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The structural evolution of the equilibrium and supercooled Cu46Zr54 liquids was investigated with a combination of elastic neutron scattering (with isotopic substitution) and synchrotron x-ray scattering studies. The partial pair correlation functions were determined over a wide temperature range (∼270 °C). These show that the Cu–Cu and Zr–Zr ordering increases as the temperature decreases, while the Cu–Zr ordering decreases. This surprising result is in contradiction with the results from molecular dynamics studies. [ABSTRACT FROM AUTHOR]

Even though the viscosity is one of the most fundamental properties of liquids, the connection with the atomic structure of the liquid has proven elusive. By combining inelastic neutron scattering with the electrostatic levitation technique, the time-dependent pair-distribution function (i.e., the Van Hove function) has been determined for liquid Zr80Pt20. We show that the decay time of the first peak of the Van Hove function is directly related to the Maxwell relaxation time of the liquid, which is proportional to the shear viscosity. This result demonstrates that the local dynamics for increasing or decreasing the coordination number of local clusters by one determines the viscosity at high temperature, supporting earlier predictions from molecular dynamics simulations. [ABSTRACT FROM AUTHOR]

Detailed high-energy x-ray diffraction studies were performed to gain insight into the evolution of phase formation in undercooled Fe83B17 and the mechanism for the stabilization of face-centered cubic (fcc) Fe in the presence of Fe23B6. Fe83B17 solidifies directly into either the equilibrium Fe2B + Fe phases or the metastable Fe23B6 + Fe phases. When formed, the metastable Fe23B6 phase either rapidly transforms into the equilibrium Fe2B phase within the solidification plateau or can persist down to ambient temperature. Here, we detail these different solidification behaviors in a set of thermal cycles taken from one sample and demonstrate the absence of a direct correlation with cooling rate and thermal history. We show that the coherent growth of Fe23B6 and fcc Fe suppresses the allotropic transition from fcc Fe to bcc Fe. The temperature evolution of the phase fractions and lattice parameters is also presented. [ABSTRACT FROM AUTHOR]

The crystallographic and physical properties of polycrystalline and single-crystal samples of BaCo2As2 and K-doped Ba1-xKxCo2As2 (x = 0.06,0.22) are investigated by x-ray and neutron powder diffraction, magnetic susceptibility x, magnetization, heat capacity Cp, 75As nuclear magnetic resonance (NMR), and electrical resistivity p measurements versus temperature T . The crystals were grown using both Sn flux and CoAs self-flux, where the Sn-grown crystals contain 1.6-2.0 mol% Sn. All samples crystallize in the tetragonal ThC22Si2-type structure (space group IA/mmm). For BaCo2As2, powder neutron diffraction data show that the c-axis lattice parameter exhibits anomalous negative thermal expansion from 10 K to 300 K, whereas the a axis lattice parameter and the unit cell volume show normal positive thermal expansion over this T range. No transitions in BaCo2As2 were found in this T range from any of the measurements. Below 40-50 K, we find p α T2, indicating a Fermi liquid ground state. A large density of states at the Fermi energy D(EF ) ≈ 18 states/(eV f.u.) for both spin directions is found from low-T Cp(T) measurements, whereas the band-structure calculations give D(EF) = 8.23 states/(eV f.u.). The enhancement of the former value above the latter is inferred to arise from electron-electron correlations and the electron-phonon interaction. The derived intrinsic X(T) monotonically increases with decreasing T, with anisotropy Xab > Xc. The 75As-NMR shift data versus T have the same T dependence as the derived x(T) data, demonstrating that the derived x(T) data are intrinsic. The observed 75As nuclear spin dynamics rule out the presence of Néel-type antiferromagnetic electronic spin fluctuations, but are consistent with the presence of ferromagnetic and/or stripe-type antiferromagnetic spin fluctuations. The crystals of Ba0.78K0.22Co2As2 were grown in Sn flux and show properties very similar to those of undoped BaCo2As2. On the other hand, the crystals from two batches of Ba0.94K0.06Co2As2 grown in CoAs self-flux show evidence of weak ferromagnetism at T ≲ 10 K with small ordered moments at 1.8 K of ≈0.007 and 0.03μ.b per formula unit, respectively. [ABSTRACT FROM AUTHOR]

Neutron and x-ray diffraction measurements are presented for powders and single crystals of CaCo2As2. The crystal structure is a collapsed-tetragonal ThCr2Si2-type structure as previously reported, but with 7(1)% vacancies on the Co sites corresponding to the composition CaCo1.86(2)As2. The thermal expansion coefficients for both the a and c axes are positive from 10 to 300 K. Neutron diffraction measurements on single crystals demonstrate the onset of A-type collinear antiferromagnetic order below the Néel temperature TN=52(1) K with the ordered moments directed along the tetragonal c axis, aligned ferromagnetically in the ab plane and antiferromagnetically stacked along the c axis. [ABSTRACT FROM AUTHOR]

