  Point defects play a crucial role in crystalline materials as they do not
only impact the thermodynamic properties but are also central to kinetic
processes. While they are necessary in thermodynamic equilibrium spontaneous
defect formation in the bulk is normally considered highly improbable except
for temperatures close to the melting point. Here, we demonstrate by means of
atomistic simulations that processes involving concerted atomic motion that
give rise to defect formation are in fact frequent in body-centered cubic
metals even down to about 50% of the melting temperature. It is shown that this
behavior is intimately related to the anharmonicity of the lattice vibrations
and a flat energy landscape along certain crystallographic directions, a
feature that is absent in, e.g., face-centered cubic lattice structures. This
insight has implications for our general understanding of these materials and
furthermore provides a complementary explanation for the so-called anomalous
diffusion in group 4 transition metals.
