The La (Ba2-xLax)Cu3- yOz single crystals : structures and conductivity

The crystal structure of the orthorhombic and tetragonal phases of La(Ba2-xLax)Cu3-yO6+x/2-y+z are determined on twinned crystals. The orthorhombic structure, obtained for low x, is close to the regular Y-Ba-Cu-O type (twin a * b * c-b * a * c), but is highly copper deficient on the Cu(1) site (~ 30 %). The local correlations (03BE ~ 20 Å) between copper atoms and vacancies, as deduced from X-ray diffuse scattering, correspond to a shortrange segregation of vacancies in chains. As a consequence of the large amount of defects, these crystals are non-typical semiconductors. The tetragonal structure, x ~ 0.50, leads to tri-twinned crystals with 90° faulting, a * a * 3 a-a * 3 a * a-3 a * a * a (a, the perovskite lattice constant). In these materials the copper sites are found to be strongly anharmonic. This is due to the disorder introduced by the La-Ba substitution. These crystals are also semiconductors with a T-1/4 activation law for the conductivity which indicates that variable range hopping is expected to set in, a consequence of localization by the disorder. J. Phys. France 50 (1989) 77-90 ler JANVIER 1989,