[Show abstract][Hide abstract] ABSTRACT: The effects of laser wavelength (355nm and 532nm) and laser pulse energy on the quantitative analysis of LiFePO4 by atom probe tomography are considered. A systematic investigation of ultraviolet (UV, 355nm) and green (532nm) laser assisted field evaporation has revealed distinctly different behaviors. With the use of a UV laser, the major issue was identified as the preferential loss of oxygen (up to 10at%) while other elements (Li, Fe and P) were observed to be close to nominal ratios. Lowering the laser energy per pulse to 1pJ/pulse from 50pJ/pulse increased the observed oxygen concentration to nearer its correct stoichiometry, which was also well correlated with systematically higher concentrations of (16)O2(+) ions. Green laser assisted field evaporation led to the selective loss of Li (~33% deficiency) and a relatively minor O deficiency. The loss of Li is likely a result of selective dc evaporation of Li between or after laser pulses. Comparison of the UV and green laser data suggests that the green wavelength energy was absorbed less efficiently than the UV wavelength because of differences in absorption at 355 and 532nm for LiFePO4. Plotting of multihit events on Saxey plots also revealed a strong neutral O2 loss from molecular dissociation, but quantification of this loss was insufficient to account for the observed oxygen deficiency.
[Show abstract][Hide abstract] ABSTRACT: It is demonstrated that Na ions are mobile at room temperature in the nitridophosphate compound Na3TiP3O9N, with a diffusion pathway that is calculated to be fully three-dimensional and isotropic. When used as a cathode in Na-ion batteries, Na3TiP3O9N has an average voltage of 2.7 V vs Na+/Na and cycles with good reversibility through a mechanism that appears to be a single solid solution process without any intermediate plateaus. X-ray and neutron diffraction studies as well as first-principles calculations indicate that the volume change that occurs on Na-ion removal is only about 0.5%, a remarkably small volume change given the large ionic radius of Na+. Rietveld refinements indicate that the Na1 site is selectively depopulated during sodium removal. Furthermore, the refined displacement parameters support theoretical predictions that the lowest energy diffusion pathway incorporates the Na1 and Na3 sites while the Na2 site is relatively inaccessible. The measured room temperature ionic conductivity of Na3TiP3O9N is substantial (4 × 10–7 S/cm), though both the strong temperature dependence of Na-ion thermal parameters and the observed activation energy of 0.54 eV suggest that much higher ionic conductivities can be achieved with minimal heating. Excellent thermal stability is observed for both pristine Na3TiP3O9N and desodiated Na2TiP3O9N, suggesting that this phase can serve as a safe Na-ion battery electrode. Moreover, it is expected that further optimization of the general cubic framework of Na3TiP3O9N by chemical substitution will result in thermostable solid state electrolytes with isotropic conductivities that can function at temperatures near or just above room temperature.
Full-text · Article · May 2014 · Chemistry of Materials
[Show abstract][Hide abstract] ABSTRACT: Improved methods for the flux growth of single crystals of the important battery material LiFePO4 have been developed, allowing the facile preparation of single crystals up to 1 cm across with well-developed facets at relatively low temperatures. The structural characterization of these samples by both powder X-ray diffraction and single crystal diffraction (X-ray and neutron) indicates that the samples are typically stoichiometric with a very low concentration of Fe defects on the Li site, though crystals with larger concentrations of defects can be specifically grown using Fe-rich fluxes. These defects occur through the formation of a Fe-rich (Li1–2xFex)FePO4 partial solid solution, in contrast to the antisite defects more commonly discussed in the literature which would preserve the ideal LiFePO4 stoichiometry. The LiFePO4 defects are shown to be sarcopside-like (2 Li+ → Fe2+ + vacancy) based on compositions refined from single crystal diffraction data, the observed dependence of unit cell parameters on defect concentration, and their observed phase behavior (defects only appear in growths from fluxes which are Fe-rich relative to stoichiometric LiFePO4). The distribution of defects has been studied by aberration corrected scanning transmission electron microscopy and was found to be highly inhomogenous, suggesting that defect-containing crystals may consist of endotaxial intergrowths of olivine LiFePO4 and sarcopside Fe3(PO4)2 in a manner that minimizes the detrimental influence of FeLi defects on the rate of Li-ion transport within crystallites.
