Thermodynamic Stability and Hydrogen Bonds in Mixed Halide Perovskites

Area of Science:

• Materials Science
• Solid-State Chemistry
• Thermodynamics

Background:

• Mixed halide perovskites are vital for optoelectronics, but their phase stability is complex.
• Understanding the factors governing stability against phase separation is crucial for device performance.

Purpose of the Study:

• To develop a thermodynamic framework for analyzing the stability of mixed halide perovskites.
• To elucidate the contributions of enthalpic, configurational, and rotational entropic effects on phase stability.

Main Methods:

• Utilized *ab initio* molecular dynamics simulations for perovskite compositions (FAPbI3)1-x(APbY3)x.
• Decomposed the free energy of mixing into enthalpic, configurational, and rotational entropic components.
• Applied a regular-solution model for miscibility gap analysis.

Main Results:

• Enthalpy of mixing is small, ranging from near zero to +1.06 kJ mol⁻¹.
• Large configurational entropy (TΔS_mix^conf ≈ 1.10–4.39 kJ mol⁻¹ at 350 K) drives thermodynamic stability.
• Mixing reduces rotational entropy of organic cations (TΔS_mix^rot ≈ -0.35 to -0.14 kJ mol⁻¹), but does not overcome configurational stabilization.
• No miscibility gap predicted for considered mixing channels.
• Hydrogen bonding dynamics do not control phase stability; Cs-containing mixtures are stable without hydrogen bonds.

Conclusions:

• Phase stability in mixed halide perovskites is governed by configurational entropy, not hydrogen bonding.
• The balance between strong configurational entropy and a smaller rotational entropy correction determines overall stability.
• The presented thermodynamic framework provides insights into rationalizing perovskite stability for optoelectronic applications.