Time-dependent structural phenomena at room temperature in quenched YBa"2Cu"3O"6"."4"1 - Local oxygen ordering and superconductivity - Jorgensen, J.D.; Pei, S.; Lightfoor, P.; Shi, H.; Paulikas, A.P.; Veal, B.W. (Elsevier)
The superconducting transition temperature, T"c, of YBa"2Cu"3O"6"."4"1 quenched from 500^oC into liquid nitrogen increases from 0 to 20 K while annealing at room temperature during the first few days following the quench. Using neutron powder diffraction and Rietveld refinement we show that this time-dependent increase of T"c is accompanied by changes in the structural properties. The a-and c-axes contract by 0.04%, while the percentage shortening of the b-axis is three times smaller. We attribute this behavior to charge transfer between the chains and planes, with the smaller contraction along the b-axis arising from oxygen ordering in the chains. Clear evidence for this charge transfer is seen in the relevant Cu-O and Cu-Cu distances. No significant changes in the average occupancies of oxygen sites are observed. We attribute the increase in T"c to local ordering of oxygen atoms around the Cu(1) atoms with no change in the average site occupancies.
http://linkinghub.elsevier.com/retrieve/pii/0921-4534(90)90676-6
Keywords: U