The Experts below are selected from a list of 1284 Experts worldwide ranked by ideXlab platform
Zhaoming Zhang - One of the best experts on this subject based on the ideXlab platform.
-
Tilting and Distortion in Rutile-Related Mixed Metal Ternary Uranium Oxides: A Structural, Spectroscopic, and Theoretical Investigation.
Inorganic chemistry, 2021Co-Authors: Gabriel L Murphy, Zhaoming Zhang, Rebekka Tesch, Piotr M Kowalski, Maxim Avdeev, Eugenia Y Kuo, Daniel J Gregg, Philip Kegler, Evgeny V Alekseev, Brendan J. KennedyAbstract:A systematic investigation examining the origins of structural distortions in rutile-related ternary uranium AUO4 oxides using a combination of high-resolution structural and spectroscopic measurements supported by ab initio calculations is presented. The structures of β-CdUO4, MnUO4, CoUO4, and MgUO4 are determined at high precision by using a combination of neutron powder diffraction (NPD) and synchrotron X-ray powder diffraction (S-XRD) or single crystal X-ray diffraction. The structure of β-CdUO4 is best described by space group Cmmm whereas MnUO4, CoUO4, and MgUO4 are described by the lower symmetry Ibmm space group and are isostructural with the previously reported β-NiUO4 [Murphy et al. Inorg. Chem. 2018, 57, 13847]. X-ray absorption spectroscopy (XAS) analysis shows all five oxides contain hexavalent uranium. The difference in space group can be understood on the basis of size mismatch between the A2+ and U6+ cations whereby unsatisfactory matching results in structural distortions manifested through tilting of the AO6 polyhedra, leading to a change in symmetry from Cmmm to Ibmm. Such tilts are absent in the Cmmm structure. Heating the Ibmm AUO4 oxides results in reduction of the tilt angle. This is demonstrated for MnUO4 where in situ S-XRD measurements reveal a second-order phase transition to Cmmm near T = 200 °C. Based on the extrapolation of variable temperature in situ S-XRD data, CoUO4 is predicted to undergo a continuous phase transition to Cmmm at ∼1475 °C. Comparison of the measured and computed data highlights inadequacies in the DFT+U approach, and the conducted analysis should guide future improvements in computational methods. The results of this investigation are discussed in the context of the wider AUO4 family of oxides.
-
Crystal Structures and Phase Transitions in A-Site Deficient Perovskites Ln1/3TaO3
Chemistry of Materials, 2008Co-Authors: Qingdi Zhou, Paul J. Saines, Neeraj Sharma, Jimmy Ting, Brendan J. Kennedy, Zhaoming Zhang, Raymond Withers, Kia S. WallworkAbstract:The synthesis and structures of the perovskites Ln1/3TaO3 are described. As the size of the Ln cation is reduced, the compounds display a sequence of structure: P4/mmm/La → Cmmm/Ce−Gd → Pmma/Tb, Dy → Pmc21/Ho, Er. Although apparently tetragonal in P4/mmm, electron diffraction patterns of Tm1/3TaO3 reveal this has a complex incommensurate structure. Likewise Gd1/3TaO3 appears metrically tetragonal, but electron diffraction and synchrotron X-ray powder diffraction demonstrate this is actually orthorhombic. The suppression of the spontaneous orthorhombic strain in Gd1/3TaO3 is thought to be due to the proximity to the first-order Cmmm−Pmma transition. Variable temperature studies show both Tb1/3TaO3 and Dy1/3TaO3 undergo a first-order Cmmm−Pmma transition upon heating.
-
Structures and phase diagram for the system CaTiO3–La2/3TiO3
Journal of Solid State Chemistry, 2007Co-Authors: Zhaoming Zhang, Gregory R. Lumpkin, Christopher J. Howard, Kevin S. Knight, Karl R. Whittle, Keiichi OsakaAbstract:High-resolution neutron and synchrotron X-ray powder diffraction have been used to examine various compositions across the system (1x)CaTiO3xLa2/3TiO3. The structures at room temperature were determined according to composition: in Pbnm for 0pxp0.5, Ibmm for 0.5oxo0.7, then I4/mcm for 0.7txo0.9, and finally in Cmmm for xX0.9. Although the four structures are the same as those proposed previously in an X-ray diffraction study, the phase boundaries are somewhat different, in particular the Pbnm2Ibmm phase boundary has been extended from xt0.4 to x40.5 in the current study based on our high-resolution neutron diffraction data. From in situ measurements to identify structures above room temperature, an approximate composition–temperature phase diagram has been constructed, involving four temperature-induced phase transitions: Pbnm-I4/mcm, Ibmm-I4/mcm, I4/mcm-Pm3¯ m and Cmmm- P4/mmm
Tian Cui - One of the best experts on this subject based on the ideXlab platform.
-
Stable structures and superconductivity of an At-H system at high pressure.
