Multiple Lattice Instabilities and Complex Ground State in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><mml:msub><mml:mi>Cs</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mrow><mml:mi>Ag</mml:mi><mml:mi>Bi</mml:mi><mml:mi>Br</mml:mi></mml:mrow><mml:mn>6</mml:mn></mml:msub></mml:math>

Abstract

Metal-halide perovskites (MHPs) are attracting considerable interest for optoelectronic applications, with <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><a:msub><a:mi>Cs</a:mi><a:mn>2</a:mn></a:msub><a:msub><a:mrow><a:mi>Ag</a:mi><a:mi>Bi</a:mi><a:mi>Br</a:mi></a:mrow><a:mn>6</a:mn></a:msub></a:math> one of the main contenders among lead-free systems. <d:math xmlns:d="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><d:msub><d:mi>Cs</d:mi><d:mn>2</d:mn></d:msub><d:msub><d:mrow><d:mi>Ag</d:mi><d:mi>Bi</d:mi><d:mi>Br</d:mi></d:mrow><d:mn>6</d:mn></d:msub></d:math> 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 spatiotemporal correlations of these fluctuations remain largely unknown. Here, we reveal these correlations using comprehensive neutron and x-ray scattering measurements on <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><g:msub><g:mi>Cs</g:mi><g:mn>2</g:mn></g:msub><g:msub><g:mrow><g:mi>Ag</g:mi><g:mi>Bi</g:mi><g:mi>Br</g:mi></g:mrow><g:mn>6</g:mn></g:msub></g:math> 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 two-dimensional fluctuations of <j:math xmlns:j="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><j:mi>Br</j:mi></j:math> octahedra at finite temperature, arising from soft optic phonons that are strongly broadened by anhamonicity, 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. Published by the American Physical Society 2024

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