Fig. 8-2 Initial Nb mean valence (n) vs Nb/A ratio (r) (A=Ca,Sr) in the starting mixtures. Open and half-filled symbols represent the superconducting and nonsuperconducting phases,respectively. The hatched area shows the highest shielding-effect-producing region.

Superconductors are materials with characteristics that include diamagnetism and zero electrical resistance at low temperatures, that is, below the critical temperature (Tc). New superconductors with Tc values above 30K were discovered in the materials having copper oxide systems in 1986. Since then, Tc values have reached 130 ~ 160K in copper oxide system superconductors. However, except Ba(K)BiO3 (Tc=35K), no noncopper oxide systems have been discovered. On the other hand, Nb is one of the more active elements for super- conductors and many Nb compounds (Nb, NbC, NbN, NbSe2, Nb3Sn(Ge)) are utilized as practical superconductors at present. If Nb oxide systems exhibited superconductive behavior, the Tc might be increased significantly. Only a few Nb oxide substances, NbOx (Tc=1.6K) and LixNbO2 (Tc=6K), however have been found. In this context, a series of new reduced niobate superconductors with the distorted defect-perovskite structure (D.P) A2Nb1+xOy (A=Ca,Sr) has been discovered. In usual superconductors of oxide systems, including copper oxide systems, superconductive characteristics are generally represented at the extremely limited narrow range of cation valence (for instance, Cu1.75+ - 2.3+) and composition. However, the new reduced niobate superconductors show contrasting behavior to the former. As seen in Fig.8-2; the superconducting phases are extended over broad ranges of valence and composition (Nb valence; Nb5+ - 1+, Nb/A ratio; 0.5 ~ 3) in both systems (Ca and Sr), the region forming the highest superconducting phase exists in strongly reduced Nb valences like Nb1+ - 2+, in the both systems, Ca and Sr, the value of Tc is 9.2K on the Nb-deficient side of Nb/A=1 and 8.3K on Nb-excess side. This discovery strongly suggests the possibility of creating attractive new Nb oxide super-conductors that contain no Cu but are practical super-conductivematerials.

Reference A. Nakamura, New Reduced Nlobate Superconductors A2Nb1+xOy(A=Ca and Sr) with the Distorted Defect Perovskite Structure, Jpn. J. Appl. Phys. 33, L583 (1994).

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