Thermal Expansion Analysis of the Low Temperature Anomalies in Non-Superconducting PrBCO
We report in this work, the linear thermal expansion coefficient α(T ) measured at low temperatures for two nonsuperconducting PrBa<sub>2</sub> Cu<sub>3</sub> O<sub>6+x</sub> ceramics in the tetragonal (x = 0.44) and orthorhombic (x = 0.95) phases. Two transitions are well observed simultaneously in the α(T ) versus T<sup>2</sup> representation below the large anomaly associated with the antiferromagnetic ordering of the Pr sublattice at the Nέel temperature TN,which varies from 9 to 14 K, respectively. The first one is characterized by an inflexion point at T<sup>2</sup> = 6 - 7 and 10 ± 0.5 K, respectively, whereas for the second one, a very narrow and pointed minimum occurs independently of x at the low-critical temperature T<sub>cr</sub>(x) = 4 - 5 K. These facts suggest the appearance of a spin reorientation at T2 which is followed by a reordering of the Pr moments below Tcr, in agreement with the specific heat and magnetic susceptibility data. Additionally, the magnetic contribution due to the Pr ions only to α(T ), α<sub>m</sub>(T ) reveals the existence of positive values with a pronounced minimum just 3 K above the sharp peak at T<sub>N</sub> , for both states. All results are discussed in terms of the weak Pr-Cu coupling and compared with those found in earlier studies.