Praseodymium and high-temperature superconductivity: Thermodynamic, structural, and critical correlations
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Praseodymium and high-temperature superconductivity: Thermodynamic, structural, and critical correlations V. E. Lamberti, M. A. Rodriguez, J. D. Trybulski, and A. Navrotsky Princeton Materials Institute and the Department of Geosciences, Princeton University, Princeton, New Jersey 08544 (Received 21 September 1995; accepted 15 January 1996)
The enthalpies of formation and the partial molar enthalpies of oxidation of polycrystalline LnBa2 Cu3 Oy (Ln Pr, Nd, Eu, Gd, Dy, Ho, Tm) and Y12x Pr x Ba2 Cu3 Oy (x 0.0, 0.1, 0.2, 0.5, 0.8, 0.9, 1.0) have been determined at 298 K by drop-solution calorimetry. The thermodynamic characteristics of Pr123 follow the trends of the trivalent-ion-based Ln123 compounds. The thermodynamic data for the (Y, Pr)123 solid solutions show nonideal solution behavior, but no x-dependent valence instability. The superconducting critical temperatures and the enthalpies of oxidation of the (Y, Pr)123 solid solutions are linearly related.
PrBa2 Cu3 Oy (Pr123) is distinguished from the other lanthanide derivatives of YBa2 Cu3 Oy (Ln123) by a variety of physical and structural anomalies. The most noted physical anomaly is certainly the absence of any superconducting phase,1 but also characteristic of Pr123 are an exceptionally high temperature (17 K) for lanthanide antiferromagnetic ordering,2 a heavy-fermion-like specific heat,3 and enhanced magnetic-transition widths in inelastic neutron scattering spectra.4 The structural irregularities include a rare-earth polyhedral volume significantly smaller than that of Nd1235 and subtle contractions in the Cu(2) – O(4)5–7 and Pr – O(2)8 bond lengths. [The apical, or bridging, oxygen is designated O(4) in our notation.] The implications of these features for the mechanism of high-Tc superconductivity, as well as for the design of practical devices such as nearly lattice-matched S-I-S junctions, have made Pr123 the subject of a great number of investigations.9 Discussions of the structure and physics of Pr123 are mainly framed in terms of the formal valence of the Pr ion (31, intermediate, or 41).4,5,9–11 Models based on a formal Pr valence at or near 3.0 generally rely upon hybridization between the Pr 4 f states and the conduction band. Arguments that favor tetravalent Pr generally assume that the additional dissociated electron of this ion is affiliated with the copper-oxygen planes, where superconductivity is destroyed by electron-hole recombination. The hybridization picture appears to be supported by the majority of experiments and calculations,9,12 although it must also be admitted that the superconducting critical temperature increases slightly with lanthanide radius,13 and that the effective ionic radii of Pr are only about 1% larger than those of Nd for all common coordination numbers.14 The lanthanide ion is almost certainly trivalent in the Nd, Gd, Ho, Er, and Yb derivatives.15 Experimental efforts have very often centered on the solid solutions R12x Pr x Ba2 Cu3 Oy , in which R is Y or, less comm
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