The structural and physical properties of Ba_1-xSr _xFe₂As₂(0 ≤ x ≤ 1) and Ba _1-xSr_xFe_1.8Co_0.2As₂(0 ≤ x ≤ 1)

Polycrystalline samples of Ba_1-xSr_xFe ₂As₂(0≤x≤1) and Ba_1-xSr _xFe_1.8Co_0.2As₂ (0≤x≤1) have been synthesized by a solid state reaction method. Structural analysis by means of x-ray diffraction shows that the lattice parameters and unit cell volume decrease monotonically with the increase of x for Ba_1-xSr _xFe₂As₂. The measurements of transport properties demonstrate that the average size of the Ba(Sr)-site cations could evidently influence the spin density wave (SDW) behavior in Ba _1-xSr_xFe₂As₂ and superconductivity in Ba_1-xSr_xFe_1.8Co_0.2As₂ as well. The critical temperature for SDW (T_SDW) increases with the Sr substitution for Ba in Ba_1-xSr_xFe₂As ₂ and, on the other hand, the superconducting T_c decreases with the increase of Sr content in Ba_1-xSr_xFe _1.8Co_0.2As₂. The inhomogeneous distributions of Ba/Sr ions and structural distortions in Ba_0.5Sr _0.5Fe₂As₂ have been investigated by transmission-electron microscopy (TEM) observations.