Suchergebnisse

Using ab initio calculations, we have analyzed the influence of anharmonic effects on the electronic structure and the phonon-dispersion relations of body-centered-cubic (bcc) niobium (Nb) and investigated the temperature dependence of the Kohn anomaly in this metal. A comparison of the results obtained in the framework of the temperature-dependent effective potential method with those derived within the quasiharmonic approximation demonstrates the importance of the explicit treatment of the finite-temperature effects upon the theoretical description of bcc Nb lattice dynamics. In agreement with experimental results, the inclusion of anharmonic vibrations in our calculations leads to the disappearance of the Kohn anomaly for the acoustic mode in a vicinity of the Gamma point with increasing temperature. Moreover, the calculated phonon self-energy indicates that the origin of the temperature dependence is related to the change of the electronic structure. We have calculated the temperature dependence of the electronic spectral function and analyzed the Fermi surface of Nb. A significant temperature-induced smearing of the electronic states has been identified as the origin of the disappearance of the Kohn anomaly in Nb at elevated temperature. ; Funding Agencies|Swedish Research Council (VR)Swedish Research Council [2015-04391, 2019-05551]; Swedish Government Strategic Research Areas in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009-00971]; Knut and Alice Wallenberg Foundation (Wallenberg Scholar Grant) [KAW-2018.0194]; Swedish Foundation for Strategic Research (SSF)Swedish Foundation for Strategic Research [EM16-0004]; Russian Science FoundationRussian Science Foundation (RSF) [18-12-00492]

We develop a method to accurately and efficiently determine the vibrational free energy as a function of temperature and volume for substitutional alloys from first principles. Taking Ti1-xAlxN alloy as a model system, we calculate the isostructural phase diagram by finding the global minimum of the free energy corresponding to the true equilibrium state of the system. We demonstrate that the vibrational contribution including anharmonicity and temperature dependence of the mixing enthalpy have a decisive impact on the calculated phase diagram of a Ti1-xAlxN alloy, lowering the maximum temperature for the miscibility gap from 6560 to 2860 K. Our local chemical composition measurements on thermally aged Ti0.5Al0.5N alloys agree with the calculated phase diagram. ; Funding Agencies|Swedish Foundation for Strategic Research Programs (Stiftelsen for Strategisk Forskning) [SRL10-0026, RMA08-0069]; FUNCASE; Swedish Research Council (Vetenskapsradet) [2012-4401, 621-2011-4426, 2015-04391, 637-2013-7296, 621-2011-4417, 330-2014-6336]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009 00971]; Swedish Governmental Agency for Innovation Systems (Vinnova) through the M-ERA.net project MC2; SECO Tools AB; Marie Sklodowska Curie Actions, Cofund [INCA 600398]; Erasmus Mundus Joint European Doctoral Programme DocMASE; Ministry of Education and Science of the Russian Federation [K2-2016-013]; Deutsche Forschungsgemeinschaft; Federal State Government of Saarland [INST 256/298-1 FUGG]

Theoretical modelling predicts very unusual structures and properties of materials at extreme pressure and temperature conditions(1,2). Hitherto, their synthesis and investigation above 200 gigapascals have been hindered both by the technical complexity of ultrahigh-pressure experiments and by the absence of relevant in situ methods of materials analysis. Here we report on a methodology developed to enable experiments at static compression in the terapascal regime with laser heating. We apply this method to realize pressures of about 600 and 900 gigapascals in a laser-heated double-stage diamond anvil cell(3), producing a rhenium-nitrogen alloy and achieving the synthesis of rhenium nitride Re7N3-which, as our theoretical analysis shows, is only stable under extreme compression. Full chemical and structural characterization of the materials, realized using synchrotron single-crystal X-ray diffraction on microcrystals in situ, demonstrates the capabilities of the methodology to extend high-pressure crystallography to the terapascal regime. ; Funding Agencies: Alexander von Humboldt Foundation; Deutsche Forschungsgemeinschaft (DFG) [LA-4916/1-1, BY112/2-1, DU 954-11/1, DU 393-9/2, DU 393-13/2]; Federal Ministry of Education and Research, Germany (BMBF) [05K19WC1]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University (Faculty Grant SFO-Mat-LiU) [200900971]; National Science Foundation -Earth Sciences [EAR -1634415]; DOE Office of Science [DE-AC02-06CH11357]; Russian Science Foundation [18-12-00492]; Knut and Alice Wallenberg Foundation (Wallenberg Scholar grant) [KAW-2018.0194]; Swedish Government Strategic Research Areas in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009 00971]; Swedish e-science Research Center (SeRC); Swedish Research Council (VR) [2019-05600]; Swedish Foundation for Strategic Research (SSF) [EM160004]; Swedish Research Council [2016-07213]; VINN Excellence Center Functional Nanoscale ...