Moessbauer study of oxygen-related lattice instabilities in oriented 1-2-3 HTSC

57Fe Moessbauer effect measurements as a function of temperature between 78 and 293 K in grain-aligned YBaCuO (Tc = 89 K, ΔTc < 2 K) with both c parallel γ and c perpendicular to γ orientations revealed a sequence of anomalies in the recoilless fraction, center shift and linewidth of all 57Fe quadrupole doublets in the host lattice. These anomalies are believed to arise due to antiferroelectric phase transitions driven by a nonlinear anharmonic vibrational motion of oxygen atoms in a breathing mode configuration. The results obtained are indicative of a possible coexistence of high-Tc superconductivity and antiferroelectricity in 1-2-3 ceramics. (orig.)