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Theoretical Investigations of Compressed Materials
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
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: urn:nbn:se:kth:diva-24641ISBN: 9789174157352 (print)OAI: oai:DiVA.org:kth-24641DiVA: diva2:352371
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
List of papers
1. Dynamical stability of body center cubic iron at the Earth's core conditions
Open this publication in new window or tab >>Dynamical stability of body center cubic iron at the Earth's core conditions
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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, 9962-9964 p.Article in journal (Refereed) Published
Abstract [en]

Here, using self-consistent ab initio lattice dynamical calculations that go beyond the quasiharmonic approximation, we show that the high-pressure high-temperature bcc-Fe phase is dynamically stable. In this treatment the temperature-dependent phonon spectra are derived by exciting all the lattice vibrations, in which the phonon-phonon interactions are considered. The high-pressure and high-temperature bcc-Fe phase shows standard bcc-type phonon dispersion curves except for the transverse branch, which is overdamped along the high symmetry direction Gamma-N, at temperatures below 4,500 K. When lowering the temperature down to a critical value T-C, the lattice instability of the bcc structure is reached. The pressure dependence of this critical temperature is studied at conditions relevant for the Earth's core.

Keyword
inner-core, high-pressures, bcc phase, temperature, fe, transformation, lattice, alloy, zr
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-24607 (URN)10.1073/pnas.1004076107 (DOI)000278246000012 ()2-s2.0-77953427338 (Scopus ID)
Note
QC 20100920Available from: 2010-09-20 Created: 2010-09-20 Last updated: 2017-12-12Bibliographically approved
2. Distortions and stabilization of simple-cubic calcium at high pressure and low temperature
Open this publication in new window or tab >>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.

Keyword
lattice-dynamics, phase, ca, vanadium, metals, sodium
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-24635 (URN)10.1073/pnas.1005279107 (DOI)000278246000013 ()2-s2.0-77953466588 (Scopus ID)
Note
QC 20100920Available from: 2010-09-20 Created: 2010-09-20 Last updated: 2017-12-12Bibliographically approved
3. Electronic and elastic properties of CaF2 under high pressure from ab initio calculations
Open this publication in new window or tab >>Electronic and elastic properties of CaF2 under high pressure from ab initio calculations
2009 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 21, no 41Article in journal (Refereed) Published
Abstract [en]

Calcium fluoride CaF2 has been studied by using density functional theory (DFT) with the generalized gradient approximation (GGA). Our results demonstrate that the sequence of the pressure-induced structural transition of CaF2 is the fluorite structure (Fm (3) over barm), the orthorhombic cotunnite-type structure (Pnma), and the hexagonal Ni2In-type structure (P6(3)/mmc). The two transitions occur at pressures of 8 GPa and 105 GPa, accompanied by volume collapses of 8.4% and 1.2%, respectively. The energy band gap increases with pressure in the (Fm (3) over barm) and the forepart of Pnma phases. However, on increasing the pressure beyond 60 GPa, the gap decreases, which is due to the fluorine p(z)-states shifting toward the Fermi energy. In addition, the elastic properties versus pressure are also discussed. Our calculated elastic constants for the cubic phase at ambient pressure are in agreement with the experimental values. The stress-strain coefficient calculations show that shear transformations in the Pnma phase are more difficult than in the cubic phase and the compressibility along the c(h) (or a(o)) direction for the orthorhombic phase is stronger than that in the hexagonal crystal.

Keyword
augmented-wave method, optical-properties, fluorite caf2, phase, crystal, transformation, constants, solids
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-18787 (URN)10.1088/0953-8984/21/41/415501 (DOI)000270110000009 ()2-s2.0-70349331505 (Scopus ID)
Note

QC 20100525

Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved
4. Substitutional alloy of Ce and Al
Open this publication in new window or tab >>Substitutional alloy of Ce and Al
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2009 (English)In: Proceedings of the National Academy of Sciences of the United States of America, ISSN 0027-8424, E-ISSN 1091-6490, Vol. 106, no 8, 2515-2518 p.Article in journal (Refereed) Published
Abstract [en]

