Comparative Studies of Frustrated Cobaltites YBaCo 4 O 7 + x and DyBaCo 4 O 7 + x with Small Oxygen Nonstoichiometry ( x
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DISORDER, AND PHASE TRANSITION IN CONDENSED SYSTEM
Comparative Studies of Frustrated Cobaltites YBaCo4O7 + x and DyBaCo4O7 + x with Small Oxygen Nonstoichiometry (x ≤ 0.2) Z. A. Kazeia,*, V. V. Snegireva, and M. S. Stolyarenkoa a
Moscow State University, Moscow, 119992 Russia *e-mail: [email protected]
Received February 4, 2020; revised March 24, 2020; accepted April 2, 2020
Abstract—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/ 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 orthorhombic lattice distortions εo = (a – b/ 3)/a = –3.9 × 10–3 and εo = –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. DOI: 10.1134/S1063776120080014
1. INTRODUCTION Since the discovery of the layered YBaCo4O7 + x (where x is the oxygen nonstoichiometry) cobaltite, researchers have synthesized a whole family of isomorphic compounds using cation substitution. These compounds exhibit a variety of interesting physical and structural properties, which are caused by a combination of the mixed valence of cobalt and frustrated triangular and Kagome lattices in a structure. On the whole, layered rare-earth (RE) RBaCo4O7 + x (R = RE ion, Ca or Y) with structure of R114 cobaltites can be used to study the fundamental problems of solid-state physics and magnetism, such as nontrivial ground states, phase transitions and short-range effects in the presence of frustrated and competing exchange interactions, spin crossover, and metal–insulator transition [1–8]. In addition, YBaCo4O7 and its derivatives have a high chemical mobility and exhibit reversible oxygen absorption and desorption [9–11]. Using an ultrahigh pressure during oxygen saturation, the authors of [12] achieved record oxygen excess for YBaCo4O7 + x (x ≈ 1.56); that is, its ability to absorb oxygen is higher than those known for other oxide materials. Therefore, the layered Y cobaltite is a promising material for
practical applications in oxygen storage systems, oxygen s
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