Publications (10)7.37 Total impact
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Article: Oxygen non-stoichiometry and the origin of Na ion ordering in gamma-NaxCoO2
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ABSTRACT: The impact of oxygen deficiency on physical properties of Na2/3CoO2-x has been investigated. From the combined thermogravimetric, magnetic susceptibility and synchrotron X-ray Laue diffraction studies, it is demonstrated that Na2/3CoO2 shows no superlattice ordering due to Na ions; however Na2/3CoO1.98, which has the same Co valence as that of Na0.71CoO2, shows nearly identical magnetic and transport properties and the same simple hexagonal superlattice ordering of sqrt(12)a. It is proposed that the Na ion ordering found in Na2/3CoO1.98 is identical to the ideal Na0.71CoO2 of large sqrt(12)a x sqrt(12)a x 3c superlattice but with additional Na vacancies which are bound to the oxygen defects at room temperature. We conclude that oxygen vacancies play a key role in stabilizing the superlattice structure and must be accounted for in its modeling. Comment: This paper has been withdrawn by the author due to we can directly change our old version of 1005.252906/2010; -
Article: Oxygen non-stoichiometry and the origin of Na ion ordering in P2-NaxCoO2
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ABSTRACT: The impact of oxygen deficiency on physical properties of Na2/3CoO2-x has been investigated. From the combined thermogravimetric, magnetic susceptibility and synchrotron X-ray Laue diffraction studies, it is demonstrated that Na2/3CoO2 shows no superlattice ordering due to Na ions; however Na2/3CoO1.98, which has the same Co valence as that of Na0.71CoO2, shows nearly identical magnetic and transport properties and the same simple hexagonal superlattice ordering of sqrt(12)a. It is proposed that the Na ion ordering found in Na2/3CoO1.98 is identical to the ideal Na0.71CoO2 of large sqrt(12)a x sqrt(12)a x 3c superlattice but with additional Na vacancies which are bound to the oxygen defects at room temperature. We conclude that oxygen vacancies play a key role in stabilizing the superlattice structure and must be accounted for in its modeling. Comment: 5 pages, 4 figures05/2010; -
Article: X-ray and electron diffraction studies of superlattices and long-range three-dimensional Na ordering in γ-Na_ {x} CoO_ {2}(x= 0.71 and 0.84)
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ABSTRACT: We have recently demonstrated that x=0.71 and 0.84 are the two most stable single-phase compounds above x=0.5 in γ-NaxCoO2 [ G. J. Shu et al. Phys. Rev. B 76 184115 (2007); F. C. Chou et al. Phys. Rev. Lett. 101 127404 (2008)], and this structural investigation was performed on the single crystals and pulverized samples elaborately synthesized therein. Using the complementary techniques of x-ray and electron diffractions, we unambiguously established the existence of superlattices in x=0.71 and 0.84, √12ap×√12ap×3cp, and √13ap×√13ap×3cp (ap and cp, the hexagonal primitive cell parameters), respectively. The exceptionally large superlattice for x=0.71 arises from the long-range three-dimensional sodium ordering, consisting of the spiral-like sodium-trimer chain screwing along c axis and being decorated with alternating truncated-triangle and honeycomb sodium sublattices in ab plane. The trimers in the neighboring sodium planes show corner-shared-like characteristics along the chain direction. A larger interplane separation of the trimers that was expected to minimize the trimer interlayer Coulomb repulsion was, however, not observed, and all Co ions in the superlattice show similar charge characteristics. In x=0.84, the large superlattice is rather a result of the long-range ordering of sodium monomers both in ab plane and along c axis. The differences in the superlattice structure and the corresponding long-range sodium ordering between x=0.71 and 0.84 may provide the critical information in understanding their distinctly different physical properties from structural aspects.Phys. Rev. B. 01/2009; 79(1). -
Article: Staging model of the ordered stacking of vacancy layers and phase separation in the layered NaCoO(x > 0.71) single crystals
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ABSTRACT: Phase diagram of Na$_x$CoO$_2$ (x $\gtrsim$ 0.71) has been reinvestigated using electrochemically fine tuned single crystals. Both phase separation and staging phenomena as a result of sodium multi-vacancy cluster ordering have been found. Phase separation phenomenon is observed in the narrow ranges of 0.76 $\lesssim$ x $\lesssim$ 0.82 and 0.83 $\lesssim$ x $\lesssim$ 0.86. While x = 0.820 shows A-type antiferromagnetic (A-AF) ordering below 22K, x = 0.833 is confirmed to have a magnetic ground state of A-AF ordering below $\sim$8K and is only reachable through slow cooling. In addition, x = 0.859 is found to be responsible for the highest A-AF transition temperature at about 29K. Staging model based on ordered stacking of multi-vacancy layers is proposed to explain the hysteretic behavior and A-AF correlation length for x $\sim$ 0.82-0.86. Comment: 6 pages, 6 figures, to be published in PRB01/2009; -
Article: Sodium ion ordering and vacancy cluster formation in NaxCoO2 (x=0.71 and 0.84) single crystals by synchrotron X-Ray diffraction.
