Sung-Yoon Chung

Microscale Measurements of the Electrical Conductivity of Doped LiFePO4

Electrical conductivity at the microscopic grain level has been measured in densely sintered polycrystalline samples of LiFePO 4 and Li 0 . 9 9 Nb 0 . 0 1 FePO 4 , using a four-point microcontact technique. The absolute value of conductivity as well as its spatial variability have been measured, and are consistent with previously reported bulk measurements. The results support the interpretation of the conductivity increase upon doping of LiFePO 4 as a lattice effect.

Effects of donor concentration and oxygen partial pressure on interface morphology and grain growth behavior in SrTiO3

Abstract Change in interface morphology with ionic vacancy concentration and the correlation between interface structure and grain growth behavior in strontium titanate (SrTiO 3 ) have been investigated using SrTiO 3 single crystals and powder compacts. Under experimental conditions where SrTiO 3 contains a negligible amount of ionic vacancies, the shape of the single crystal embedded in matrix grains was well faceted, showing a strong anisotropy in interfacial energy. However, as strontium or oxygen vacancies increased with the addition of an Nb 2 O 5 donor dopant or reduction of oxygen partial pressure, the faceted shape changed to a smoothly curved rough one indicating that an interface roughening transition occurred and, as a result, the anisotropy in interfacial energy was considerably reduced. Grain growth behavior was also strongly dependent on the interface structures; while normal grain growth occurred when the interfaces were rough, abnormal grain growth behavior was observed in the samples with faceted interfaces. It appears, therefore, that the ionic vacancies in SrTiO 3 can cause the interface roughening transition and change the resultant grain growth behavior.

Electronic Structure and Electrical Conductivity of Undoped LiFePO4

The electronic structure of LiFePO 4 underpins transport properties important to its use as a lithium storage electrode. Here we have calculated the electronic structure of LiFePO 4 in the ordered olivine structure by a first-principles method to determine (i) the effective mass of carriers and (ii) the nature of the band structure. The electrical conductivity in high purity undoped LiFePO 4 has also been measured experimentally. Spin-polarized calculations show a large electron effective mass and a much smaller but highly anisotropic hole effective mass, suggesting that hole-doped compositions should have the greater electronic conductivity. More surprisingly, the calculations show that this polyanion compound is a half-metal with spin-sensitive band structure, like some other oxides being studied for spintronics applications. This previously unappreciated aspect of the LiFePO 4 electronic structure may play a role in determining transport properties including those relevant to electrochemical applications.

Orientation‐Dependent Arrangement of Antisite Defects in Lithium Iron(II) Phosphate Crystals

The distribution and local concentration of point defects in crystal lattices such as dopants and atomic vacancies have been recognized as significant factors that govern the overall electrical and optical properties of inorganic crystals. The intentional use of impurities in semiconductors and the formation of ionic vacancies in ion-conducting metal oxides are well-known examples of displaying the correlation between atomic-scale chemical variations and resulting physical properties. Furthermore, as the chemically different environment induced by point defects leads to breaking of the ordered arrangement of atoms in crystals, mass and charge transport behaviors are also considerably affected by the presence of the defects. In many lithium intercalation compounds, an ordered array of lithium is usually maintained. Therefore, the control of point defects, including cation disorder, is of major significance for application to electrodes in rechargeable batteries. A variety of investigations on lithium vacancies and cation intermixing have been reported for layered oxides. In contrast, few experimental details showing the atomic-scale point defects in olivine-type lithium metal phosphates LiMPO4 (where M = Fe, Mn, Ni, Co), are yet available, although these phosphates have attracted a great deal of attention as alternative cathode materials in lithium-ion batteries over the past decade. As illustrated in Figure 1a, the lithium and the metal (M) ion in LiMPO4 having an ordered olivine structure occupy different octahedral inter-

Lattice distortion and polarization switching in calcium copper titanate

It is shown that CaCu3Ti4O12 grains in a polycrystalline specimen have domains and comparatively thick domain walls. The orientations of the domains within a grain were observed to be slightly tilted toward each other, suggesting the presence of lattice distortions. Multiple reflection spots in the electron diffraction pattern and misfit dislocations were also found in the domain walls. Kelvin probe force microscopy showed the variation in the surface potential of a grain by external electric fields, demonstrating the polarization of dipole moments. The influence of the local distortions on the formation of dipole moments is discussed in terms of the deviation from the cubic symmetry due to the small misfit between domains in a single grain of CaCu3Ti4O12 polycrystals.

