Impact of metal substitutions for cobalt in YBaCo4O7

The “114” YBaCo4O7 cobaltite undergoes structural transition just beyond room temperature at TS∼310 K. Correspondingly, its signature in the physical properties is detected by T-dependent measurements of electrical resistivity, magnetic susceptibility and thermoelectric power. It is found that low-level substitutions of divalent (M=Zn2+) or trivalent (M=Ga3+, Al3+) cations for cobalt according to the YBaCo4−xMxO7 formula with x⩽0.4 have a strong impact upon this transition. On the one hand, Zn2+ substitutions preserve the transition but with TS decreasing as x increases. On the other hand, for x=0.2 Ga3+ or Al3+, the transition is suppressed, i.e., for only 5% trivalent foreign cation substituted for cobalt. Though at first, this contrasted behaviour between divalent and trivalent substituting cations appears to be linked to the opposite evolution of hole carriers “Co3+” concentration with x, a possible destabilization of 3Co2+: 1Co3+ charge ordering induced by the M3+ cations is considered.