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ABSTRACT: In the rich phase diagram of NaxCoO2, x=0.71 enjoys special stability and is called the Curie-Weiss metal due to its anomalous properties. Similarly, x=0.84 prepared from high temperature melt is a special end point beyond which the system phase separates. Using synchrotron x-ray diffraction on single crystals, we discovered sqrt[12]a and sqrt[13]a superlattice structures which we interpret as the ordering of Na (vacancy) clusters. These results lead to a picture of coexisting local moments and itinerant carriers and form the first step towards understanding the many anomalous properties of cobaltates.Physical Review Letters 10/2008; 101(12):127404. · 7.37 Impact Factor -
Article: Sodium vacancy ordering and the co-existence of localized spins and itinerant charges in NaxCoO2
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ABSTRACT: The sodium cobaltate family (NaxCoO2) is unique among transition metal oxides because the Co sits on a triangular lattice and its valence can be tuned over a wide range by varying the Na concentration x. Up to now detailed modeling of the rich phenomenology (which ranges from unconventional superconductivity to enhanced thermopower) has been hampered by the difficulty of controlling pure phases. We discovered that certain Na concentrations are specially stable and are associated with superlattice ordering of the Na clusters. This leads naturally to a picture of co-existence of localized spins and itinerant charge carriers. For x = 0.84 we found a remarkably small Fermi energy of 87 K. Our picture brings coherence to a variety of measurements ranging from NMR to optical to thermal transport. Our results also allow us to take the first step towards modeling the mysterious ``Curie-Weiss'' metal state at x = 0.71. We suggest the local moments may form a quantum spin liquid state and we propose experimental test of our hypothesis.10/2007; -
Article: Thermodynamic and Transport Measurements on Superconducting Na_xCoO_2 + yH_2O Single Crystals Prepared by Electrochemical De-intercalation
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ABSTRACT: Superconducting single crystal samples of Na_xCoO_2 + yH_2O have been produced using an electrochemical technique which dispenses with the usual bromine chemical de-intercalation step and permits much more precise control of the Na content. After careful hydration, nearly single-phase crystals have been obtained in which over 90% of the sample's volume corresponds to the superconducting Na_{0.3}CoO_2 + 1.3H_2O structure. Susceptibility and specific heat measurements confirm that bulk superconductivity has been achieved. The extracted normal state density of states indicates Fermi-liquid behavior with strong mass enhancement and a modest Wilson ratio. Measurements of H_{c2} for H||c and H||ab reveal significant anisotropy. The estimated value of H_{c2} for H||c yields a coherence length of ~100 A, consistent with an extremely narrow bandwidth. Comment: 5 pages; 5 figures; new data in figures; revised and updated text06/2003; -
Article: Staging model of the ordered stacking of vacancy layers and phase separation in layered NaxCoO2 (x≳0.71) single crystals
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ABSTRACT: Phase diagram of Na[subscript x]CoO[subscript 2](x≳0.71) has been reinvestigated using electrochemically fine-tuned single crystals. Both phase-separation and staging phenomena as a result of sodium multivacancy cluster ordering have been found. Phase-separation phenomenon is observed in the narrow ranges of 0.76≲x≲0.82 and 0.83≲x≲0.86. While x=0.820 shows A-type antiferromagnetic (A-AF) ordering below 22 K, x=0.833 is confirmed to have a magnetic ground state of A-AF ordering below ∼8 K and is only reachable through slow cooling. In addition, x=0.859 is found to be responsible for the highest A-AF transition temperature at about 29 K. Staging model based on ordered stacking of multivacancy layers is proposed to explain the hysteretic behavior and A-AF correlation length for x ∼0.82–0.86. Department of Energy National Science Council of TaiwanAPS. -
