamics in CH₃NH₃PbCl₃

Area of Science:

• Materials Science
• Solid-State Physics
• Crystallography

Background:

• Organic-inorganic lead-halide perovskites exhibit high photovoltaic efficiencies.
• Hydrogen bonding between organic cations and halide anions is proposed to influence optoelectronic properties.

Purpose of the Study:

• To investigate the molecular dynamics and their relationship with structural phase transitions in methylammonium lead chloride (MAPbCl3).
• To determine the role of hydrogen bonding versus lead-halide bonding in the optoelectronic properties of lead-halide perovskites.

Main Methods:

• Time-of-flight neutron spectroscopy (backscattering and higher-energy) to probe molecular dynamics.
• Neutron powder diffraction to identify structural phase transitions.

Main Results:

• Thermally activated molecular dynamics observed around 95 K.
• Anomalous broadening in molecular vibrations at ~95 K, indicating reduced lifetime.
• Structural phase transitions occurred at higher temperatures (178 K and 173 K).
• Decoupled dynamics between molecular motion and inorganic lattice transitions.

Conclusions:

• Molecular and lattice dynamics in MAPbCl3 are decoupled.
• The energy scale relevant to photovoltaic properties is likely determined by lead-halide bonds, not hydrogen bonds.