PHYSICAL REVIEW B VOLUME 45, NUMBER 2 1 JANUARY 1992-II Low-temperature phase transformations in YBa2Cu306+ by anelastic relaxation measurements and possible formation of ferroelectric and antiferroelectric domains G. Cannelli, M. Canali and R. Cantelli Dipartimento di Energetica, Universita di Roma "La Sapienza, " Via A. Scarpa 14, I-00161 Roma, Italy F. Cordero CNR, Istituto di Acustica "O. M. Corbino, " Via Cassia 1216, I-00189 Roma, Italy S. Ferraro and M. Ferretti Istituto di Chimica Fisica, Universita di Genova, I-16132 Genova, Italy F. Trequattrini CNR, Istituto di Acustica "O. M. Corbino, " Via Cassia 1216, I-00189 Roma, Italy (Received 18 July 1991; revised manuscript received 16 September 1991) We report on three phase transformations occurring between 120 and 170 K in YBa2Cu306+„ for x ) 0. 2. It is proposed that one or two of them are due to the ordering of the 0 atoms among the off- center positions in the Cu-0 zigzag chains. Since an electric dipole is associated to 0 ions in such off- center positions, ferroelectric (oxygens on the same side of the chain) and antiferroelectric (oxygens on opposite sides in zigzag fashion) domains should also form. The proposed interpretation correlates well with the overall phenomenology, including an anelastic relaxation process previously interpreted in terms of the nearly uncorrelated jumps of such oxygens. The phase transition around 170 K may corre- spond to a ferroelectric displacive transition appearing in the dielectric function. INTRODUCTION The YBa2Cu306+ ceramic has a rich and mostly unexplored phase diagram in the (x, T) plane, and, due to the unusually high mobility of 0 in the CuO„planes, ' 0 ordering has been observed to take place also at room temperature, drastically influencing the superconductive properties of 0-deficient samples. ' A phase transition occurring around 230 K has been attributed to 0 ordering in the basal planes, because of its coincidence with reversible twin refinements, which imply jumps of the 0 atoms between the 0(1) and 0(5) positions in the CuO„planes. However, it is unlikely that all the observed phase transitions are connected to reordering of 0 between 0(1) and 0(5) positions. This is especially true for those transitions occurring below 200 K in samples with x — 1, where it is presumed that most of the material is in the ortho-I phase and there is little possibility for further ordering. In superconducting YBa2Cu30 7 ferroelectric phase transitions have been revealed at 90 and 170 K by measuring the dielectric constant through the ultrasound-induced rf field. Coexistence of ferroelectri- city, presumably in the CuO„planes, with metallic con- ductivity and superconductivity in the Cu02 planes may be of relevance for the coupling mechanism which leads to superconductivity. On the other side, an anomaly around T, — 110-130K (subscript r stands for "transformation") has been ob- served by acoustic measurements both at kHz (Refs. 8 — 10), and the MHz (Ref. 11), and by a jump in the lat- tice parameters by x-ray diffraction measurements. ' Such an anomaly has been attributed to a martensiticlike phase transition, " and put in close connection with the 92 K superconductivity. " It has been argued that the state below T, is the true normal state which becomes su- perconducting;" moreover, it has been suggested that su- perconductivity could arise from the lattice instability as- sociated with the phase transition at T„similarly to the case of the 315 superconductors. ' Such an anomaly at T, — 120 K has been reported to disappear in concomi- tance with the disappearance of the 92-K superconduc- tivity, and a new elastic anomaly at 70 K has been ob- served to be connected with the 60-K superconductivi- ty. ' Similar results have been obtained regarding the 107-K and 83-K superconductivity in Bi-Sr-Ca-Cu-0 and the 115-K superconductivity in Tl-Ba-Ca-Cu-O, thus pointing to superconductivity induced by a structural in- stability. On the other hand, in most experiments there is no trace of the 110 — 130 K elastic anomaly; moreover, at vi- bration frequencies in the kHZ range at which most ex- periments are conducted, the anomaly occurs at a tem- perature close to that of a thermally activated process, 45 931 1992 The American Physical Society