  Cubic energy materials such as thermoelectrics or hybrid perovskite materials
are often understood to be highly disordered. In GeTe and related IV-VI
compounds, this is thought to provide the low thermal conductivities needed for
thermoelectric applications. Since conventional crystallography cannot
distinguish between static disorder and atomic motions, we develop the
energy-resolved variable-shutter pair distribution function technique. This
collects structural snapshots with varying exposure times, on timescales
relevant for atomic motions. In disagreement with previous interpretations, we
find the time-averaged structure of GeTe to be crystalline at all temperatures,
but with anisotropic anharmonic dynamics at higher temperatures that resemble
static disorder at fast shutter speeds, with correlated ferroelectric
fluctuations along the $<$100$>$c direction. We show that this anisotropy
naturally emerges from a Ginzburg-Landau model that couples polarization
fluctuations through long-range elastic interactions. By accessing
time-dependent atomic correlations in energy materials, we resolve the
long-standing disagreement between local and average structure probes, and show
that spontaneous anisotropy is ubiquitous in cubic IV-VI materials.
