  The underlying dielectric properties of materials, intertwined with
intriguing phenomena such as topological polariton modes and anisotropic
thermal conductivities, stem from the anisotropy in atomic vibrations.
Conventionally, X-ray diffraction techniques have been employed to estimate
thermal ellipsoids of distinct elements, albeit lacking the desired spatial and
energy resolutions. Here we introduce a novel approach utilizing the dark-field
monochromated electron energy-loss spectroscopy for momentum-selective
vibrational spectroscopy, enabling the cartographic delineation of variations
of phonon polarization vectors. By applying this technique to centrosymmetric
cubic-phase strontium titanate, we successfully discern two types of oxygen
atoms exhibiting contrasting vibrational anisotropies below and above 60 meV
due to their frequency-linked thermal ellipsoids. This method establishes a new
pathway to visualize phonon eigenvectors at specific crystalline sites for
diverse elements, thus delving into uncharted realms of dielectric, optical,
and thermal property investigations with unprecedented spatial resolutions.
