Single crystals of BaFe 2 As 2 and Ba 0.55 K 0.45 Fe 2 As 2 have been grown out of excess Sn with 1% or less incorporation of solvent. The crystals are exceptionally micaceous, are easily exfoliated, and can have dimensions as large as 3 ϫ 3 ϫ 0.2 mm 3 . The BaFe 2 As 2 single crystals manifest a structural phase transition from a high-temperature tetragonal phase to a low-temperature orthorhombic phase near 85 K and do not show any sign of superconductivity down to 1.8 K. This transition can be detected in the electrical resistivity, Hall resistivity, specific heat, and the anisotropic magnetic susceptibility. In the Ba 0.55 K 0.45 Fe 2 As 2 single crystals this transition is suppressed and instead superconductivity occurs with a transition temperature near 30 K. Whereas the superconducting transition is easily detected in resistivity and magnetization measurements, the change in specific heat near T c is small, but resolvable, giving ⌬C p / ␥T c Ϸ 1. The application of a 140 kOe magnetic field suppresses T c by only ϳ4 K when applied along the c axis and by ϳ2 K when applied perpendicular to the c axis. The ratio of the anisotropic upper critical fields, ␥ = H c2Ќc / H c2 ʈc , varies between 2.5 and 3.5 for temperatures down to ϳ2 K below T c .

We present a systematic investigation of the antiferromagnetic ordering and structural distortion for the series of Ba(Fe1−xRux)2As2 compounds (0 ≤ x ≤ 0.246). Neutron and x-ray diffraction measurements demonstrate that, unlike for the electron-doped compounds, the structural and magnetic transitions remain coincident in temperature. Both the magnetic and structural transitions are gradually suppressed with increased Ru concentration and coexist with superconductivity. For samples that are superconducting, we find strong competition between superconductivity, the antiferromagnetic ordering, and the structural distortion.

The atomic structures in equilibrium and supercooled liquids of Zr 80 Pt 20 were determined as a function of temperature by in-situ high-energy synchrotron diffraction studies of the levitated liquids (containerless processing) using the beamline electrostatic levitation (BESL) technique. The presence of a pronounced pre-peak at q ~ 1.7 Å -1 in the static structure factor indicates medium range order (MRO) in the liquid. The position and intensity of the pre-peak remain constant with cooling, indicating that the MRO is already present in the liquid above its melting temperature. An analysis of the liquid atomic structures obtained using the Reverse Monte Carlo (RMC) method utilizing both the structure factor, S(q), from x-ray diffraction experiments and the partial pair-correlation functions from ab initio molecular dynamics (MD) simulations show that the pre-peak arises from a Pt-Pt correlation that can be indentified with icosahedral short-range order around the Pt atoms. The local atomic ordering is dominated by icosahedral-like structures, raising the nucleation barrier between the liquid and these phases, thus assisting glass formation.