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Distortions and stabilization of simple-cubic calcium at high pressure and low temperature
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2010 (English)In: Proceedings of the National Academy of Sciences of the United States of America, ISSN 0027-8424, E-ISSN 1091-6490, Vol. 107, no 22, 9965-9968 p.Article in journal (Refereed) Published
Abstract [en]

Ca-III, the first superconducting calcium phase under pressure, was identified as simple-cubic (sc) by previous X-ray diffraction (XRD) experiments. In contrast, all previous theoretical calculations showed that sc had a higher enthalpy than many proposed structures and had an imaginary (unstable) phonon branch. By using our newly developed submicrometer high-pressure single-crystal XRD, cryogenic high-pressure XRD, and theoretical calculations, we demonstrate that Ca-III is neither exactly sc nor any of the lower-enthalpy phases, but sustains the sc-like, primitive unit by a rhombohedral distortion at 300 K and a monoclinic distortion below 30 K. This surprising discovery reveals a scenario that the high-pressure structure of calcium does not go to the zero-temperature global enthalpy minimum but is dictated by high-temperature anharmonicity and low-temperature metastability fine-tuned with phonon stability at the local minimum.

Place, publisher, year, edition, pages
2010. Vol. 107, no 22, 9965-9968 p.
Keyword [en]
lattice-dynamics, phase, ca, vanadium, metals, sodium
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:kth:diva-24635DOI: 10.1073/pnas.1005279107ISI: 000278246000013Scopus ID: 2-s2.0-77953466588OAI: oai:DiVA.org:kth-24635DiVA: diva2:352247
Note
QC 20100920Available from: 2010-09-20 Created: 2010-09-20 Last updated: 2010-09-20Bibliographically approved
In thesis
1. Theoretical Investigations of Compressed Materials
Open this publication in new window or tab >>Theoretical Investigations of Compressed Materials
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The use of high pressure as a tool to design new materials as well as to investigatematerials properties has become increasingly important during last one decade. The maingoal of the present thesis is to enhance the significance of the high pressure method as aquantitative tool in solid state investigations. Virtually all of the properties of solids aredirectly determined by their electronic structure. Similarly, the changes in the propertiesof solids under pressure are determined by the changes in the electronic structure underpressure. We have attempted to provide a comprehensive description of the resulting theoryin a electronic structure and the properties of condensed matter.

The theoretical basis for these investigations is the density functional theory, in combinationwith ab initio method. The study of pressure induced phase transitions for thecompounds of CaF2, Cr2GeC, Ti3SiC2, as well as V at 0 K are presented. The latticeparameters, the phase transition pressures, the equation of states, the electronic structureshave been calculated and shown a good agreement with experimental results.

A lattices dynamic study of the body center cubic (bcc) Fe under high pressure andhigh temperature is presented. The bcc iron could dynamical stabilize in the Earth innercore conditions. The unusual phase transition of bcc V under high pressure is investigatedand it is shown that the driving mechanism is electron-phonon interaction.

Finally, a method based on the LDA+U approach has been applied to study spin statetransition in FeCO3. Our results show that magnetic entropy play a significant role in spinstate transition.

Place, publisher, year, edition, pages
Stockholm: KTH, 2010. 34 p.
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-24641 (URN)9789174157352 (ISBN)
Public defence
2010-10-08, Sal D3, Lindstedtsvägen 5, KTH, Stockholm, 10:00
Opponent
Supervisors
Note
QC 20100920Available from: 2010-09-20 Created: 2010-09-20 Last updated: 2010-09-22Bibliographically approved

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