The Experts below are selected from a list of 5688 Experts worldwide ranked by ideXlab platform
K. A. Leblanc - One of the best experts on this subject based on the ideXlab platform.
-
Thermodynamic ground state of MgB6 predicted from first principles structure search methods.
Journal of Chemical Physics, 2014Co-Authors: Hui Wang, K. A. LeblancAbstract:Crystalline structures of magnesium hexaboride, MgB6, were investigated using unbiased structure searching methods combined with first principles density functional calculations. An orthorhombic Cmcm structure was predicted as the thermodynamic ground state of MgB6. The energy of the Cmcm structure is significantly lower than the theoretical MgB6 models previously considered based on a primitive cubic arrangement of boron octahedra. The Cmcm structure is stable against the decomposition to elemental magnesium and boron solids at atmospheric pressure and high pressures up to 18.3 GPa. A unique feature of the predicted Cmcm structure is that the boron atoms are clustered into two forms: localized B6 octahedra and extended B∞ ribbons. Within the boron ribbons, the electrons are delocalized and this leads to a metallic ground state with vanished electric dipoles. The present prediction is in contrast to the previous proposal that the crystalline MgB6 maintains a semiconducting state with permanent dipole mome...
-
thermodynamic ground state of mgb6 predicted from first principles structure search methods
Journal of Chemical Physics, 2014Co-Authors: Hui Wang, K. A. Leblanc, Bo Gao, Yansun YaoAbstract:Crystalline structures of magnesium hexaboride, MgB6, were investigated using unbiased structure searching methods combined with first principles density functional calculations. An orthorhombic Cmcm structure was predicted as the thermodynamic ground state of MgB6. The energy of the Cmcm structure is significantly lower than the theoretical MgB6 models previously considered based on a primitive cubic arrangement of boron octahedra. The Cmcm structure is stable against the decomposition to elemental magnesium and boron solids at atmospheric pressure and high pressures up to 18.3 GPa. A unique feature of the predicted Cmcm structure is that the boron atoms are clustered into two forms: localized B6 octahedra and extended B∞ ribbons. Within the boron ribbons, the electrons are delocalized and this leads to a metallic ground state with vanished electric dipoles. The present prediction is in contrast to the previous proposal that the crystalline MgB6 maintains a semiconducting state with permanent dipole moments. MgB6 is estimated to have much weaker electron-phonon coupling compared with that of MgB2, and therefore it is not expected to be able to sustain superconductivity at high temperatures.
Hui Wang - One of the best experts on this subject based on the ideXlab platform.
-
Thermodynamic ground state of MgB6 predicted from first principles structure search methods.
Journal of Chemical Physics, 2014Co-Authors: Hui Wang, K. A. LeblancAbstract:Crystalline structures of magnesium hexaboride, MgB6, were investigated using unbiased structure searching methods combined with first principles density functional calculations. An orthorhombic Cmcm structure was predicted as the thermodynamic ground state of MgB6. The energy of the Cmcm structure is significantly lower than the theoretical MgB6 models previously considered based on a primitive cubic arrangement of boron octahedra. The Cmcm structure is stable against the decomposition to elemental magnesium and boron solids at atmospheric pressure and high pressures up to 18.3 GPa. A unique feature of the predicted Cmcm structure is that the boron atoms are clustered into two forms: localized B6 octahedra and extended B∞ ribbons. Within the boron ribbons, the electrons are delocalized and this leads to a metallic ground state with vanished electric dipoles. The present prediction is in contrast to the previous proposal that the crystalline MgB6 maintains a semiconducting state with permanent dipole mome...
-
thermodynamic ground state of mgb6 predicted from first principles structure search methods
Journal of Chemical Physics, 2014Co-Authors: Hui Wang, K. A. Leblanc, Bo Gao, Yansun YaoAbstract:Crystalline structures of magnesium hexaboride, MgB6, were investigated using unbiased structure searching methods combined with first principles density functional calculations. An orthorhombic Cmcm structure was predicted as the thermodynamic ground state of MgB6. The energy of the Cmcm structure is significantly lower than the theoretical MgB6 models previously considered based on a primitive cubic arrangement of boron octahedra. The Cmcm structure is stable against the decomposition to elemental magnesium and boron solids at atmospheric pressure and high pressures up to 18.3 GPa. A unique feature of the predicted Cmcm structure is that the boron atoms are clustered into two forms: localized B6 octahedra and extended B∞ ribbons. Within the boron ribbons, the electrons are delocalized and this leads to a metallic ground state with vanished electric dipoles. The present prediction is in contrast to the previous proposal that the crystalline MgB6 maintains a semiconducting state with permanent dipole moments. MgB6 is estimated to have much weaker electron-phonon coupling compared with that of MgB2, and therefore it is not expected to be able to sustain superconductivity at high temperatures.