No preview · Article · Oct 2013 · Chemistry of Materials
[Show abstract][Hide abstract] ABSTRACT: We present here the results of electrical resistivity ρ, magnetization M, ac susceptibility χac′, and specific heat CM measurements that have been carried out on single crystals of Yb3Pt4 over a wide range of fields and temperatures. The 2.4-K Néel temperature that is found in zero field collapses under field to a first-order transition TN=0 at BCEP=1.85 T. In the absence of antiferromagnetic order, the specific heat CM(T,B), the magnetization M(T,B), and even the resistivity ρ(T,B) all display B/T scaling, indicating that they are dominated by strong paramagnetic fluctuations, where the only characteristic energy scale results from the Zeeman splitting of an energetically isolated, Yb doublet ground state. This paramagnetic scattering disappears with the onset of antiferromagnetic order, revealing Fermi liquid behavior Δρ=AT2 that persists up to the antiferromagnetic phase line TN(B), but not beyond. The first-order character of TN=0 and the ubiquity of the paramagnetic fluctuations imply that non-Fermi-liquid behaviors are absent in Yb3Pt4. In contrast to heavy fermions such as YbRh2Si2, Yb3Pt4 represents an extremely simple regime of f-electron behavior where the Yb moments and conduction electrons are almost decoupled, and where Kondo physics plays little role.
[Show abstract][Hide abstract] ABSTRACT: The crystal structure of the promising Li-ion battery cathode material LiFeBO(3) has been redetermined based on the results of single crystal X-ray diffraction data. A commensurate modulation that doubles the periodicity of the lattice in the a-axis direction is observed. When the structure of LiFeBO(3) is refined in the 4-dimensional superspace group C2/c(α0γ)00, with α = 1/2 and γ = 0 and with lattice parameters of a = 5.1681 Å, b = 8.8687 Å, c = 10.1656 Å, and β = 91.514°, all of the disorder present in the prior C2/c structural model is eliminated and a long-range ordering of 1D chains of corner-shared LiO(4) is revealed to occur as a result of cooperative displacements of Li and O atoms in the c-axis direction. Solid-state hybrid density functional theory calculations find that the modulation stabilizes the LiFeBO(3) structure by 1.2 kJ/mol (12 meV/f.u.), and that the modulation disappears after delithiation to form a structurally related FeBO(3) phase. The band gaps of LiFeBO(3) and FeBO(3) are calculated to be 3.5 and 3.3 eV, respectively. Bond valence sum maps have been used to identify and characterize the important Li conduction pathways, and suggest that the activation energies for Li diffusion will be higher in the modulated structure of LiFeBO(3) than in its unmodulated analogue.
No preview · Article · Jun 2012 · Journal of the American Chemical Society
[Show abstract][Hide abstract] ABSTRACT: Lithium iron borate (LiFeBO3) is a particularly desirable cathode material for lithium-ion batteries due to its high theoretical capacity (220 mA h g−1) and its favorable chemical constituents, which are abundant, inexpensive and non-toxic. However, its electrochemical performance appears to be severely hindered by the degradation that results from air or moisture exposure. The degradation of LiFeBO3 was studied through a wide array of ex situ and in situ techniques (X-ray diffraction, nuclear magnetic resonance, X-ray absorption spectroscopy, electron microscopy and spectroscopy) to better understand the possible degradation process and to develop methods for preventing degradation. It is demonstrated that degradation involves both Li loss from the framework of LiFeBO3 and partial oxidation of Fe(II), resulting in the creation of a stable lithium-deficient phase with a similar crystal structure to LiFeBO3. Considerable LiFeBO3 degradation occurs during electrode fabrication, which greatly reduces the accessible capacity of LiFeBO3 under all but the most stringently controlled conditions for electrode fabrication. Comparative studies on micron-sized LiFeBO3 and nanoscale LiFeBO3–carbon composite showed a very limited penetration depth (30 nm) of the degradation phase front into the LiFeBO3 core under near-ambient conditions. Two-phase reaction regions during delithiation and lithiation of LiFeBO3 were unambiguously identified through the galvanostatic intermittent titration technique (GITT), although it is still an open question as to whether the two-phase reaction persists across the whole range of possible Li contents. In addition to the main intercalation process with a thermodynamic potential of 2.8 V, there appears to be a second reversible electrochemical process with a potential of 1.8 V. The best electrochemical performance of LiFeBO3 was ultimately achieved by introducing carbon to minimize the crystallite size and strictly limiting air and moisture exposure to inhibit degradation.
