Phonon lifetimes and mode softening in cubic Cs2AgBiBr6

Zihan Zhang; Peter M. Gehring; Adam H. Slavney; Julian A. Vigil; Tao Hong; Jose A. Rodriguez-Rivera; Johan Klarbring; Hemamala I. Karunadasa; Michael F. Toney; Nicholas J. Weadock

doi:10.1016/j.newton.2025.100097

Phonon lifetimes and mode softening in cubic Cs₂AgBiBr₆

Zihan Zhang
, Peter M. Gehring
, Adam H. Slavney
, Julian A. Vigil
, Tao Hong
, Jose A. Rodriguez-Rivera
, Johan Klarbring
, Hemamala I. Karunadasa
, Michael F. Toney
, Nicholas J. Weadock

Triple-Axis

Research output: Contribution to journal › Article › peer-review

2 Scopus citations

Abstract

Metal halide perovskites (MHPs) have emerged as noteworthy candidates for photovoltaic applications in recent years. Their high power-conversion efficiency is largely attributed to the structure-function relationship, which is not well understood. In this study, we examine the low-frequency phonons of the double perovskite Cs₂AgBiBr₆ using neutron inelastic scattering. We find that the acoustic phonon lifetimes decrease from 16 to 3 ps along Γ to X, which is indicative of significant anharmonicity that contributes to the ultralow thermal conductivity. Additionally, we observe a linear temperature dependence of the square of the zone-center soft optical phonon energy, which is consistent with a weakly first-order displacive cubic-tetragonal structural phase transition. These results provide a deeper understanding of the lattice dynamics and phase transitions in MHPs as well as the effects of anharmonicity by comparison with prototypical hybrid organic-inorganic MHPs, (CH₃NH₃) PbX₃ (X = Br, I).

Original language	English
Article number	100097
Journal	Newton
Volume	1
Issue number	4
DOIs	https://doi.org/10.1016/j.newton.2025.100097
State	Published - Jun 2 2025

Funding

This work (conception, neutron scattering, and analysis) was supported by the Center for Hybrid Organic Inorganic Semiconductors for Energy, an Energy Frontier Research Center funded by the Office of Basic Energy Sciences, an office of science within the US Department of Energy (DOE). H.I.K. acknowledges funding through the DOE Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under contract DE-AC02-76SF00515 for the materials synthesis. Access to MACS was provided by the Center for High-Resolution Neutron Scattering, a partnership between the National Institute of Standards and Technology and the National Science Foundation under agreement DMR-2010792. Any mention of commercial products here is for information only; it does not imply recommendation or endorsement by the National Institute of Standards and Technology. A portion of this research used resources at the High-Flux Isotope Reactor, a DOE Office of Science User Facility operated by Oak Ridge National Laboratory. The beam time was allocated to CTAX on proposal IPTS-26851. J.A.V. acknowledges fellowship support from the Stanford University Office of the Vice Provost of Graduate Education and the National Science Foundation Graduate Research Fellowship Program under grant no. DGE-1656518.

Keywords

displacive phase transition
lead-free perovskite
neutron inelastic scattering
soft optical phonon

Access to Document

10.1016/j.newton.2025.100097

Cite this

@article{0b1c022498cf47b78a0919a15f961979,

title = "Phonon lifetimes and mode softening in cubic Cs2AgBiBr6",

abstract = "Metal halide perovskites (MHPs) have emerged as noteworthy candidates for photovoltaic applications in recent years. Their high power-conversion efficiency is largely attributed to the structure-function relationship, which is not well understood. In this study, we examine the low-frequency phonons of the double perovskite Cs2AgBiBr6 using neutron inelastic scattering. We find that the acoustic phonon lifetimes decrease from 16 to 3 ps along Γ to X, which is indicative of significant anharmonicity that contributes to the ultralow thermal conductivity. Additionally, we observe a linear temperature dependence of the square of the zone-center soft optical phonon energy, which is consistent with a weakly first-order displacive cubic-tetragonal structural phase transition. These results provide a deeper understanding of the lattice dynamics and phase transitions in MHPs as well as the effects of anharmonicity by comparison with prototypical hybrid organic-inorganic MHPs, (CH3NH3) PbX3 (X = Br, I).",

keywords = "displacive phase transition, lead-free perovskite, neutron inelastic scattering, soft optical phonon",

author = "Zihan Zhang and Gehring, \{Peter M.\} and Slavney, \{Adam H.\} and Vigil, \{Julian A.\} and Tao Hong and Rodriguez-Rivera, \{Jose A.\} and Johan Klarbring and Karunadasa, \{Hemamala I.\} and Toney, \{Michael F.\} and Weadock, \{Nicholas J.\}",

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year = "2025",

month = jun,

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doi = "10.1016/j.newton.2025.100097",

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T1 - Phonon lifetimes and mode softening in cubic Cs2AgBiBr6

AU - Zhang, Zihan

AU - Gehring, Peter M.

AU - Slavney, Adam H.

AU - Vigil, Julian A.

AU - Hong, Tao

AU - Rodriguez-Rivera, Jose A.

AU - Klarbring, Johan

AU - Karunadasa, Hemamala I.

AU - Toney, Michael F.

AU - Weadock, Nicholas J.

PY - 2025/6/2

Y1 - 2025/6/2

N2 - Metal halide perovskites (MHPs) have emerged as noteworthy candidates for photovoltaic applications in recent years. Their high power-conversion efficiency is largely attributed to the structure-function relationship, which is not well understood. In this study, we examine the low-frequency phonons of the double perovskite Cs2AgBiBr6 using neutron inelastic scattering. We find that the acoustic phonon lifetimes decrease from 16 to 3 ps along Γ to X, which is indicative of significant anharmonicity that contributes to the ultralow thermal conductivity. Additionally, we observe a linear temperature dependence of the square of the zone-center soft optical phonon energy, which is consistent with a weakly first-order displacive cubic-tetragonal structural phase transition. These results provide a deeper understanding of the lattice dynamics and phase transitions in MHPs as well as the effects of anharmonicity by comparison with prototypical hybrid organic-inorganic MHPs, (CH3NH3) PbX3 (X = Br, I).

AB - Metal halide perovskites (MHPs) have emerged as noteworthy candidates for photovoltaic applications in recent years. Their high power-conversion efficiency is largely attributed to the structure-function relationship, which is not well understood. In this study, we examine the low-frequency phonons of the double perovskite Cs2AgBiBr6 using neutron inelastic scattering. We find that the acoustic phonon lifetimes decrease from 16 to 3 ps along Γ to X, which is indicative of significant anharmonicity that contributes to the ultralow thermal conductivity. Additionally, we observe a linear temperature dependence of the square of the zone-center soft optical phonon energy, which is consistent with a weakly first-order displacive cubic-tetragonal structural phase transition. These results provide a deeper understanding of the lattice dynamics and phase transitions in MHPs as well as the effects of anharmonicity by comparison with prototypical hybrid organic-inorganic MHPs, (CH3NH3) PbX3 (X = Br, I).

KW - displacive phase transition

KW - lead-free perovskite

KW - neutron inelastic scattering

KW - soft optical phonon

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