Multiple Lattice Instabilities and Complex Ground State in
Metal-halide perovskites (MHPs) are attracting considerable interest for optoelectronic applications, with Cs2AgBiBr6 one of the main contenders among lead-free systems. Cs2AgBiBr6 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 Cs2AgBiBr6 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-phononmore »