[Paper Review] Ineffectiveness of Formamidine in Suppressing Ultralow Thermal Conductivity in Cubic Hybrid Perovskite FAPbI3

Jiongzhi Zheng, Zheng Chang+6 more|arXiv (Cornell University)|Jun 3, 2024

Thermal Expansion and Ionic ConductivityMaterials Science3 citations

TL;DR

This study reveals that formamidine ligands fail to suppress the ultra-low thermal conductivity (~0.63 Wm⁻¹K⁻¹ at 300 K) in cubic FAPbI3 perovskite, which instead arises primarily from Pb-I covalent bonding and anti-bonding states in [PbI3]1− units. Using temperature-dependent effective potential (TDEP) and linearized Wigner transport, the authors show phonon transport is particle-like and governed by strong anharmonicity, with thermal conductivity underestimated when using 0 K force constants due to temperature-sensitive anharmonic interactions.

ABSTRACT

Fundamentally understanding the lattice dynamics and microscopic mechanisms of thermal transport in cubic hybrid organic-inorganic perovskites remains elusive, primarily due to their strong anharmonicity and frequent phase transitions. In this work, we comprehensively investigate the thermal transport behavior in cubic hybrid perovskite FAPbI3, integrating first principles-based anharmonic lattice dynamics with a linearized Wigner transport formula. The Temperature Dependent Effective Potential (TDEP) technique allows us to stabilize the negative soft modes, primarily dominated by organic cations, at finite temperatures in cubic FAPbI3. We then predict an ultra-low thermal conductivity of ~0.63 Wm^(-1) K^(-1) in cubic FAPbI3 at 300 K, with a temperature dependence of T^(-0.740), suggesting a good crystalline nature of phonon transport. Notably, the ultra-low thermal conductivity in cubic FAPbI3 is primarily attributed to the [PbI3]1- units, challenging the conventional focus on organic FA+ cations. This shift in focus is due to the presence of Pb(s)-I(p) anti-bonding sates within the [PbI3]1- units. Furthermore, thermal transport in cubic FAPbI3 is predominantly governed by the particle-like phonon propagation channel across the entire temperature range of 300-500 K, a result of diminished suppression of low-frequency phonons by FA+ cations and large inter-branch spacings. Finally, our findings underscore that the anharmonic force constants are highly temperature-sensitive, leading to underestimations of thermal conductivity when relying on 0-K anharmonic force constants. Our study not only elucidates the microscopic mechanisms of thermal transport in cubic FAPbI3 but also provides a crucial framework for the discovery, design, and understanding of hybrid organic-inorganic compounds with ultra-low thermal conductivity.

Motivation & Objective

• To understand the microscopic origins of ultra-low thermal conductivity in cubic hybrid perovskite FAPbI3.
• To investigate the role of organic FA+ cations and [PbI3]1− units in phonon scattering and thermal transport.
• To evaluate the effectiveness of formamidine in suppressing thermal conductivity through lattice dynamics and anharmonic force constants.
• To develop a framework for predicting thermal conductivity in hybrid perovskites using temperature-dependent anharmonic models.
• To correct the underestimation of thermal conductivity when relying on 0 K anharmonic force constants.

Proposed method

• Employed first-principles anharmonic lattice dynamics with the Temperature Dependent Effective Potential (TDEP) method to stabilize soft modes at finite temperatures.
• Applied the linearized Wigner transport formula to compute thermal conductivity from phonon dispersion and scattering rates.
• Calculated temperature-dependent anharmonic force constants using TDEP to capture the strong temperature sensitivity of lattice dynamics.
• Tracked phonon transport behavior across 300–500 K to assess the dominance of particle-like phonon propagation.
• Used 0 K anharmonic force constants as a baseline for comparison to highlight systematic underestimation of thermal conductivity.
• Performed electronic structure analysis to identify Pb(s)-I(p) anti-bonding states as key contributors to phonon scattering.

Experimental results

Research questions

• RQ1 What is the dominant microscopic origin of ultra-low thermal conductivity in cubic FAPbI3, and is it primarily driven by organic cations or inorganic [PbI3]1− frameworks?
• RQ2 To what extent does formamidine ligand incorporation suppress thermal conductivity in FAPbI3, and why is it ineffective?
• RQ3 How does the temperature dependence of anharmonic force constants affect the accuracy of thermal conductivity predictions?
• RQ4 What is the nature of phonon transport (e.g., wave-like vs. particle-like) in cubic FAPbI3 across 300–500 K?
• RQ5 Why is thermal conductivity systematically underestimated when using 0 K anharmonic force constants in this system?

Key findings

• The thermal conductivity of cubic FAPbI3 is predicted to be ~0.63 Wm⁻¹K⁻¹ at 300 K, with a temperature dependence of T⁻⁰.⁷⁴⁰.
• The ultra-low thermal conductivity is primarily attributed to the [PbI3]1− units, not the organic FA+ cations, due to Pb(s)-I(p) anti-bonding states.
• Phonon transport in FAPbI3 is dominated by particle-like propagation across 300–500 K, due to weak suppression of low-frequency phonons by FA+ cations.
• Anharmonic force constants in FAPbI3 are highly temperature-sensitive, leading to significant underestimation of thermal conductivity when using 0 K force constants.
• The TDEP method successfully stabilizes negative soft modes associated with FA+ cations at finite temperatures, enabling accurate thermal transport modeling.
• Large inter-branch spacings in the phonon spectrum further suppress scattering, reinforcing the particle-like nature of phonon transport.