  In cosmology, the axion is a hypothetical particle that is currently
considered as candidate for dark matter. In condensed matter, a counterpart of
the axion (the "axion quasiparticle") has been predicted to emerge in
magnetoelectric insulators with fluctuating magnetic order and in
charge-ordered Weyl semimetals. To date, both the cosmological and
condensed-matter axions remain experimentally elusive or unconfirmed. Here, we
show theoretically that ordinary lattice vibrations can form an axion
quasiparticle in Dirac insulators with broken time- and space-inversion
symmetries, even in the absence of magnetic fluctuations. The physical
manifestation of the phononic axion is a magnetic-field-induced phonon
effective charge, which can be probed in optical spectroscopy. By replacing
magnetic fluctuations with lattice vibrations, our theory widens the scope for
the observability of the axion quasiparticle in condensed matter.
