Suchergebnisse

Using the disordered local moments approach in combination with the ab initio molecular dynamics method, we simulate the behavior of a paramagnetic phase of NiO at finite temperatures to investigate the effect of magnetic disorder, thermal expansion, and lattice vibrations on its electronic structure. In addition, we study its lattice dynamics. We verify the reliability of our theoretical scheme via comparison of our results with available experiment and earlier theoretical studies carried out within static approximations. We present the phonon dispersion relations for the paramagnetic rock-salt (B1) phase of NiO and demonstrate that it is dynamically stable. We observe that including the magnetic disorder to simulate the paramagnetic phase has a small yet visible effect on the band gap. The amplitude of the local magnetic moment of Ni ions from our calculations for both antiferromagnetic and paramagnetic phases agree well with other theoretical and experimental values. We demonstrate that the increase of temperature up to 1000 K does not affect the electronic structure strongly. Taking into account the lattice vibrations and thermal expansion at higher temperatures have amajor impact on the electronic structure, reducing the band gap from similar to 3.5 eV at 600 K to similar to 2.5 eV at 2000 K. We conclude that static lattice approximations can be safely employed in simulations of the paramagnetic state of NiO up to relatively high temperatures (similar to 1000 K), but as we get closer to the melting temperature vibrational effects become quite large and therefore should be included in the calculations. ; Funding Agencies|Swedish Research Council (VR) [2015-04391, 621-2011-4417, 330-2014-6336]; Ministry of Education and Science of the Russian Federation [14.Y26.31.0005, K2-2017-080]; Marie Sklodowska Curie Actions [INCA 600398]; Swedish Foundation for Strategic Research (SSF) through the future research leaders 6 program; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFOMatLiU) [2009 00971]

Beryllium oxides have been extensively studied due to their unique chemical properties and important technological applications. Typically, in inorganic compounds beryllium is tetrahedrally coordinated by oxygen atoms. Herein based on results of in situ single crystal X-ray diffraction studies and ab initio calculations we report on the high-pressure behavior of CaBe2P2O8, to the best of our knowledge the first compound showing a step-wise transition of Be coordination from tetrahedral (4) to octahedral (6) through trigonal bipyramidal (5). It is remarkable that the same transformation route is observed for phosphorus. Our theoretical analysis suggests that the sequence of structural transitions of CaBe2P2O8 is associated with the electronic transformation from predominantly molecular orbitals at low pressure to the state with overlapping electronic clouds of anions orbitals. ; Funding Agencies|Ministry of Science and High Education of the Russian Federation [K2-2019-001]; Russian Foundation for Basic Research [19-02-00871]; Swedish Research Council (VR) [2015-04391]; Swedish Government Strategic Research Areas in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009-00971]; SeRC

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 ...