Physical chemistry chemical physics : PCCP, 2018Co-Authors: Ziji Shao, Yanping Huang, Defang Duan, Hui Xie, Fubo Tian, Bingbing Liu, Tian CuiAbstract:The phase diagram, electronic properties and superconductivity of an At–H system at high pressure are investigated through first principles calculation considering the effect of spin–orbit coupling (SOC). The Cmcm-AtH2, Pnma-AtH2, P6/mmm-AtH4, and Cmmm-AtH4 phases are uncovered above 50 GPa. Metallization is realized at 50 GPa for AtH2 and 60 GPa for AtH4, with Tc values of approximately 5–10 K and 30–50 K, respectively. In P6/mmm-AtH4, phonon softening induced by Fermi surface nesting occurs as the pressure increases, which is closely related to the structural phase transition of P6/mmm → Cmmm and plays a crucial role in the superconductivity of the P6/mmm phase. In addition, the spin–orbit coupling effect considerably influences the energy of ground states, pressure points of phase transitions, electronic structures, and even the electron–phonon coupling of the At–H system. Such an influence may also occur in other heavy atomic hydrides.
-
Design of Superhard Ternary Compounds under High Pressure: SiC2N4 and Si2CN4
The Journal of Physical Chemistry C, 2010Co-Authors: Hongbo Wang, Hui Wang, Hanyu Liu, Tian CuiAbstract:Ab initio evolutionary methodology for crystal structure prediction is performed to explore the high-pressure structures of two ternary compounds, SiC2N4 and Si2CN4. For SiC2N4, we found intriguing high-pressure polymorphs with monoclinic C2/m and orthorhombic Cmmm symmetries containing tetrahedral CN4 and octahedral SiN6 units, respectively. For Si2CN4, two high-pressure monoclinic C2/m and P21/m structures both consisting of octahedral SiN6 units were discovered. Thermodynamic study demonstrated that it is energetically desirable to synthesize the Cmmm structured SiC2N4 and P21/m structured Si2CN4 at above 29 and 19 GPa, respectively. We have ruled out the earlier proposed high-pressure monoclinic structures for the two ternary compounds borrowed from known structural information. The newly predicted high-pressure phases of the two ternary compounds contain short, strong, and three-dimensional covalent bonding, which are responsible for the predicted superior mechanical properties, e.g., very large bulk...
Defang Duan - One of the best experts on this subject based on the ideXlab platform.
-
Stable structures and superconductivity of an At-H system at high pressure.
Physical chemistry chemical physics : PCCP, 2018Co-Authors: Ziji Shao, Yanping Huang, Defang Duan, Hui Xie, Fubo Tian, Bingbing Liu, Tian CuiAbstract:The phase diagram, electronic properties and superconductivity of an At–H system at high pressure are investigated through first principles calculation considering the effect of spin–orbit coupling (SOC). The Cmcm-AtH2, Pnma-AtH2, P6/mmm-AtH4, and Cmmm-AtH4 phases are uncovered above 50 GPa. Metallization is realized at 50 GPa for AtH2 and 60 GPa for AtH4, with Tc values of approximately 5–10 K and 30–50 K, respectively. In P6/mmm-AtH4, phonon softening induced by Fermi surface nesting occurs as the pressure increases, which is closely related to the structural phase transition of P6/mmm → Cmmm and plays a crucial role in the superconductivity of the P6/mmm phase. In addition, the spin–orbit coupling effect considerably influences the energy of ground states, pressure points of phase transitions, electronic structures, and even the electron–phonon coupling of the At–H system. Such an influence may also occur in other heavy atomic hydrides.
-
Structures and properties of binary MgBi compounds under pressure
Solid State Communications, 2018Co-Authors: Yunxian Liu, Chao Wang, Hairui Sun, Mei Wang, Defang DuanAbstract:Abstract The phase diagram, crystal structures, electronic properties, bonding nature and superconductivities of binary Mg Bi compounds were theoretically explored under pressure. Upon compression, Mg3Bi, MgBi, MgBi2, and MgBi3 were found to be thermodynamically stable. The stable phases present metallic features as a result of notable band structures and the finite DOS at the Fermi level. The calculated ELF and Bader charge analysis indicate that there was no covalent bond with charges transferring from Mg atoms to Bi atoms. The superconducting critical temperature (Tc) value of Cmmm-MgBi is 8.19 K at 10 GPa. Further research reveals that bismuth phonon modes play a major role in the superconductivity of Cmmm-MgBi. This study provides the crystal structures and corresponding properties of magnesium-bismuth compounds, which has broad implications for further exploration on other bismuthides.
-
The stability of B₆ octahedron in BaB₆ under high pressure
RSC Advances, 2016Co-Authors: Xiaoli Huang, Yanping Huang, Defang Duan, Mingkun Liu, Quan Zhuang, Shuli Wei, Qiang ZhouAbstract:We have performed in situ synchrotron X-ray diffraction and first-principles calculations to explore the compression behavior of barium hexaboride (BaB₆) under high pressure. No phase transitions in our experiment are observed up to 49.3 GPa at ambient temperature. It is found that the ambient cage structure (Pm3m) is still stable with a basic covalent network during the experimental pressure run. The results of our theoretical calculations show that the ambient structure might transform into three dynamically stable structures (Cmmm, Cmcm and I4/mmm) at 78 GPa, 97 GPa and 105 GPa respectively. The energy band calculations indicate that the sample is still a semiconductor with a narrow gap at 50 GPa.
