Comparative Studies of Frustrated Cobaltites YBaCo4O7 +x and DyBaCo4O7 +x with Small Oxygen Nonstoichiometry (x ≤ 0.2)

Comparative experimental studies of the structural and elastic characteristics of the YBaCo4O7 + x and DyBaCo4O7 + x (x = 0, 0.1, 0.2) cobaltite systems reveal their different behavior depending on excess oxygen content x. A relation between structural parameters aav = (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 eo = (a – b/ $$\sqrt 3 $$ )/a = –3.9 × 10–3 and eo = –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 TN, 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 TN are rapidly smeared and disappear. The qualitatively different behavior of the DyBaCo4O7 + x series is likely to be related to the rare-earth-ion-induced magnetic anisotropy.