ION-SIZE EFFECTS ON THE STRUCTURE AND SUPERCONDUCTIVITY OF LA-214 COMPOUNDS AT 1 8 DOPING/

A structural transition to a low-temperature tetragonal (TLT) phase oc curs below T(d2) almost-equal-to 60 K in La2-xBaxCuO4 only around x = 0.125, accompanied by sharp suppression of superconductivity. In this study we investigated the effects of partial substitution of smaller d ivalent cations for Ba2+ ions (Ba2+ > Sr2+ > Ca2+) on these anomalous low-temperature properties. The suppression of superconductivity is st rongly affected by the appearance of the TLT phase for this carrier co ncentration. We also show that the variations of T(d2) and T(c) are qu antitatively scaled by the averaged size of cations at the La site. Th is dependence can be interpreted as chemical effects acting effectivel y as applied pressure. We discuss how our observation is related to re cent reports which claim that the TLT structure itself does not necess arily destroy the superconductivity.