Papers by Keyword: Structural Phase Transitions

Abstract: It has been found that the temperature dependences of the Young’s modulus for rare-earth cobaltites RBaCo₄O₇ (R = Dy - Er, Y) exhibit significant hysteresis and irreversible effects over a wide range (80–280) K between the structural and magnetic phase transition temperatures. At the magnetic phase transition temperature, there are weak and smoothed anomalies of the Young’s modulus, which correlate with the low dimensionality and frustration of the exchange interactions in the Kagome lattice of the Co subsystem for the studied rare-earth cobaltites.

Abstract: The mechanically activated oxides MnO₂, Mn₂O₃, Mn₃O₄ and MnO were studied under ambient and non-ambient temperature conditions. Regimes of mechanical treatment were found that allowed one to preserve the single-phase state without an essential change of oxide chemical composition. New data about the temperatures and sequence of phase transitions in mechanically activated manganese oxides as well as about the kinetics of isotope exchange were obtained. Magnetic properties of the treated oxides were measured in the temperature range of 4-300К. It is shown that mechano-activation essentially influenced the temperature and field dependences of magnetization, temperatures of magnetic phase transitions and leads to the appearance of additional magnetic phases. New data on the rates of surface reactions during isotope exchange and oxygen self-diffusion coefficients are obtained.

Abstract: Various calcium borohydride samples were investigated by means of combined measurements of thermogravimetry and mass spectrometry, and anelastic spectroscopy. On heating, the release of 2-5% tetrahydrofuran (THF) is detected in all the samples at temperatures below ~480 K, even in those which were previously thermally treated, according to procedures known from the literature, in order to remove the solvent. Dehydrogenation takes place above 480 K. Above room temperature the temperature dependence of the Young modulus of Ca (BH₄)₂ clearly monitors the release of THF and two irreversible structural phase transitions: from the α to the α’ phase around 460 K and from the α’ to the β phase, nearly completely evolved around 590 K. Moreover, the coefficient of elastic energy dissipation presents two dynamic processes below room temperature; a peak around 120 K characterized by an activation energy of 0.20 eV and a pre-exponential factor typical of atom-cluster relaxations, that we attributed to the dynamics of THF molecules retained in the borohydride lattice, and a peak around 200 K, possibly due to the relaxation of H vacancies.

Abstract: An investigation has been undertaken of the structural characteristics of the manganese oxides to understand these characteristics affected by mechanochemical treatment conditions. Chemically pure manganese (II, III) oxides and their mixtures were used as the initial components.

Abstract: In this work with the help of the phenomenological Ginzburg-Landau theory of structural and magnetic phase transitions the phase diagrams of Heusler Ni-Mn-X (X = In, Sn, Sb) alloys with the inversion of exchange interaction are investigated. The investigation shows that the type of the phase diagrams in Heusler Ni-Mn-X (X = In, Sn, Sb) alloys depends on the value and sign of the free energy parameters. As it is seen from the analysis of the phase diagrams with the determined values and signs of the parameters of Landau functional there are thermodynamic paths which allow to explain experimental phase transitions in Heusler alloys Ni-Mn-X (X = In, Sn, Sb) and Ni-Co- Mn-X (X = In, Sn, Sb) qualitatively.

Abstract: Phase diagrams of mixed crystal systems exhibiting the cooperative Jahn–Teller effect are investigated. The competition of Jahn–Teller interaction with a) the preference energy of cation distribution over nonequivalent sublattices or b) stabilization energy of 3d-ion valence configuration is considered. The developed model enables to explain the nature of equilibrium and metastable states, the variety of phase diagrams and its special features in crystals with the competing interactions.