The formation of substitutional alloys has been restricted to elements with similar atomic radii and electronegativity. Using high-pressure at 298 K, we synthesized a face-centered cubic disordered alloy of highly dissimilar elements (large Ce and small Al atoms) by compressing the Ce3Al intermetallic compound > 15 GPa or the Ce3Al metallic glass > 25 GPa. Synchrotron X-ray diffraction, Ce L-3-edge absorption spectroscopy, and ab initio calculations revealed that the pressure-induced Kondo volume collapse and 4f electron delocalization of Ce reduced the differences between Ce and Al and brought them within the Hume-Rothery (HR) limit for substitutional alloying. The alloy remained after complete release of pressure, which was also accompanied by the transformation of Ce back to its ambient 4f electron localized state and reversal of the Kondo volume collapse, resulting in a non-HR alloy at ambient conditions.

Keyword
4f electron delocalization, Ce-Al solid solution alloy, high pressure, Hume-Rothery rules, metallic glass, high-pressure, metallic glasses, crystal-structure, room-temperature, alpha transition, cerium metal, behavior, approximation, potassium, chemistry
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-18204 (URN)10.1073/pnas.0813328106 (DOI)000263652900011 ()2-s2.0-62449280994 (Scopus ID)
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved
5. Magnetic Fe(n+1)AC(n) (n=1, 2, 3, and A = Al, Si, Ge) phases: from ab initio theory
Open this publication in new window or tab >>Magnetic Fe(n+1)AC(n) (n=1, 2, 3, and A = Al, Si, Ge) phases: from ab initio theory
2008 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 20, no 6Article in journal (Refereed) Published
Abstract [en]

We have investigated the structural stability and magnetism for a set of compounds Fe(n+1)AC(n) (n = 1, 2, 3, and A = Al, Si, Ge) using ab initio theory. From our calculation, we have shown that some Fe(n+1)AC(n) phases (n = 2) with the general MAX phase formula and a layered hexagonal structure that belongs to space group D-6h(4)-P6(3)/mmc can have a combination of properties of the MAX phase at the same time as having magnetism. The Fe3AlC2 phase shows the most stable ferromagnetic properties among these MAX phases and the magnetic moment is 0.73 mu(B)/Fe atom. In addition, the phase stability is predicted by comparing the total energy of the Fe2AlC and Fe2SiC phases with the total energy of the competing equilibrium phases at the corresponding composition.

Keyword
C alloys, spectroscopy, nitride, TI3SIC2
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-24642 (URN)10.1088/0953-8984/20/6/064217 (DOI)000252927300018 ()
Note
QC 20100920Available from: 2010-09-20 Created: 2010-09-20 Last updated: 2017-12-12Bibliographically approved
6. NANOLAYERED MAX PHASES FROM ab initio CALCULATIONS
Open this publication in new window or tab >>NANOLAYERED MAX PHASES FROM ab initio CALCULATIONS
2008 (English)In: International Journal of Modern Physics B, ISSN 0217-9792, Vol. 22, no 25-26, 4495-4499 p.Article in journal (Refereed) Published
Abstract [en]

The advancement in new materials processing and fabrication techniques has made it possible to better control the atomistic level of structures in a way, which was not feasible only a decade ago. If one can couple this atomic control with a good understanding of the relationship between structure and properties, this will in the future lead to a significant contribution to the synthesizing of tailor-made materials. In this paper we have focused on, the structurally related nanolayered ternary compounds M(N+1)AX(N), (MAX) where N = 1, 2 or 3, M is an early transition metal, A is an A-group (mostly IIIA and IVA) element, and X is either C and/or N, which has attracted increasing interest owing to their unique properties. The general relations between the electronic structure and materials properties of MAX phases have been elaborated based on ab initio calculations.