Tunable current-voltage characteristics in polycrystalline calcium copper titanate

The nonlinear current-voltage relationship and the subsequent threshold voltage in polycrystalline CaCu3Ti4O12 are found to be easily controlled by simply doping a small amount of supervalent cations, Nb5+ and Ta5+. The authors show that the dopants have a significant effect on the reduction of the electrostatic potential at the grain boundaries, scarcely changing the conductance of the bulk grains. Through microcontact analysis and impedance spectroscopy, the control of the potential barrier at the boundaries was demonstrated to be a key factor for governing the overall current-voltage characteristics. Plausible compensation mechanisms for the excess charge of the added dopants are also discussed.

Site-selectivity of 3d metal cation dopants and dielectric response in calcium copper titanate

By doping a few atomic percent of 3d-block cations, we demonstrate that the high dielectric response in CaCu3Ti4O12 can be reduced by a factor of ∼103 at room temperature. Each of the added dopants shows its own preferential substitution on either Cu or Ti sites. The dopants that act as acceptors have a critical impact on the disappearance of the electrostatic potential barrier at grain boundaries, resulting in drastically decreased permittivity values of <90 without voltage dependence. The present doping experiment directly shows that the potential barrier at internal interfaces is a key factor for the peculiar dielectric phenomena.

Intergranular amorphous films and dislocations-promoted grain growth in SrTiO3

Abstract To investigate the effect of dislocations on grain growth in polycrystals, two sets of experiments were performed using SrTiO 3 single crystals and SrTiO 3 powder compacts. In the first set, with single crystals embedded in 2.0-mol%-TiO 2 -exess powder compacts, the growth of the single crystal was not affected by dislocations at 1300 °C, below the eutectic temperature, while it was enhanced by dislocations at 1470 °C, above the eutectic temperature, where the grain boundaries are wetted by an amorphous phase. In the second set, with 0.5-mol%-Nb 2 O 5 -doped powder compacts, the single crystal grew considerably into the SrTiO 3 matrix grains in the presence of an amorphous film between the grains at 1470 °C both in 95N 2 -5H 2 and in air, similar to the case of the TiO 2 -exess samples. However, when annealed at 1470 °C in 95N 2 -5H 2 after pre-annealing at 1250 °C in 95N 2 -5H 2 , the amorphous phase remained at triple junctions and did not penetrate the grain boundaries, implying that this boundary configuration also is thermodynamically stable above the eutectic temperature. In this case, growth of the embedded single crystal was insignificant in spite of the presence of dislocations. These experimental observations indicate that the growth of SrTiO 3 is promoted by dislocations only when an intergranular amorphous film is present at grain boundaries. The apparent ineffectiveness of dislocations on grain growth promotion without an intergranular amorphous film is discussed in terms of a low fraction of facetted grain boundaries and grain boundary drag by triple junctions.

Initial cation stoichiometry and current-voltage behavior in Sc-doped calcium copper titanate

It is demonstrated that either a strong nonlinear or a nearly Ohmic behavior in current-voltage relationship can be distinctively represented in Sc-doped polycrystalline CaCu3Ti4O12, with variation in the initial composition of samples. Through conductive atomic force microscopy and impedance spectroscopy, such a definitive difference in the electrical property was found to depend on the presence of a large electrostatic barrier at the grain boundaries. The effect of Sc ions on the boundary potential barrier and the conductance of single grains is discussed. Findings suggest a different site selectivity of Sc in the lattice in terms of the initial cation stoichiometry.

Cation Disordering by Rapid Crystal Growth in Olivine-Phosphate Nanocrystals

On the basis of Paulings first rule for ionic bonding, the coordination number of cations with oxygen anions can be determined by comparison of their relative ionic size ratio. In contrast to simple oxides, various site occupancies by multicomponent cations with similar sizes usually occur in complex oxides, resulting in distinct physical properties. Through an unprecedented combination of in situ high-temperature high-resolution electron microscopy, crystallographic image processing, geometric phase analysis, and neutron powder diffraction, we directly demonstrate that while the initial crystallites after nucleation during crystallization have a very high degree of ordering, significant local cation disordering is induced by rapid crystal growth in Li-intercalation metal-phosphate nanocrystals. The findings in this study show that control of subsequent crystal growth during coarsening is of great importance to attain a high degree of cation ordering, emphasizing the significance of atomic-level visualization in real time.