PHYSICAL CHARACTERIZATION OF PURE AND SUBSTITUTED TETRAGONAL LaBaCaCu3O7-δ SUPERCONDUCTOR

Abstract

The high T c superconductor, forming the focus of the present review, presents an interesting situation where, despite its oxygen content close to 7.0, the system prefers to crystallize in tetragonal structure, as distinct from exhaustively studied orthorhombic RE:123 ( REBa 2 Cu 3 O 7, RE = rare earth) compounds. Its optimum T c is decided by the cation intermixing at various sites, unlike that in case of normal RE:123, where all cationic sites are fully occupied and fixed, leaving the anionic sites free for oxygen filling. In this, we review the rich crystal chemistry of this compound and discuss it in conjunction with its unusual superconducting properties. The article is divided in five parts: (1) first we discuss variously reported interesting finer structural details and superconductivity of pristine LaBaCaCu 3 O 7-δ (La:1113) superconductor. The La, Ba and Ca sites are found to be intermixed with respect to their nominal sites of the standard RE:123 structure. (2) We examine the structural aspects and superconductivity of La 1-x RE x BaCaCu 3 O 7 (RE = Nd, Dy, Sm and Pr). Unlike RE:123 and 124 compounds, the RE:1113 compounds form only with light rare earths such as La, Pr and Nd. Pr:1113 compound is found to be an insulator with Pr moments ordering magnetically at around 8 K, unlike at 17 K for Pr:123. (3) Studies of Pr substitution reveal that relative T c depression due to Pr in the (La,Nd) 1-x Pr x BaCaCu 3 O 7 system is less in comparison to that found for La or Nd site Pr substituted (La,Nd) 1-x Pr x Ba 2 Cu 3 O 7. (4) The results of superconductivity and structural details are reviewed for LaBaCaCu 3-x M x O 7 (M = Fe, Co, Ni, Ga and Zn) and finally (5) we summarize our conclusions. The results in terms of T c depression due to these impurities in the parent La:1113 system are seen as similar to those observed for RE:124 ( REBa 2 Cu 4 O 8) and Nd:214 ( Nd 2-x Ce x CuO 4), but are different from RE:123 and La:214. The results of structural details, superconductivity and magnetic ordering presented in the four parts for La 1-x RE x BaCaCu 3-y M y O 7 (RE = Nd, Dy, Sm, Pr with 1.0 ≥ x ≥ 0.0, and M = Fe, Co, Ni, Ga and Zn with 0.24 ≥ y ≥ 0.0 were obtained from X-ray and neutron diffraction and using AC and DC magnetic susceptibility, electrical resitivity and heat capacity data as a function of temperature. This review is mainly a consolidated work of the authors, including their collaboration from various laboratories and the reported data of other workers, all being duly acknowledged in the text.