In tetragonal SrCo2As2 single crystals, inelastic neutron scattering measurements demonstrated that strong stripe-type antiferromagnetic (AFM) correlations occur at a temperature T=5 K [Jayasekara et al., arXiv:1306.5174] that are the same as in the isostructural AFe2As2 (A = Ca, Sr, Ba) parent compounds of high-Tc superconductors. This surprising discovery suggests that SrCo2As2 may also be a good parent compound for high-Tc superconductivity. Here structural and thermal expansion, electrical resistivity ρ, angle-resolved photoemission spectroscopy (ARPES), heat capacity Cp, magnetic susceptibility χ, 75As NMR, and neutron diffraction measurements of SrCo2As2 crystals are reported together with LDA band structure calculations that shed further light on this fascinating material. The c-axis thermal expansion coefficient αc is negative from 7 to 300 K, whereas αa (the a-axis thermal expansion coefficient) is positive over this T range. The ρ(T) shows metallic character. The ARPES measurements and band theory confirm the metallic character and in addition show the presence of a flat band near the Fermi energy EF. The band calculations exhibit an extremely sharp peak in the density of states D(E≈EF) arising from a flat dx2−y2 band, where the x and y axes are along the a and b axes of the Co square lattice, respectively. A comparison of the Sommerfeld coefficient of the electronic specific heat with χ(T→0) suggests the presence of strong ferromagnetic itinerant spin correlations, which on the basis of the Stoner criterion predicts that SrCo2As2 should be an itinerant ferromagnet, in conflict with the magnetization data. The χ(T) does have a large magnitude, but also exhibits a broad maximum at ≈115 K suggestive of dynamic short-range AFM spin correlations, in agreement with the neutron scattering data. The measurements show no evidence for any type of phase transition between 1.3 and 300 K and we suggest that metallic SrCo2As2 has a gapless quantum spin-liquid ground state. [ABSTRACT FROM AUTHOR]

The short-range order (SRO) and medium-range order of electrostatically levitated Zr80Pt20 and Zr77Rh23 liquids are presented based on a combination of high-energy x-ray diffraction and time-of-flight neutron diffraction studies. The atomic structures of the Zr80Pt20 liquids were determined as a function of temperature from constrained reverse Monte Carlo simulations using x-ray and elastic neutron scattering measurements and two partial pair-distribution functions obtained from molecular dynamics (MD) simulations. Analysis of both the Faber-Ziman and Bhatia-Thornton partial structure factors shows that the Zr80Pt20 and Zr77Rh23 liquids have similar topological SRO. Interestingly, the chemical SRO appears to be much more strongly tied to the topological order in the Zr77Rh23 liquid than in the Zr80Pt20. These results demonstrate that the combination of experimental scattering measurements with MD results provides a powerful approach for obtaining details of chemical and topological ordering in metallic glasses and liquids. [ABSTRACT FROM AUTHOR]

The structural evolution of the equilibrium and supercooled Cu 46 Zr 54 liquids was investigated with a combination of elastic neutron scattering (with isotopic substitution) and synchrotron x-ray scattering studies. The partial pair correlation functions were determined over a wide temperature range (∼270 °C). These show that the Cu-Cu and Zr-Zr ordering increases as the temperature decreases, while the Cu-Zr ordering decreases. This surprising result is in contradiction with the results from molecular dynamics studies.

Even though the viscosity is one of the most fundamental properties of liquids, the connection with the atomic structure of the liquid has proven elusive. By combining inelastic neutron scattering with the electrostatic levitation technique, the time-dependent pair-distribution function (i.e., the Van Hove function) has been determined for liquid Zr 80 Pt 20 . We show that the decay time of the first peak of the Van Hove function is directly related to the Maxwell relaxation time of the liquid, which is proportional to the shear viscosity. This result demonstrates that the local dynamics for increasing or decreasing the coordination number of local clusters by one determines the viscosity at high temperature, supporting earlier predictions from molecular dynamics simulations.

High-energy x-ray diffraction measurements of undercooled, electrostatically levitated Ni50Zr50 liquid droplets were performed. The observed solidification pathway proceeded through the nucleation and growth of the metastable B2 phase, which persisted for several seconds before the rapid appearance of the stable B33 phase. This sequence is shown to be consistent with predictions from classical nucleation theory using data obtained from molecular dynamics (MD) simulations. A plausible mechanism for the B2-B33 transformation is proposed and investigated through further MD simulations.