  Higgs and Goldstone modes, well known in high energy physics, have been
realized in a number of condensed matter physics contexts, including
superconductivity and magnetism. The Goldstone-Higgs concept is also applicable
to and gives rise to new insights on structural phase transitions. Here, we
show that the Leggett mode, a collective mode observed in multi-band
superconductors, also has an analog in crystallographic phase transitions. Such
structural Leggett modes can occur in the phase channel as in the original work
of Leggett, \href{https://doi.org/10.1143/PTP.36.901}{Prog.\ Theor.\ Phys.\
\textbf{36}, 901 (1966)}. That is, they are antiphase Goldstone modes
(anti-phasons). In addition, a new collective mode can also occur in the
amplitude channel, an out-of phase (antiphase) Higgs mode, that should be
observable in multi-band superconductors as well. We illustrate the existence
and properties of these structural Leggett modes using the example of the
pyrochlore relaxor ferroelectric, Cd$_2$Nb$_2$O$_7$.
