Anomalies in the Young modulus at structural phase transitions in rare-earth cobaltites RBaCo 4 O 7 (R = Y, Tm-Lu)
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Anomalies in the Young Modulus at Structural Phase Transitions in RareEarth Cobaltites RBaCo4O7 (R = Y, Tm–Lu) Z. A. Kazeia, V. V. Snegireva, A. S. Andreenkoa, and L. P. Kozeevab a
Moscow State University, Moscow, 119992 Russia email: [email protected] b Nikolaev Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630090 Russia Received August 11, 2010
Abstract—The elastic properties of rareearth cobaltites RBaCo4O7 (R = Y, Tm–Lu) have been experimen tally studied in the temperature range of 80–300 K. The strong softening of the Young modulus ΔE(T)/E0 ≈ –(0.1–0.2) of cobaltites with Lu and Yb ions has been revealed, which is due to the instability of the crystal structure upon cooling and is accompanied by an inverse jump at the secondorder structural phase transi tion. The softening of the Young modulus and the jump at the phase transition decrease by an order of mag nitude and the transition temperature Ts and hysteresis ΔTs increase from a compound with Lu to that with Tm. A large softening of the Young modulus at the structural transition in Lu and Yb cobaltites indicates that the corresponding elastic constant goes to zero, whereas this constant in Tm cobaltite is not a “soft” mode of the phase transition. It has been found that the structural phase transition in Lu and Yb cobaltites is accom panied by a large absorption maximum at the phase transition point and an additional maximum in the low temperature phase and absorption anomalies in Tm cobaltite is an order of magnitude smaller. DOI: 10.1134/S1063776111070041
1. INTRODUCTION The investigation of compounds that have two and more coupled subsystems and exhibit simultaneously several ordered states (magnetic, quadrupole, charge, ferroelectric, etc.) and phase transitions of various natures is the topical field of solid state physics. It is no wonder, that these systems have an unusual combina tion of physical properties (structural, magnetic, mag netoelastic, elastic, electric, and ferroelectric) and demonstrate new physical phenomena. These com pounds were studied in numerous experimental and theoretical works, results were analyzed and summa rized in reviews [1]. Rareearth cobaltites RBaCo4Ox (x = 7.0 ± δ, 8.0 ± δ), having coupled 3d and 4f mag netic subsystems [2, 3], belong to systems of such type. The family of rareearth cobaltites RBaCo4O7 + δ, which exhibit an unusual magnetic behavior due to the frustration of exchange interactions and mixed valence in the 3d subsystem, has been intensely stud ied for several years [4–6]. At high temperatures, these compounds have the hexagonal structure P63mc [7–9] undergoing small orthorhombic distortions upon cooling. In particular, the orthorhombic distortion in Yb cobaltite is (a – b/ 3 )/a = –6.1 × 10–3 (a ≈ ah, b ≈ 3 ah, ah = bh = 6.26539 Å) [10]. The temperature of the structural phase transition Ts decreases monotoni cally with a decrease in the radius of the rareearth ion, so that the phase transition occurs below 300 K begi
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