Keyword
Molecular-dynamics, transition-metals, ti3sic2, nitrides, carbides, nacl
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-24643 (URN)10.1142/S0217979208050243 (DOI)000260804900022 ()
Note
QC 20100920Available from: 2010-09-20 Created: 2010-09-20 Last updated: 2017-12-12Bibliographically approved
7. First-principles calculations of the electronic structure and pressure-induced magnetic transition in siderite FeCO3
Open this publication in new window or tab >>First-principles calculations of the electronic structure and pressure-induced magnetic transition in siderite FeCO3
2008 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 78, no 15Article in journal (Refereed) Published
Abstract [en]

Rhombohedral siderite FeCO3 has been studied by using density-functional theory with the generalized gradient approximation (GGA). In order to take into account the strong on-site Coulomb interaction U present in FeCO3, we also performed the GGA+U calculations. We observe a pressure-induced magnetic transition (high spin -> low spin) at pressures of 15 and 28 GPa, which are underestimated with respect to the experiment, for the GGA and GGA+U calculations, respectively. This phase transition was with a volume collapse of 10% around, also accompanied by increases in bulk modulus, Young's modulus and sound velocity. The electronic density of states and charge-density calculations revealed that the magnetic transition was due to the pressure-induced Fe 3d electron delocalization.

Keyword
x-ray-emission, augmented-wave method, low-spin transition, earths, lower mantle, metal oxides, spectroscopy, iron, phase, feo, approximation
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-17937 (URN)10.1103/PhysRevB.78.155119 (DOI)000260574400058 ()2-s2.0-55349116704 (Scopus ID)
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved
8. Study of Ti2SC under compression up to 47 GPa
Open this publication in new window or tab >>Study of Ti2SC under compression up to 47 GPa
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2008 (English)In: Journal of Alloys and Compounds, ISSN 0925-8388, E-ISSN 1873-4669, Vol. 448, no 1-2, L1-L4 p.Article in journal (Refereed) Published
Abstract [en]

The pressure dependence of the lattice parameters of the ternary layered carbide, Ti2SC, was measured by using synchrotron radiation X-ray diffraction and a diamond anvil cell setup. The experiment was conducted at room temperature and no phase transformation was observed up to the maximum pressure of 47 GPa. The a and c lattice parameters at room condition are 3.216 (A) over circle and 11.22 (A) over circle, respectively. The bulk modulus, calculated using the Birch-Murnaghan equation of state, is 191 +/- 3 GPa, with a pressure derivative of 4.0 +/- 0.3 and that obtained by our ab initio calculations is 183 GPa, with a pressure derivative of 4.1. L Like the majority of the ternary layered carbides (MAX phases), compressibility along the c-axis was higher than that along the a-axis.

Keyword
High-pressure, h-phases, ti3sic2, ti3gec2, si
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-24645 (URN)10.1016/j.jallcom.2006.10.086 (DOI)000252373600001 ()
Note
QC 20100920Available from: 2010-09-20 Created: 2010-09-20 Last updated: 2017-12-12Bibliographically approved
9. Synthesis and compressive behavior of Cr2GeC up to 48 GPa
Open this publication in new window or tab >>Synthesis and compressive behavior of Cr2GeC up to 48 GPa
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2008 (English)In: Journal of Alloys and Compounds, ISSN 0925-8388, E-ISSN 1873-4669, Vol. 463, no 1-2, 220-225 p.Article in journal (Refereed) Published
Abstract [en]

M(n+1)AX(n) compounds have gathered huge momentum because of its exciting properties. In this paper we report the synthesis of ternary layered ceramic Cr2GeC, a 211 M(n+1)AX(n) compound by hot-pressing. Scanning electron microscopy and X-ray diffraction have been employed to characterize the new synthesized phase. High-pressure compressibility of Cr2GeC were measured using diamond anvil cell and synchrotron C radiation at room temperature up to 48 GPa. No phase transformation was observed in the experimental pressure range. The bulk modulus of Cr2GeC calculated using the Birch-Murnaghan equation of state is 169 +/- 3 GPa, with K' = 3.05 +/- 0.15.

