Ashfia Huq

Superconductivity in LaFe 1 − x Co x AsO

Here we report the synthesis and basic characterization of LaFe1-xCoxAsO for several values of x. The parent phase LaFeAsO orders antiferromagnetically (TN{approx}145 K). Replacing Fe with Co is expected both to electron dope and introduce disorder in the FeAs layer. For x=0.05 antiferromagnetic order is destroyed and superconductivity is observed at Tconset=11.2 K. For x=0.11 superconductivity is observed at Tconset=14.3 K and for x=0.15 it is observed at Tconset=6.0 K. For x=1, and the material appears to be ferromagnetic as judged by magnetization measurements. We conclude that Co is an effective dopant to induce superconductivity. Somewhat surprisingly, the system appears to tolerate considerable disorder in the FeAs planes.

Tetragonal vs. cubic phase stability in Al – free Ta doped Li7La3Zr2O12 (LLZO)

Li7La3Zr2O12 (LLZO) garnet is attracting interest as a promising Li-ion solid electrolyte. LLZO exists in a tetragonal and cubic polymorph where the cubic phase exhibits ∼2 orders of magnitude higher Li-ion conduction. It has been suggested that a critical Li vacancy concentration (0.4–0.5 atoms per formula unit) is required to stabilize the cubic polymorph of Li7La3Zr2O12. This has been confirmed experimentally for Al3+ doping on the Li+ site. Substitution of M5+ (M = Ta, Nb) for Zr4+ is an alternative means to create Li vacancies and should have the same critical Li vacancy concentration, nevertheless, subcritically doped compositions (0.25 moles of Li vacancies per formula unit) have been reported as cubic. Adventitious Al, from alumina crucibles, was likely present in these studies that could have acted as a second dopant to introduce vacancies. In this work, Al-free subcritically doped (Li6.75La3Zr1.75Ta0.25O12) and critically doped (Li6.5La3Zr1.5Ta0.5O12) compositions are investigated. X-ray diffraction indicates that both compositions are cubic. However, upon further materials characterization, including SEM analysis, Raman spectroscopy, Electrochemical Impedance Spectroscopy, and neutron diffraction it is evident that the subcritically doped composition is a mixture of cubic and tetragonal phases. The results of this study confirm that 0.4–0.5 Li vacancies per formula unit are required to stabilize the cubic polymorph of LLZO.

Glass-like phonon scattering from a spontaneous nanostructure in AgSbTe2.

Materials with very low thermal conductivity are of great interest for both thermoelectric and optical phase-change applications. Synthetic nanostructuring is most promising for suppressing thermal conductivity through phonon scattering, but challenges remain in producing bulk samples. In crystalline AgSbTe2 we show that a spontaneously forming nanostructure leads to a suppression of thermal conductivity to a glass-like level. Our mapping of the phonon mean free paths provides a novel bottom-up microscopic account of thermal conductivity and also reveals intrinsic anisotropies associated with the nanostructure. Ground-state degeneracy in AgSbTe2 leads to the natural formation of nanoscale domains with different orderings on the cation sublattice, and correlated atomic displacements, which efficiently scatter phonons. This mechanism is general and suggests a new avenue for the nanoscale engineering of materials to achieve low thermal conductivities for efficient thermoelectric converters and phase-change memory devices.

Lithium metal fluorosulfate polymorphs as positive electrodes for Li-ion batteries: synthetic strategies and effect of cation ordering

Transition-metal fluorosulfates are currently being extensively explored for their use as cathodes in Li-ion batteries. Several new polymorphs of LiMSO4F (M = Fe, Mn, Zn) crystallizing in the tavorite, triplite and sillimanite structures have captured much recent interest, but synthetic access is limited and the underlying phase stability and ion transport in these materials are poorly understood. Here we report that solvothermal routes to LiMSO4F (M = Fe, Mn, Zn) offer significant advantage over both exotic ionothermal methods and solid state synthesis by enabling greater control of the chemistry. We show new limits for the onset of triplite crystallization, and report new phases in the Li[Fe,Zn]SO4F system that enable a fuller understanding of the complex chemistry and thermodynamics underlying these fascinating materials. The transformation of LiFeSO4F from the tavorite to the triplite polymorph is triggered in the absence of any substituents, proving that tavorite is an intermediate in the reaction pathway. As a result of structural changes between tavorite and triplite, their Li+ transport paths are quite different. Combined X-ray/neutron diffraction studies of the triplites suggest that distinct inter-site zig-zag paths must be involved, owing to complete cation disorder that impacts the electrochemical behavior.

Crystal Structure, Lattice Vibrations,and Superconductivity of LaO1-xFxBiS2

Neutron scattering measurements have been performed on polycrystalline samples of the newly discovered layered superconductor LaO0:5F0:5BiS2, and its nonsuperconducting parent compound LaOBiS2. The crystal structures and vibrational modes have been examined. Upon F-doping, while the lattice contracts signicantly along c and expands slightly along a, the buckling of the BiS2 plane remains almost the same. In the inelastic measurements, a large dierence in the high energy phonon modes was observed upon F substitution. Alternatively, the low energy modes remain almost unchanged between non-superconducting and superconducting states either by F- doping or by cooling through the transition temperature. Using density functional perturbation theory we identify the phonon modes, and estimate the phonon density of states. We compare these calculations to the current measurements and other theoretical studies of this new superconducting material.

Thermoelectric transport properties of CaMg2Bi2, EuMg2Bi2, and YbMg2Bi2

The thermoelectric transport properties of CaMg

Visualizing oxygen anion transport pathways in NdBaCo2O5+δ by in situ neutron diffraction

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Properties of single crystalline AZn2Sb2 (A=Ca,Eu,Yb)

Bi

Spin-ordering and magnetoelastic coupling in the extended kagome system YBaCo4O7

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Superconductivity in LaFe1-xCoxAsO

, EuMg