INFLUENCE OF THE SIZE OF ALKALINE-EARTH CATIONS ON THE STRUCTURAL DISTORTION AND TRANSPORT-PROPERTIES OF THE SEMICONDUCTING 1212-TYPE CUPRATES TLBA2-XSRXLNCU2O7

The systematic X.R.D. and E.D. study of the oxide TlBa2-xSrxLnCu2O7 (Ln = Y or Nd) for 0 less-than-or-equal-to x less-than-or-equal-to 2 has been performed. It is shown that the presence of strontium induces a distortion of the ideal tetragonal cell ''3.9 x 3.9 x 12 angstrom3'', leading to a supercell ''3.9 square-root 2 x 3.9 square-root 2 x 24 angstrom3'' and the existence of diffuse streaks. The H.R.E.M. investigation shows that this effect is correlated with a local perturbation of the rock-salt type layers ''SrO-TlO-SrO''. These materials are semiconductors, with a resistivity decreasing as the strontium content increases. The latter behaviour is interpreted as a modification of the hybridization of the copper oxygen orbitals induced by the replacement of barium by strontium. The effect of oxygen pressure annealing on the transport properties is also demonstrated leading, for pure strontium phases, to lower activation energies, which may be due to tiny deviations from thallium and oxygen stoichiometry.