  The potential of lead-halide perovskites for realistic applications is
currently hindered by their limited long-term stability under functional
activation. While the role of lattice flexibility in the thermal response of
perovskites has become increasingly evident, the description of
thermally-induced distortions is still unclear. In this work, we provide a
unified picture of thermal activation in CsPbBr3 across length scales, showing
that lattice symmetry does not increase at high temperatures. We combine
temperature-dependent XRD, Br K-edge XANES, ab initio MD simulations, and
calculations of the XANES spectra by first-principles, accounting for both
thermal fluctuations and core hole final state effects. We find that the
octahedral tilting of the Pb-Br inorganic framework statistically adopts
multiple local configurations over time - in the short-range. In turn, the
stochastic nature of the local thermal fluctuations uplifts the longer-range
periodic octahedral tilting characterizing the low temperature structure, with
the statistical mean of the local configurations resulting in a cubic-like
time-averaged lattice. These observations can be rationalized in terms of
displacive thermal phase transitions through the soft mode model, in which the
phonon anharmonicity of the flexible inorganic framework causes the excess free
energy surface to change as a function of temperature. Our work demonstrates
that the effect of thermal dynamics on the XANES spectra can be effectively
described for largely anharmonic systems, provided ab initio MD simulations are
performed to determine the dynamically fluctuating structures, and core hole
final state effects are included in order to retrieve an accurate XANES line
shape. Moreover, it shows that the soft mode model, previously invoked to
describe displacive thermal phase transitions in oxide perovskites, carries a
more general validity.
