Papers by Keyword: Phase Transition

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Authors: Sergey M. Zharkov, Roman R. Altunin, Evgeny T. Moiseenko, Galina M. Zeer, Sergey N. Varnakov, Sergey G. Ovchinnikov
Abstract: Solid-state reaction processes in Fe/Si multilayer nanofilms have been studied in situ by the methods of transmission electron microscopy and electron diffraction in the process of heating from room temperature up to 900ºС at a heating rate of 8-10ºС/min. The solid-state reaction between the nanolayers of iron and silicon has been established to begin at 350-450ºС increasing with the thickness of the iron layer.
Authors: Mikhail A. Zagrebin, Vladimir V. Sokolovskiy, Vasiliy D. Buchelnikov
Abstract: In this work the magnetic properties (exchange parameters, magnetic moment of non-stoichiometric Ni–Mn–Ga Heusler alloys with structural disorder by the help of ab initio calculations have been performed. Theoretical composition dependences of the total magnetic moment are in qualitative agreement in qualitative agreement with experimental data. Calculations of exchange parameters show that interactions between the Mn atoms in regular positions and Mn atoms in Ni and Ga positions are antiferromagnetic.
Authors: Archana Singh, Mahendra Aynyas, S.P. Sanyal
Abstract: We report a first principles calculation of pressure-induced structural phase transition properties of uranium chalcogenides (UX; X=S, Se and Te). The total energies as a function of volume are obtained by means of self-consistent tight binding linear muffin orbital method (TB-LMTO) by performing spin and non-spin polarized calculations to determine the magnetic and structural stabilities. From the present study, we predict a magnetic phase transition from ferromagnetic (FM) to non-magnetic (NM) state around 67.7 and 10.2 GPa for US and USe, respectively. The pressure-induced magnetic transitions are found second-order in nature. We have also predicted structural phase transition from FM-NaCl-type (B1 phase) structure to NM-CsCl-type (B2 phase) structure at around 77.5, 23.5 for US and USe, respectively, while UTe undergoes from FM-B1 to FM-B2 phase around 12.0 GPa.
Authors: Hiromi Nakano, Nobuo Ishizawa, Hirohisa Sato, Naoki Kamegashira
Abstract: The BaLn2Mn2O7 (Ln = rare earth) has a Sr3Ti2O7-type structure with double block oxygen octahedra belonging to the Ruddlesden-Popper-Type homologous series AO(ABO3)2. In-situ measurement of the phase transition for BaLn2Mn2O7 was performed using single-crystal X-ray diffraction and a high-temperature transmission electron microscope (TEM). Two types of transitions were observed in BaPr2Mn2O7: the transition from primitive tetragonal (P42/mnm) to body-centered tetragonal (I4/mmm) at around 400 K and the first-order phase transition at around 1040 K. Multiple phase transitions were also observed in BaEu2Mn2O7, with one from P42/mnm to I4/mmm at around 400 K and another, above 550 K, as a first-order phase transition. The high-temperature phase had a 1.5% lattice mismatch along the c-axis compared with the low-temperature phase. We succeeded in recording for the first time in-situ structural change in BaGd2Mn2O7 as a movie by high-temperature TEM. The high-temperature phase nucleated parallel to the (00l) plane as a layer above 550 K and grew until covering the entire inspected region at around 1023 K. The first-order phase transition was caused by the structural and/or electrical distortion of the layered perovskite structure composed of Jahn-Teller ion Mn3+.
Authors: Radosław Zaleski, Jacek Goworek
Abstract: The ordering of n-nonadecane confined in the pores of MCM-41 silica was investigated by Positron Annihilation Lifetime Spectroscopy. No irradiation effects caused by the positron source were observed. The ortho-positronium components found in the lifetime spectra correspond to the free volumes observed in the bulk alkane plus additional space at the interface between n-nonadecane and the silica wall. Phase transitions rigid-rotator and rotator-liquid were shifted towards lower temperatures by 28 and 16 degrees respectively.
Authors: A.I. Harkunov, V.E. Antonov, O.I. Barkalov, M. Calvo-Dahlborg, U. Dahlborg, V.K. Fedotov, E.G. Ponyatovsky, M. Winzenick
Authors: Lei Zhang, Sheng Li Jiang, Jun Chen
Abstract: The issue of HMX phase transition under hydrostatic compression is not clear and experiments show conflicting results. Effective solution via first-principles simulation is challenged by difficulty of accurate prediction of Van der Waals interaction, which exists ubiquitously and is crucial for determining the structure of molecules and condensed matter. We have contributed to this by constructing a set of pseudopotentials and pseudoatomic orbital basis, specialized for molecular systems with C/H/N/O elements. The reliability of the method is verified from the interaction energies of 45 complexes (comparing to the results of coupled cluster with singles and doubles (Triple) (CCSD)(T)) and the crystalline structures of 7 typical explosives (comparing to experiments). Using this method, we complete the phase diagram of HMX under static compression up to 50 GPa. We make it clear that no β→δ/ε→δ phase transition occurs at 27 GPa, which has long been a hot debate in experiments. A possible γ→β phase transition is found at around 2.10 GPa in the environment of vapour. We have also predicted the equation of states for α-, δ-, and γ-HMX, which are experimentally absent.
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