Comparative Studies of Frustrated Cobaltites YBaCo₄O_{7 +x} and DyBaCo₄O_{7 +x} with Small Oxygen Nonstoichiometry (x ≤ 0.2)

Comparative experimental studies of the structural and elastic characteristics of the YBaCo₄O_{7 +x} and DyBaCo₄O_{7 +x} (x = 0, 0.1, 0.2) cobaltite systems reveal their different behavior depending on excess oxygen content x. A relation between structural parameters a_av = (a + b/$\sqrt{3}$)/2 and c and index x is determined, and the nonstoichiometric cobaltites are found to be separated into two phases with different values of x under certain synthesis and heat-treatment conditions. Stoichiometric samples with Y and Dy ions have ortho-rhombic lattice distortions ε_o = (a – b/$\sqrt{3}$)/a = –3.9 × 10^–3 and ε_o = –4.1 × 10^–3, respectively. These distortions remove frustrations and promote the formation of a long-range magnetic order in the cobalt subsystem. The Y cobaltite exhibits pronounced anomalies of the elastic properties at the magnetic phase transition temperature T_N, and only their traces are detected in the Dy cobaltite. The structural distortion disappears in the samples of both series at a small oxygen nonstoichiometry, the frustrations are retained, the further development of a long-range magnetic order is hindered, and the anomalies of the elastic properties at T_N are rapidly smeared and disappear. The qualitatively different behavior of the DyBaCo₄O_{7 +x} series is likely to be related to the rare-earth-ion-induced magnetic anisotropy.