Oxygen nonstoichiometry, structures, and physical properties of YBaCo2O5+x (0.00 ≤ x ≤ 0.52)

We synthesized YBaCo2O5+x with various oxygen contents x and investigated physical and structural properties. YBaCo2O5+x can be classified into three parts of composition region in the structural properties. Two kinds of superstructure, 2 x 1 x 1 and 3 x 3 x 1, originating in the ordering of excess oxygen in Y(O)(x) layer appear in the range 0.50 less than or equal to x and 0.25 less than or equal to x less than or equal to 0.44, respectively. The 2 x 1 x 1 phase shows the metal-insulator transition accompanied with the large lattice distortion and the magnetic transition. The 3 x 3 x 1 phase (stoichiometric composition, x = 0.44) also shows some magnetic and structural transitions. A plausible model of the 3 x 3 x 1 superstructure is proposed. In the range 0.00 less than or equal to x less than or equal to 0.19, YBaCo2O5+x has no longer superstructure. YBaCo2O5+x in x = 0.00 shows successive phase transitions, a paramagnetic tetragonal to antiferromagnetic orthorhombic transition, and a charge order transition. The present study reveals a simultaneous phase separation with the charge order transition. (C) 2001 Academic Press.