Murat Durandurdu - One of the best experts on this subject based on the ideXlab platform.
-
Formation of a Cmcm phase in SnS at high pressure; an ab initio constant pressure study
Solid State Communications, 2010Co-Authors: Sebahaddin Alptekin, Murat DurandurduAbstract:The stability of SnS at high pressure is studied using a constant pressure ab initio technique. For the first time, a pressure-induced phase transformation from the Pnma structure to a Cmcm structure with the application of pressure is predicted through the simulations in this material. The Cmcm phase is still a layered structure, consisting of rocksalt-like bilayers, similar to that formed at high temperatures. The Cmcm structure is fivefold coordinated. This phase transformation gradually proceeds and is due to the significant decrease of the second neighbor distances. This phase change is also studied by total energy calculations.
-
Cmcm phase of GeS at high pressure
Physical Review B, 2005Co-Authors: Murat DurandurduAbstract:We study the pressure-induced phase transition of the layered GeS structure using a constant-pressure ab initio technique. For the first time, we predict a gradual phase transition to a Cmcm structure with the application of hydrostatic pressure. The high-pressure phase is still a layered structure, consisting of rocksaltlike bilayers, but it has an unusual fivefold coordination. The transition is due to the significant decrease of second neighbor distances. We also find the metallization of GeS prior to transforming into the Cmcm phase.
-
Ab initio simulation of high-pressure phases of GaAs
Physical Review B, 2002Co-Authors: Murat Durandurdu, David A. DraboldAbstract:The pressure-induced phase transition in GaAs is studied using an ab initio constant-pressure relaxation simulation. GaAs undergoes a first-order phase transition to $\mathrm{Cmcm}$ at 54 GPa. Upon further increase of pressure a gradual phase change to $\mathrm{Imm}2$ structure is seen at 57 GPa, which confirms an earlier experiment and clears some doubts about the existence and identity of $\mathrm{Imm}2.$ The transition pressures are also calculated from the Gibbs free energy, and it is found that the structural phase change occurs at 23.5 GPa for $\mathrm{Cmcm}$ and at 24 GPa for $\mathrm{Imm}2.$ The transformation path from $\mathrm{Cmcm}$ and $\mathrm{Imm}2$ proceeds through sliding of some $\mathrm{Cmcm}$ planes and relatively large sliding yields a transition from $\mathrm{Imm}2$ to simple hexagonal structure. We find that $\mathrm{Cmcm}$ and $\mathrm{Imm}2$ phases are semimetals.
Yansun Yao - One of the best experts on this subject based on the ideXlab platform.
-
thermodynamic ground state of mgb6 predicted from first principles structure search methods
Journal of Chemical Physics, 2014Co-Authors: Hui Wang, K. A. Leblanc, Bo Gao, Yansun YaoAbstract:Crystalline structures of magnesium hexaboride, MgB6, were investigated using unbiased structure searching methods combined with first principles density functional calculations. An orthorhombic Cmcm structure was predicted as the thermodynamic ground state of MgB6. The energy of the Cmcm structure is significantly lower than the theoretical MgB6 models previously considered based on a primitive cubic arrangement of boron octahedra. The Cmcm structure is stable against the decomposition to elemental magnesium and boron solids at atmospheric pressure and high pressures up to 18.3 GPa. A unique feature of the predicted Cmcm structure is that the boron atoms are clustered into two forms: localized B6 octahedra and extended B∞ ribbons. Within the boron ribbons, the electrons are delocalized and this leads to a metallic ground state with vanished electric dipoles. The present prediction is in contrast to the previous proposal that the crystalline MgB6 maintains a semiconducting state with permanent dipole moments. MgB6 is estimated to have much weaker electron-phonon coupling compared with that of MgB2, and therefore it is not expected to be able to sustain superconductivity at high temperatures.