No preview · Article · May 2012 · Journal of Materials Chemistry
[Show abstract][Hide abstract] ABSTRACT: We present measurements of the specific heat, magnetization, magnetocaloric effect, and magnetic neutron diffraction carried out on single crystals of antiferromagnetic Yb3Pt4, where highly localized Yb moments order at TN=2.4 K in zero field. The antiferromagnetic order was suppressed to TN-->0 by applying a field of 1.85 T in the ab plane. Magnetocaloric effect measurements show that the antiferromagnetic phase transition is always continuous for TN>0, although a pronounced step in the magnetization is observed at the critical field in both neutron diffraction and magnetization measurements. These steps sharpen with decreasing temperature, but the related divergences in the magnetic susceptibility are cut off at the lowest temperatures, where the phase line itself becomes vertical in the field-temperature plane. As TN-->0, the antiferromagnetic transition is increasingly influenced by a quantum critical end point, where TN ultimately vanishes in a first-order phase transition.
[Show abstract][Hide abstract] ABSTRACT: We present measurements of the magnetization M, ac susceptibility χ′, electrical resistivity ρ, and specific heat C in single crystals of metallic YFe2Al10. The magnetic susceptibility follows a Curie-Weiss temperature dependence for 75K≤T≤750 K, with a fluctuating Fe moment of 0.45μB/Fe, and the ac susceptibility χ′ diverges at lower temperatures χ′∼T-1.28±0.04 when the ac field is in the basal plane. The field B and temperature T dependencies of the magnetization M are well described by the scaling expression MT -β = F(B/Tβ+γ) for 1.8K≤T≤30 K and for fields larger than 0.1T. These results indicate that strong quasi-two-dimensional critical fluctuations are present that can be suppressed by magnetic fields. The magnetic and electronic parts of the specific heat CM show a similar divergence for 0.4K≤T≤12 K, where C M/T∼T-0.47±0.03. The divergences in χ′ and CM/T indicate that YFe2Al 10 is located near a quantum critical point, and no magnetic order is observed above 0.09 K. We argue that our results are inconsistent with quantum impurity or disorder models, suggesting instead that YFe2Al 10 is on the verge of bulk magnetic ordering, and that the critical fluctuations that are associated with this quantum critical point lead to the divergencies in CM/T and χ′.
[Show abstract][Hide abstract] ABSTRACT: We present a report on the physical properties of the transition-metal-based ferromagnets HfFeGa2 and HfMnGa2. The magnetic susceptibility in both displays Curie-Weiss behavior at high temperature that is replaced by the critical susceptibility just above the Curie temperatures, which are 47.9 K in HfFeGa2 and 25.6 K in HfMnGa2. The ferromagnetically ordered state has a coercive field of 1700 Oe in HfFeGa2 and 320 Oe in HfMnGa2, with strong anisotropy that largely confines the moments to the b axis. Critical exponents that are derived from neutron diffraction measurements and Arrott plot analyses of the magnetization confirm the mean-field character of the ferromagnetic transitions. Phonons dominate the specific heat at all temperatures, but clear ordering anomalies accompany the onset of ferromagnetic order, as well as an electronic component that is larger in the ordered than paramagnetic states. Both HfFeGa2 and HfMnGa2 are metallic, and we observe an anomalous exponent in the temperature-dependent resistivity ρ(T), where ρ(T)-ρ0=BT5/3, signaling that the ordered state is a marginal Fermi liquid. Overall, the robustness of ferromagnetic order, the Curie temperatures, and the impact of fluctuations in both HfFeGa2 and HfMnGa2 are very similar to those of previously studied ferromagnets, such as MnSi, ZrZn2, Ni3Al, and Sc3In.
No preview · Article · May 2011 · Physical review. B, Condensed matter
[Show abstract][Hide abstract] ABSTRACT: Single crystals of HfMnGa2, space group Pnma, were grown using a Ga self flux technique. A sharp peak in the AC susceptibility chiAC shows a phase transition at TC 26 K, followed by Curie-Weiss behavior at higher temperatures. Arrott plot analysis confirms this transition is ferromagnetic with a spontaneous moment of mu00.3 muB/ Mn. HfMnGa2 has a coercive field of ˜ 0.1 T as well as a large magnetic anisotropy that restricts the moments to point in the  direction. Both a large Rhodes-Wohlfarth parameter mufluct/mu03.6 and low TC suggest HfMnGa2 is comparable to other itinerant ferromagnets MnSi, ZrZn2 and Ni3Al. Resistivity rho(T) shows HfMnGa2 to be metallic with rho(T)-rho0 having a T^5/3 dependance in the ordered state. This non-Fermi liquid relationship was also observed in Ni3Al and ZrZn2 over a more limited range of temperatures.