Brendan J. Kennedy - One of the best experts on this subject based on the ideXlab platform.
-
Tilting and Distortion in Rutile-Related Mixed Metal Ternary Uranium Oxides: A Structural, Spectroscopic, and Theoretical Investigation.
Inorganic chemistry, 2021Co-Authors: Gabriel L Murphy, Zhaoming Zhang, Rebekka Tesch, Piotr M Kowalski, Maxim Avdeev, Eugenia Y Kuo, Daniel J Gregg, Philip Kegler, Evgeny V Alekseev, Brendan J. KennedyAbstract:A systematic investigation examining the origins of structural distortions in rutile-related ternary uranium AUO4 oxides using a combination of high-resolution structural and spectroscopic measurements supported by ab initio calculations is presented. The structures of β-CdUO4, MnUO4, CoUO4, and MgUO4 are determined at high precision by using a combination of neutron powder diffraction (NPD) and synchrotron X-ray powder diffraction (S-XRD) or single crystal X-ray diffraction. The structure of β-CdUO4 is best described by space group Cmmm whereas MnUO4, CoUO4, and MgUO4 are described by the lower symmetry Ibmm space group and are isostructural with the previously reported β-NiUO4 [Murphy et al. Inorg. Chem. 2018, 57, 13847]. X-ray absorption spectroscopy (XAS) analysis shows all five oxides contain hexavalent uranium. The difference in space group can be understood on the basis of size mismatch between the A2+ and U6+ cations whereby unsatisfactory matching results in structural distortions manifested through tilting of the AO6 polyhedra, leading to a change in symmetry from Cmmm to Ibmm. Such tilts are absent in the Cmmm structure. Heating the Ibmm AUO4 oxides results in reduction of the tilt angle. This is demonstrated for MnUO4 where in situ S-XRD measurements reveal a second-order phase transition to Cmmm near T = 200 °C. Based on the extrapolation of variable temperature in situ S-XRD data, CoUO4 is predicted to undergo a continuous phase transition to Cmmm at ∼1475 °C. Comparison of the measured and computed data highlights inadequacies in the DFT+U approach, and the conducted analysis should guide future improvements in computational methods. The results of this investigation are discussed in the context of the wider AUO4 family of oxides.
-
Crystal Structures and Phase Transitions in A-Site Deficient Perovskites Ln1/3TaO3
Chemistry of Materials, 2008Co-Authors: Qingdi Zhou, Paul J. Saines, Neeraj Sharma, Jimmy Ting, Brendan J. Kennedy, Zhaoming Zhang, Raymond Withers, Kia S. WallworkAbstract:The synthesis and structures of the perovskites Ln1/3TaO3 are described. As the size of the Ln cation is reduced, the compounds display a sequence of structure: P4/mmm/La → Cmmm/Ce−Gd → Pmma/Tb, Dy → Pmc21/Ho, Er. Although apparently tetragonal in P4/mmm, electron diffraction patterns of Tm1/3TaO3 reveal this has a complex incommensurate structure. Likewise Gd1/3TaO3 appears metrically tetragonal, but electron diffraction and synchrotron X-ray powder diffraction demonstrate this is actually orthorhombic. The suppression of the spontaneous orthorhombic strain in Gd1/3TaO3 is thought to be due to the proximity to the first-order Cmmm−Pmma transition. Variable temperature studies show both Tb1/3TaO3 and Dy1/3TaO3 undergo a first-order Cmmm−Pmma transition upon heating.
Keiichi Osaka - One of the best experts on this subject based on the ideXlab platform.
-
Structures and phase diagram for the system CaTiO3–La2/3TiO3
Journal of Solid State Chemistry, 2007Co-Authors: Zhaoming Zhang, Gregory R. Lumpkin, Christopher J. Howard, Kevin S. Knight, Karl R. Whittle, Keiichi OsakaAbstract:High-resolution neutron and synchrotron X-ray powder diffraction have been used to examine various compositions across the system (1x)CaTiO3xLa2/3TiO3. The structures at room temperature were determined according to composition: in Pbnm for 0pxp0.5, Ibmm for 0.5oxo0.7, then I4/mcm for 0.7txo0.9, and finally in Cmmm for xX0.9. Although the four structures are the same as those proposed previously in an X-ray diffraction study, the phase boundaries are somewhat different, in particular the Pbnm2Ibmm phase boundary has been extended from xt0.4 to x40.5 in the current study based on our high-resolution neutron diffraction data. From in situ measurements to identify structures above room temperature, an approximate composition–temperature phase diagram has been constructed, involving four temperature-induced phase transitions: Pbnm-I4/mcm, Ibmm-I4/mcm, I4/mcm-Pm3¯ m and Cmmm- P4/mmm