Ultrasonic measurements in the MHz frequency range have been performed
on two single-phase polycrystalline YBa2Cu4O8 superconducting samples
of different T(c) and microstructure in a temperature region between
30 and 350 K. Both longitudinal and transverse modes are employed. A p
ronounced dissipation peak at about 260 K accompanied by an increase i
n velocity is observed. This anomaly is suggested to be caused by the
formation of an antiferroelectric or ferroelectric state associated wi
th the ordering of oxygen atoms in some off-center positions of the st
ructure. Below 200 K a thermal hysteresis in velocity develops with no
anomalous behavior observed in attenuation. The area of the thermal h
ysteresis loop seems to be mainly determined by both the percentage of
pore volume in the sample, and by grain size. The hysteresis is expla
ined in terms of the reversible defects created near grain boundaries
during cooling, due to the unusual large anisotropic thermal expansion
in the material. No anomaly is found at T(c) in either sample at the
available resolution. However, a large reproducible increase of veloci
ty takes place at a temperature below T(c) together with the appearanc
e of an attenuation peak.