Keyword
High-pressure, ti3sic2, temperature, ti3gec2, ti2alc, oxidation, phases, air
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-24674 (URN)10.1016/j.jallcom.2007.09.078 (DOI)000258697300041 ()
Note
QC 20100920Available from: 2010-09-21 Created: 2010-09-21 Last updated: 2017-12-12Bibliographically approved
10. Unusual lattice dynamics of vanadium under high pressure
Open this publication in new window or tab >>Unusual lattice dynamics of vanadium under high pressure
2007 (English)In: Proceedings of the National Academy of Sciences of the United States of America, ISSN 0027-8424, E-ISSN 1091-6490, Vol. 104, no 42, 16428-16431 p.Article in journal (Refereed) Published
Abstract [en]

The electronic structures and lattice dynamics of pressure-induced complex phase transitions [bcc -> hRl(110.5 degrees) -> distorted-hRl(108.2') -> bcc] in vanadium as a function of pressure up to 400 GPa have been investigated with an ab initio method using density functional perturbation theory (DFPT). At ambient pressure, the soft transverse acoustic phonon mode corresponding to Kohn anomaly appears at a wave vector q = 2k(F) along [xi 00] Gamma -> H high symmetry direction. The nonclegenerate transverse acoustic branches TA(1) on < 110 > and TA(2) on < 001 > show an exceptionally large split at high symmetry point N (0.5 0.5 0.0). The lattice dynamical instability starts at a pressure of 62 GPa (V/V-0 = 0.78, where V-0 is experimental volume of bcc-V at ambient conditions), derived by phonon softening that results in phase transition of bcc -> hR1 (alpha = 110.5 degrees). At compression around 130 GPa (V/V-0 = 0.67), the rhombohedral angle of hR1 phase changed to 108.2 degrees, and the electronic structure changed drastically. At even higher pressure, approximate to 250 GPa (V/V-0 = 0.57), lattice dynamic calculations show that the bcc structure becomes stable again.

Keyword
ab initio calculation, phonon, structure phase transition, phonon-dispersion, pseudopotentials
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-17061 (URN)10.1073/pnas.0707377104 (DOI)000250373400010 ()2-s2.0-36749027665 (Scopus ID)
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved
11. Structural phase transition of vanadium at 69 GPa
Open this publication in new window or tab >>Structural phase transition of vanadium at 69 GPa
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2007 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 98, no 8Article in journal (Refereed) Published
Abstract [en]

A phase transition was observed at 63-69 GPa and room temperature in vanadium with synchrotron x-ray diffraction. The transition is characterized as a rhombohedral lattice distortion of the body-centered-cubic vanadium without a discontinuity in the pressure-volume data, thus representing a novel type of transition that has never been observed in elements. Instead of driven by the conventional s-d electronic transition mechanism, the phase transition could be associated with the softening of C-44 trigonal elasticity tensor that originates from the combination of Fermi surface nesting, band Jahn-Teller distortion, and electronic topological transition.

Keyword
Elastic-constants, high-pressures, electronic-structure, crystal-structure, lattice-dynamics, single-crystals, ab-initio, bcc, nb, state
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-24675 (URN)10.1103/PhysRevLett.98.085502 (DOI)000244420700039 ()
Note
QC 20100920Available from: 2010-09-21 Created: 2010-09-21 Last updated: 2017-12-12Bibliographically approved
12. Ab initio prediction of high-pressure structural phase transition in BaH2
Open this publication in new window or tab >>Ab initio prediction of high-pressure structural phase transition in BaH2
2007 (English)In: Journal of Alloys and Compounds, ISSN 0925-8388, E-ISSN 1873-4669, Vol. 446, 405-408 p.Article in journal (Refereed) Published
Abstract [en]

We have performed ab initio electronic structure calculations to calculate the structural properties and high-pressure phase transition in Barium dihydride (BaH2). Our results show that BaH2 crystallizes the orthorhombic phase (CoSi2-type structure) with Pnma space group at ambient conditions. A phase transition to the hexagonal (Ni2In-type structure) with P6(3)/mmc space group is found at pressure around 4 GPa. At the phase transition, the coordination number of hydrogen increases from 9 to 11 and the average bond length of Ba-H increases. The results show a band gap of 2.9 eV for orthorhombic and 1.8 eV for hexagonal phase. In addition, it was also found that more energy is required to desorb hydrogen atom from high-pressure phase as compared to ambient phase.

Keyword
Earth metal-hydrides, neutron-diffraction, crystal-structure
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-24676 (URN)10.1016/j.jallcom.2006.12.103 (DOI)000250822900086 ()
Note
QC 20100921Available from: 2010-09-22 Created: 2010-09-21 Last updated: 2017-12-12Bibliographically approved

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