[Show abstract][Hide abstract] ABSTRACT: Yb3Pt4 is an antiferromagnet that orders at TN=2.4K. Magnetic fields B suppress TN, and the B-T phase line TN(B) terminates almost vertically at T=0, BC=2.0 T. Specific heat measurements find a mean-field transition at TN(B), and the magnetocaloric effect shows that the antiferromagnetic transition is continuous at all fields, with no associated latent heat. However, neutron diffraction measurements performed for B˜BC find that a distinct step in the magnetization deltaM occurs near the transition, with a magnitude that increases for T0. We argue that a nonzero magnetization step deltaM is required to give deltaS=0 for T=0, since the vertical phase line at T=0 implies dTN/dB=-deltaM/deltaS->-∞. We argue that TN (B) terminates at BC in a T=0 first order transition.
[Show abstract][Hide abstract] ABSTRACT: Muon spin rotation and relaxation measurements have been carried out on the unconventional antiferromagnet Yb(3)Pt(4). Oscillations are observed below T(N) = 2.22(1) K, consistent with the antiferromagnetic (AFM) Néel temperature observed in bulk experiments. In agreement with neutron diffraction experiments the oscillation frequency ω(μ)(T)/2π follows an S = 1/2 mean-field temperature dependence, yielding a quasistatic local field of 1.71(2) kOe at T = 0. A crude estimate gives an ordered moment of ∼ 0.66 μ(B) at T = 0, comparable to 0.81 μ(B) from neutron diffraction. As [Formula: see text] from above the dynamic relaxation rate λ(d) exhibits no critical slowing down, consistent with a mean-field transition. In the AFM phase a T-linear fit to λ(d)(T), appropriate to a Fermi liquid, yields highly enhanced values of λ(d)/T and the Korringa constant K(μ)(2)T/λ(d), with K(μ) the estimated muon Knight shift. A strong suppression of λ(d) by applied field is observed in the AFM phase. These properties are consistent with the observed large Sommerfeld-Wilson and Kadowaki-Woods ratios in Yb(3)Pt(4) (although the data do not discriminate between Fermi-liquid and non-Fermi-liquid states), and suggest strong enhancement of q≈0 spin correlations between large-Fermi-volume band quasiparticles in the AFM phase of Yb(3)Pt(4).
Full-text · Article · Feb 2011 · Journal of Physics Condensed Matter
[Show abstract][Hide abstract] ABSTRACT: We review here the results of magnetization, specific heat, and inelastic neutron scattering measurements conducted on Yb3Pt4, Yb2Pt2Pb, Yb5Pt9, and YbRh2Pb, which indicate that the Yb moments in these heavy electron compounds are appreciably localized, at least in their paramagnetic
states. The magnetic ground states in each are isolated magnetic doublets, and we show that magnetic fields suppress long
ranged magnetic order and lead to a characteristic magnetic field-temperature phase diagram where order vanishes suddenly
above a critical value for the field. We argue that the stability of magnetic order in these compounds arises from the competition
between the Zeeman splitting gμ
H of the ground state doublet, which favors a spin polarized state with minimal entropy and without long range order, and
the exchange splitting Δ of the doublet, which enables long ranged magnetic order.
KeywordsQuantum critical points-Ytterbium compounds
No preview · Article · Oct 2010 · Journal of Low Temperature Physics
[Show abstract][Hide abstract] ABSTRACT: Aiming to study quantum critical behavior in itinerant-electron transition-metal ferromagnets, we have recently been first to characterize the intermetallic compound HfFeGa2. We have found that HfFeGa2 is a ferromagnet with a low ordering temperature and a small ordered magnetic moment. We present extensive magnetization and neutron diffraction studies of single crystals of pristine HfFeGa2. ac-magnetization data indicates the Curie temperature TC ˜ 49 K, and above TC we find Curie-Weiss behavior with an effective moment of ˜ 2.2 muB/Fe. HfFeGa2 is ferromagnetic with the ordered magnetic moments parallel to the easy c-axis of the orthorhombic crystal structure, and with a zero-temperature spontaneous magnetization of ˜ 0.6 muB/Fe. By a scaling analysis of magnetization and neutron diffraction data we have determined TC = 48.3 K as well as the critical exponents beta= 0.494 and gamma= 1.21. Our results indicate HfFeGa2 is comparable to the well-known itinerant ferromagnet ZrZn2.
