  There has been growing interest in structural quantum phase transitions and
quantum fluctuations of phonons in the research area of condensed matter
physics. Here, we report the observation of glass-like features in the lattice
heat capacity of a stuffed tridymite-type crystal, Ba1-xSrxAl2O4, a candidate
compound of quantum paraelectrics. Substitutional chemical suppression of the
ferroelectric phase transition temperature (TC) of Ba1-xSrxAl2O4 results in the
disappearance of the TC at x=0.07. For the compositional window of x=0.2-0.5,
the lattice heat capacity is enhanced below approximately 10 K and diverges
from the T3-scaling law below 2.5 K. Synchrotron X-ray diffraction experiments
on single crystals reveal the weakly correlated disorder in the crystal
structure that survives down to low temperature; this disorder is responsible
for the observed excess heat capacity. These features can be considered one of
the manifestations of structural quantum fluctuations.
