Effect of dilution in the cobalt subsystem on phase transitions in rare-earth cobaltites RBaCo_4–xM_xO₇ (R = Dy–Er, Yb, Y; M = Al, Zn)

We have experimentally studied the structural and elastic characteristics of rare-earth cobaltites RBaCo_4–xM_xO₇ (R = Dy–Er, Yb, Y), in which cobalt ions are partly substituted by diamagnetic Al or Zn ions. It was found that small substitution of Co³⁺ ions by Al³⁺ ions in the YbRBaCo_4–xM_xO₇ system (x = 0.1, 0.2, 0.5) leads to a rapid decrease and smearing of ΔE(T)/E₀ anomalies of the Young's modulus in the region of the structural phase transition, which is accompanied by increasing hysteresis. Pure rare-earth cobaltites RBaCo₄O₇ (R = Dy–Er, Y) exhibit a correlation between the room-temperature structure distortion and hysteresis on the ΔE(T)/E₀ curve in a temperature interval of 80–280 K. In Zn-substituted cobaltites RBaCoZn₃O₇, both the hysteresis and ΔE(T)/E₀ anomalies disappear, as do low-temperature sound absorption maxima. This behavior is evidence of the suppression of structural and magnetic phase transitions and the retention of only short-range correlations of the order parameter in Zn-substituted samples.