Single crystals of silicon doped LiFePO4 with a silicon content of 1% are grown successfully by the floating zone technique and characterized by single‐crystal and powder X‐ray diffraction, secondary ion mass spectroscopy, and chemical analysis. Electron paramagnetic resonance demonstrates the presence of only Fe2+; no traces of Fe3+ are found. Impedance spectroscopy as well as step‐function polarization/depolarization (DC) measurements are carried out using the cells Ti/LiFe(Si)PO4/Ti and LiAl/LiI/LiFe(Si)PO4/LiI/LiAl. The electronic and ionic conductivities as well as the Li‐diffusivity of the sample in the major crystallographic directions ([h00], [0k0], and [00l]) are determined. Within experimental error the transport properties along the b‐ and c‐axes are found to be the same but differ significantly from the a‐axis, which exhibits lower values. Compared to undoped LiFePO4, Si‐doping leads to an increase of the ionic conductivity while the electronic conductivity decreases, which is in agreement with a donor effect. The activation energies of conductivities and diffusivities are interpreted in terms of defect chemistry and relevant Brouwer diagrams are given.

We demonstrate the preparation of large, free standing iron pnictide single crystals with a size up to 20 x 10 x 1 mm 3 using solvents in zirconia crucibles under argon atmosphere. Transport and magnetic properties are investigated to study the effect of potassium doping on the structural and superconducting property of the compounds. The spin density wave (SDW) anomaly at T s ~138 K in BaFe 2 As 2 single crystals from self-flux shifts to T s ~85 K due to Sn solvent growth. We show direct evidence for an incorporation of Sn on the Fe site. The electrical resistivity data show a sharp superconducting transition temperature T c~3 8.5 K for the single crystal of Ba 0.68 K 0.32 Fe 2 As 2.A nearly 100% shielding fraction and bulk nature of the superconductivity for the single crystal were confirmed by magnetic susceptibility data. A sharp transition T c~2 5 K occurred for the single crystal of Sr 0.85 K 0.15 Fe 2 As 2 . There is direct evidence for a coexistence of the SDW and superconductivity in the low doping regime of Sr 1-x K x Fe 2 As 2 single crystals. Structural implications of the doping effects as well as the coexistence of the two order parameters are discussed.

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The DC and AC magnetic susceptibilities of Na0.85CoO2 single crystals were measured for the different crystal orientations of H (ab)-and H (c)-axis. The DC-magnetic susceptibility for H (c)-direction exhibited the antiferromagnetic transition at TN = 22 K. The thermal hysteresis between the zero-field-cooled (ZFC) and the field-cooled (FC) magnetization below TN and the large frustration parameter indicated the spin frustration along the c-axis. For an applied magnetic field in H (ab)-plane, the DC magnetic susceptibility exhibited the logarithmic divergent behavior at low temperatures (T ≤ 6.8 K). This could be understood by the impurity spin effect, dressed by the spin fluctuation. From the AC magnetic susceptibility measurements, the real part of the AC-susceptibility for H (ab) exhibited the spin glass-like behavior at low temperatures (T ≤ 4 K). Remarkably, for an applied AC magnetic field with H (c)-axis, the sign of the AC magnetic susceptibility changed from a positive to a negative value with increasing AC magnetic field frequency (f ≥ 3 kHz) at low temperatures (T ≤ 7 K). We interpret the sign change of AC magnetic susceptibility along the c-axis in terms of the sudden sign reversal of the phase difference φ from in-phase to out-of-phase response with an applied AC magnetic field in the AC-susceptibility phase space.