Crystallography and physical properties of BaCo2 As2, Ba0.94 K0.06 Co2 As2, and Ba0.78 K0.22 Co2 As2

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 χ, magnetization, heat capacity Cp,As75 nuclear magnetic resonance (NMR), and electrical resistivity ρ 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 ThCr2Si2-type structure (space group I4/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 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 χ(T) monotonically increases with decreasing T, with anisotropy χab>χc. The 75As-NMR shift data versus T have the same T dependence as the derived χ(T) data, demonstrating that the derived χ(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 10 K with small ordered moments at 1.8 K of ≈0.007 and 0.03μB per formula unit, respectively.