Ultrasonic measurements on two single-phase polycrystalline ${\mathrm{YBa}}_2$${\mathrm{Cu}}_4$${\mathrm{O}}_8$ superconducting samples

Ultrasonic measurements in the MHz frequency range have been performed on two single-phase polycrystalline ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_4$ ${\mathrm{O}}_8$ superconducting samples of different ${\mathit{T}}_{\mathit{c}}$ and microstructure in a temperature region between 30 and 350 K. Both longitudinal and transverse modes are employed. A pronounced dissipation peak at about 260 K accompanied by an increase in velocity is observed. This anomaly is suggested to be caused by the formation of an antiferroelectric or ferroelectric state associated with the ordering of oxygen atoms in some off-center positions of the structure. Below 200 K a thermal hysteresis in velocity develops with no anomalous behavior observed in attenuation. The area of the thermal hysteresis loop seems to be mainly determined by both the percentage of pore volume in the sample, and by grain size. The hysteresis is explained 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 ${\mathit{T}}_{\mathit{c}}$ in either sample at the available resolution. However, a large reproducible increase of velocity takes place at a temperature below ${\mathit{T}}_{\mathit{c}}$ together with the appearance of an attenuation peak.