Multiple lattice instabilities and complex ground state in Cs${}_2$AgBiBr${}_6$

Abstract

Metal halides perovskites (MHPs) are attracting considerable interest for optoelectronic applications, with Cs${}_2$AgBiBr${}_6$ one of the main contenders among lead-free systems. Cs${}_2$AgBiBr${}_6$ crystallizes in a nominally double-perovskite structure, but exhibits a soft lattice with large atomic fluctuations characteristic of MHPs. While crucial to understand electron-phonon and phonon-phonon couplings, the spatio-temporal correlations of these fluctuations remain largely unknown. Here, we reveal these correlations using comprehensive neutron and x-ray scattering measurements on Cs${}_2$AgBiBr${}_6$ single-crystals, complemented with first-principles simulations augmented with machine-learned neural-network potentials. We report the discovery of an unexpected complex modulated ground state structure containing several hundred atoms, arising from a soft-phonon instability of the low-temperature tetragonal phase. Further, our experiments and simulations both reveal extensive correlated 2D fluctuations of Br octahedra at finite temperature, arising from soft anharmonic optic phonons, reflecting very shallow potential wells. These results provide new insights into the atomic structure and fluctuations in MHPs, critical to understand and control their thermal and optoelectronic properties.

12 pages, 4 figures

References in corpus (5)

• Spontaneous Octahedral Tilting in the Cubic Inorganic Caesium Halide Perovskites CsSnX${}_3$ and CsPbX${}_3$ (X = F, Cl, Br, I)
• Visualization of Dynamic Polaronic Strain Fields in Hybrid Lead Halide Perovskites
• Dynamic Shortening of Disorder Potentials in Anharmonic Halide Perovskites
• The effects of intrinsic local distortions vs. dynamic thermal motions on the stability and band gaps of cubic oxide and halide perovskites
• Anharmonic electron-phonon coupling in ultrasoft and locally disordered perovskites

Cited by in corpus (5)

• Noncollinear electric dipoles in a polar, chiral phase of CsSnBr${}_3$ perovskite
• Topological polarons in halide perovskites
• Ineffectiveness of Formamidine in Suppressing Ultralow Thermal Conductivity in Cubic Hybrid Perovskite FAPbI3
• Local structural disorder in crystalline materials
• Low-temperature structural instabilities of the halide double perovskite Cs${}_2$AgBiBr${}_6$ investigated via x-ray diffraction and infrared phonons