Tom Nilges - One of the best experts on this subject based on the ideXlab platform.
-
direct synthesis and characterization of mixed valent li0 5 δcopo4 a li deficient derivative of the Cmcm polymorph of licopo4
RSC Advances, 2017Co-Authors: Jennifer Ludwig, Stephan Geprägs, Dennis Nordlund, Marca M. Doeff, Inés Puente Orench, Carlos Alarconsuesca, Tom NilgesAbstract:While the majority of research activities on LiCoPO4 is focussed on the thermodynamically stable olivine-type Pnma polymorph, the metastable Pna21 and Cmcm modifications have recently attracted considerable attention due to their interesting material properties. In this study, we present the first Li-deficient structural derivative of the Cmcm modification with the nominal composition Li0.5−δCoPO4. As opposed to the substoichiometric olivine (Pnma) phases LixCoPO4 (x = 0; 2/3), which are exclusively accessible by electrochemical or chemical Li extraction techniques, this is also the first time that a direct soft-chemical synthesis route towards a LixCoPO4-type material is accomplished. X-ray and neutron diffraction studies indicate that Cmcm-type Li0.5−δCoPO4 shows vacancies on both the Li and Co sites, whereas X-ray absorption spectra demonstrate that the structure features heterovalent Co ions (+2/+3) to compensate for the Li deficit. Magnetic measurements reveal a long-range antiferromagnetic order below 10.5 K. A thorough investigation of the thermal stability using thermogravimetric analysis, differential scanning calorimetry, and temperature-dependent in situ X-ray powder diffraction demonstrates that Li0.5−δCoPO4 is metastable and exhibits a complex, multi-step thermal decomposition mechanism. In the first step at 394 °C, it decomposes to α-Co2P2O7 (P21/c) and LiCoPO4 (Cmcm) upon O2 release. The LiCoPO4 (Cmcm) intermediate is then irreversibly transformed to olivine-type LiCoPO4 (Pnma) at 686 °C. The material properties of Li0.5−δCoPO4 are further compared to the fully lithiated, isostructural LiCoPO4 (Cmcm) phase, for which an improved structure solution as well as Co L2,3-edge X-ray absorption spectra are reported for the first time.
-
Direct synthesis and characterization of mixed-valent Li0.5−δCoPO4, a Li-deficient derivative of the Cmcm polymorph of LiCoPO4
RSC Advances, 2017Co-Authors: Jennifer Ludwig, Carlos Alarcón-suesca, Stephan Geprägs, Dennis Nordlund, Marca M. Doeff, Inés Puente Orench, Tom NilgesAbstract:While the majority of research activities on LiCoPO4 is focussed on the thermodynamically stable olivine-type Pnma polymorph, the metastable Pna21 and Cmcm modifications have recently attracted considerable attention due to their interesting material properties. In this study, we present the first Li-deficient structural derivative of the Cmcm modification with the nominal composition Li0.5−δCoPO4. As opposed to the substoichiometric olivine (Pnma) phases LixCoPO4 (x = 0; 2/3), which are exclusively accessible by electrochemical or chemical Li extraction techniques, this is also the first time that a direct soft-chemical synthesis route towards a LixCoPO4-type material is accomplished. X-ray and neutron diffraction studies indicate that Cmcm-type Li0.5−δCoPO4 shows vacancies on both the Li and Co sites, whereas X-ray absorption spectra demonstrate that the structure features heterovalent Co ions (+2/+3) to compensate for the Li deficit. Magnetic measurements reveal a long-range antiferromagnetic order below 10.5 K. A thorough investigation of the thermal stability using thermogravimetric analysis, differential scanning calorimetry, and temperature-dependent in situ X-ray powder diffraction demonstrates that Li0.5−δCoPO4 is metastable and exhibits a complex, multi-step thermal decomposition mechanism. In the first step at 394 °C, it decomposes to α-Co2P2O7 (P21/c) and LiCoPO4 (Cmcm) upon O2 release. The LiCoPO4 (Cmcm) intermediate is then irreversibly transformed to olivine-type LiCoPO4 (Pnma) at 686 °C. The material properties of Li0.5−δCoPO4 are further compared to the fully lithiated, isostructural LiCoPO4 (Cmcm) phase, for which an improved structure solution as well as Co L2,3-edge X-ray absorption spectra are reported for the first time.