Article: X-ray and electron diffraction studies of superlattices and long-range three-dimensional Na ordering in gamma-Na[subscript x]CoO[subscript 2] (x=0.71 and 0.84)
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ABSTRACT: We have recently demonstrated that x=0.71 and 0.84 are the two most stable single-phase compounds above x=0.5 in gamma-Na[subscript x]CoO[subscript 2] [G. J. Shu et al., Phys. Rev. B 76, 184115 (2007); F. C. Chou et al., Phys. Rev. Lett. 101, 127404 (2008)], and this structural investigation was performed on the single crystals and pulverized samples elaborately synthesized therein. Using the complementary techniques of x-ray and electron diffractions, we unambiguously established the existence of superlattices in x=0.71 and 0.84, sqrt12a[subscript p]×sqrt12a[subscript p]×3c[subscript p], and sqrt13a[subscript p]×sqrt13a[subscript p]×3c[subscript p] (a[subscript p] and c[subscript p] the hexagonal primitive cell parameters), respectively. The exceptionally large superlattice for x=0.71 arises from the long-range three-dimensional sodium ordering, consisting of the spiral-like sodium-trimer chain screwing along c axis and being decorated with alternating truncated-triangle and honeycomb sodium sublattices in ab plane. The trimers in the neighboring sodium planes show corner-shared-like characteristics along the chain direction. A larger interplane separation of the trimers that was expected to minimize the trimer interlayer Coulomb repulsion was, however, not observed, and all Co ions in the superlattice show similar charge characteristics. In x=0.84, the large superlattice is rather a result of the long-range ordering of sodium monomers both in ab plane and along c axis. The differences in the superlattice structure and the corresponding long-range sodium ordering between x=0.71 and 0.84 may provide the critical information in understanding their distinctly different physical properties from structural aspects. National Science Council of the Republic of China National Taiwan University Excellence ProjectAPS. -
Article: Oxygen nonstoichiometry and the origin of Na ion ordering in P2-Na_ {x} CoO_ {2}
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ABSTRACT: The impact of oxygen deficiency on physical properties of Na2/3CoO2−δ has been investigated. From the combined thermogravimetric, magnetic susceptibility, and synchrotron x-ray Laue diffraction studies, it is demonstrated that Na2/3CoO2 shows no superlattice ordering due to Na ions; however Na2/3CoO1.98, which has the same Co valence as that of Na0.71CoO2, shows nearly identical magnetic and transport properties and the same simple hexagonal superlattice ordering of √12a. It is proposed that the Na ion ordering found in Na2/3CoO1.98 is identical to the ideal Na0.71CoO2 of large √12a×√12a×3c superlattice but with additional Na vacancies which are bound to the oxygen defects at room temperature. We conclude that oxygen vacancies play a key role in stabilizing the superlattice structure and must be accounted for in its modeling.Phys. Rev. B. 82(5).
Top co-authors
Top Journals
Institutions
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2009
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Massachusetts Institute of Technology
- Department of Physics
Cambridge, MA, USA
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2008
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National Taiwan University
- Center for Condensed Matter Sciences
Taipei, Taipei, Taiwan
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