Distribution of Sr atoms in single crystals of (La 1− xSr x) 2CuO 4−δ and the superconducting transition temperature

Abstract

Ordering of the Sr atom distribution over La sites in single crystals of (La 1− x Sr x ) 2CuO 4−δ has been established by X-ray structural analysis. The ordering can be: full - La(La 0.76Sr 0.24)CuO 3.92, partial - (La 0.94Sr 0.06)(La 0.86Sr 0.14)CuO 4−δ or it can be missing - (La 0.97Sr 0.03) 2CuO 4−δ. The ordering leads to Sr concentration in certain layers of the structure, -La-La-Cu-(La, Sr)-(La, Sr)-Cu-La-La-, and the loss of some oxygen atoms in the same layers. The latter accounts for a lowering of T c in single crystals with an ordered Sr atom distribution. The superconducting transition temperature of (La 1− x Sr x ) 2CuO 4−δ depends not only on the Sr content in a sample but also on its distribution over La sites. That is why the superconducting transition temperature of ceramic samples is, as a rule, higher than that of single crystals with the same Sr content.