-
Direct synthesis and characterization of mixed-valent Li-0.5-delta CoPO4, a Li-deficient derivative of the Cmcm polymorph of LiCoPO4
RSC Advances, 2017Co-Authors: Jennifer Ludwig, Carlos Alarcón-suesca, Dennis Nordlund, Marca M. Doeff, Stephan Gepraegs, Ines Puente Orenchef, Tom NilgesAbstract:While the majority of research activities on LiCoPO4 is focussed on the thermodynamically stable olivine-type Pnma polymorph, the metastable Pna2(1) and Cmcm modifications have recently attracted considerable attention due to their interesting material properties. In this study, we present the first Li-deficient structural derivative of the Cmcm modification with the nominal composition Li-0.5-delta CoPO4. As opposed to the substoichiometric olivine (Pnma) phases LixCoPO4 (x = 0; 2/3), which are exclusively accessible by electrochemical or chemical Li extraction techniques, this is also the first time that a direct soft-chemical synthesis route towards a LixCoPO4-type material is accomplished. X-ray and neutron diffraction studies indicate that Cmcm-type Li-0.5-delta CoPO4 shows vacancies on both the Li and Co sites, whereas X-ray absorption spectra demonstrate that the structure features heterovalent Co ions (+2/+3) to compensate for the Li deficit. Magnetic measurements reveal a long-range antiferromagnetic order below 10.5 K. A thorough investigation of the thermal stability using thermogravimetric analysis, differential scanning calorimetry, and temperature-dependent in situ X-ray powder diffraction demonstrates that Li-0.5-delta CoPO4 is metastable and exhibits a complex, multi-step thermal decomposition mechanism. In the first step at 394 degrees C, it decomposes to alpha-Co2P2O7 (P2(1)/c) and LiCoPO4 (Cmcm) upon O-2 release. The LiCoPO4 (Cmcm) intermediate is then irreversibly transformed to olivine-type LiCoPO4 (Pnma) at 686 degrees C. The material properties of Li-0.5-delta CoPO4 are further compared to the fully lithiated, isostructural LiCoPO4 (Cmcm) phase, for which an improved structure solution as well as Co L-2,L-3-edge X-ray absorption spectra are reported for the first time
-
In Situ Studies and Magnetic Properties of the Cmcm Polymorph of LiCoPO4 with a Hierarchical Dumbbell-Like Morphology Synthesized by Easy Single-Step Polyol Synthesis
Inorganics, 2016Co-Authors: Carlos Alarcón-suesca, Jennifer Ludwig, Viktor Hlukhyy, Christoph Stinner, Tom NilgesAbstract:LiCoPO4 (LCP) exists in three different structural modifications: LCP-Pnma (olivine structure), LCP-Pn21a (KNiPO4 structure type), and LCP-Cmcm (Na2CrO4 structure type). The synthesis of the LCP-Cmcm polymorph has been reported via high pressure/temperature solid-state methods and by microwave-assisted solvothermal synthesis. Phase transitions from both LCP-Pn21a and LCP-Cmcm to LCP-Pnma upon heating indicates a metastable behavior. However, a precise study of the structural changes during the heating process and the magnetic properties of LCP-Cmcm are hitherto unknown. Herein, we present the synthesis and characterization of LCP-Cmcm via a rapid and facile soft-chemistry approach using two different kinetically controlled pathways, solvothermal and polyol syntheses, both of which only require relatively low temperatures (~200 °C). Additionally, by polyol, method a dumbbell-like morphology is obtained without the use of any additional surfactant or template. A temperature-dependent in situ powder XRD shows a transition from LCP-Cmcm at room temperature to LCP-Pnma and finally to LCP-Pn21a at 575 and 725 °C, respectively. In addition to that, the determination of the magnetic susceptibility as a function of temperature indicates a long-range antiferromagnetic order below TN = 11 K at 10 kOe and 9.1 K at 25 kOe. The magnetization curves suggests the presence of a metamagnetic transition.