[Show abstract][Hide abstract] ABSTRACT: A unique system among f-electron based quantum critical systems, Yb3Pt4 orders antiferromagnetically at 2.4 K. Heat capacity, magnetocaloric effect and neutron diffraction experiments show the magnetic order can be suppressed to lower temperatures by magnetic fields applied in the easy ab plane of the rhombohedral structure. A mean-field-like anomaly in temperature-dependent heat capacity is reduced with increasing field (H //a), and disappears at (1.5K,1.6T). However, the anomaly seen in the field-dependent heat capacity at temperatures as low as 0.1K, indicates that the phase boundary line continues, showing a possible quantum critical point at about 1.8 T. Isentropes determined by direct measurements of the magnetocaloric effect (H//a) show a slope change, consistent with a continuous phase transition at all temperatures below 2.4K. Field-dependent (H//b) diffracted magnetic peak intensity is consistent with both thermodynamic measurements down to 1.5K. At lower temperatures, observations indicate two phase transitions. The upper field transition, a step in magnetization, appears to be first order. Details of the experiments and the H-T phase diagram will be discussed.
[Show abstract][Hide abstract] ABSTRACT: We present a study of the magnetic and thermodynamic properties of HfFe1-xRuxGa2 single crystals grown using flux techniques. Having found a low temperature ferromagnetic intermetallic compound HfFeGa2 we try to suppress the Curie Temperature (Tc) by doping with Ru as a means to investigate the evolution of critical phenomena and perhaps realize a ferromagnetic quantum critical point (QCP). Magnetization measurements have shown changes in Tc of HfFe1-xRuxGa2 from approximately 48K to below 1.8K as a function of Ru concentration (x). We will show recent data as well as discuss the development of the spontaneous moment (m0), susceptibility chi along with heat capacity upon doping and present the resulting magnetic phase diagram.
[Show abstract][Hide abstract] ABSTRACT: We have used neutron-diffraction measurements to study the zero-field magnetic structure of the intermetallic compound Yb3Pt4, which was earlier found to order antiferromagnetically at the Néel temperature TN=2.4 K, and displays a field-driven quantum-critical point at 1.6 T. In Yb3Pt4, the Yb moments sit on a single low-symmetry site in the rhombohedral lattice with space group R3̅ . The Yb ions form octahedra with edges that are twisted with respect to the hexagonal unit cell, a twisting that results in every Yb ion having exactly one Yb nearest neighbor. Below TN, we found new diffracted intensity due to a k=0 magnetic structure. This magnetic structure was compared to all symmetry-allowed magnetic structures and was subsequently refined. The best-fitting magnetic-structure model is antiferromagnetic and involves pairs of Yb nearest neighbors on which the moments point almost exactly toward each other. This structure has moment components within the ab plane as well as parallel to the c axis although the easy magnetization direction lies in the ab plane. Our magnetization results suggest that besides the crystal-electric-field anisotropy, anisotropic exchange favoring alignment along the c axis is responsible for the overall direction of the ordered moments. The magnitude of the ordered Yb moments in Yb3Pt4 is 0.81μB/Yb at 1.4 K. The analysis of the bulk properties, the size of the ordered moment, and the observation of well-defined crystal-field levels argue that the Yb moments are spatially localized in zero field.
Full-text · Article · Feb 2010 · Physical Review B
[Show abstract][Hide abstract] ABSTRACT: Heavy electron systems provide ideal venues to study a range of issues associated with quantum criticality, including unconventional
electronic phases, moment formation, and complex phase diagrams with exotic critical phenomena. In the heavy electron antiferromagnets
studied so far, magnetic order occurs via a second order phase transition which can be tuned via pressure or field to a quantum
critical point. Fermi liquid behavior is found beyond the quantum critical point, and the quasiparticle mass diverges at the
quantum critical point, nucleating the moments required to enable magnetic order itself. We review here our experimental results
on a new heavy electron system, Yb3Pt4, where antiferromagnetic order is weakly first order in zero field, but becomes second order at a critical endpoint with
the application of magnetic field. No divergence of the quasiparticle mass is observed near the quantum critical field, and
instead magnetic order is driven by the exchange enhancement of the Fermi liquid itself. These data support the thesis that
there are multiple routes to quantum criticality in the heavy electron compounds.