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Low-temperature structural instabilities of the halide double perovskite Cs2AgBiBr6 investigated via x-ray diffraction and infrared phonons

Collin Tower*, Fereidoon S. Razavi, Jeremy Dion, and Maureen Reedyk

Jürgen Nuss and Reinhard K. Kremer

  • *Contact author: ct15zs@brocku.ca
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APS Open Sci. 1, 000040 – Published 10 June, 2026

DOI: https://doi.org/10.1103/fs84-8lgj

Abstract

The halide double perovskite Cs2AgBiBr6 has been proposed as a potential replacement for organic halide perovskites for photovoltaic applications. Further investigation of its dielectric response, optical properties, and structural stability is thus warranted. Cs2AgBiBr6 exhibits a well-documented structural phase transition at 120 K but indications for an additional lower-temperature (∼40 K) phase transition have also been reported. On the basis of measurements of the specific heat capacity, temperature-dependent powder x-ray diffraction, low-frequency capacitance, and infrared reflectivity, we clarify the previously reported splitting of phonon modes in the Raman spectrum at ∼40K as due to a subtle structural phase transition from the tetragonal I4/m structure to a monoclinic P121/n1 crystal structure. The infrared-active vibrational modes are experimentally investigated in the three structural regimes. In the cubic structure at room temperature, the four infrared-active modes are observed at 135, ∼85, 55, and ∼25cm−1. As the symmetry reduces to tetragonal, a minute splitting of these modes is found; however, below 40 K additional mode splitting is observed, indicating a further reduction in symmetry.

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APS Open Sci. 1, 000040– Published 10 June, 2026
Vol. 1
  • Received 19 December 2024
  • Accepted 26 May 2026
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

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