Vacancy-induced low-energy excitations in the one-dimensional ionic conductor β-eucryptite

We have measured the specific heat C of β-Li_xAl_xSi_2?xO₄ (β-eucryptite) for x = 1 and 0.93 between 0.1 and 2.5 K. Below 1 K, an anomaly is observed in C which exhibits a maximum at 0.6 K. This anomaly is enhanced by a factor of 10 in the substoichiometric sample. These results are interpreted in the Frenkel-Kontorova model of configurational excitations which arise in this material because of vacancies in the ion channels. Below 0.5 K a time dependence of the specific heat is observed which hints at rather slow dynamics of the configurational excitations.