Doping induced insulator-metal transition in yttrium cuprate perovskite

Full text: Structure and electronic phase transitions in perovskite phase of yttrium cuprate ceramics were studied by means of X-ray-diffraction and electric resistance measurements so as to reveal possible doping induced insulator-metal transition. Non-stoichiometric perovskite phase of YBa2Cu3O8.94 was sintered and described in the highly symmetric space group of Pm 3m with the lattice parameter ac = 0.4053 nm. The value of resistance R was found independent on temperature in the range of 77-300 K and much less than that typical for dielectrics. During the isothermal annealing at 500 K the perovskite phase transformed into the superconducting phase of the optimal composition YBa2Cu3O7-x (x=0.06 - 0.08) with the transition temperature of Tc = 91 K, above which the R(T) dependence had a metallic character. The cubic-orthorhombic transition happened due to decreasing of the oxygen content, ordering of yttrium and barium cations in the metallic sub-lattice and tripleting of the original cubic cell along c-axis. Under the thermally stimulated transition there occurred the formation of the BaCO3 impurity phase of 2 %. While 60Co-gamma-irradiation of the perovskite samples stimulated the formation of optimally doped superconducting grains covered with a layer of the underdoped ortho-phase. The observed insulator-metal transition is caused by oxygen depletion and atomic ordering in yttrium and barium sublattices, i.e. induced by doping. The carrier localization is driven by cation disorder. The work is supported by grant 3505 from the international STCU fund. (authors)