  Single crystals of mixed alkaline earth metal iron arsenide materials of
Ba1-xSrxFe2As2 and Ba0.5Sr0.5(Fe1-yCoy)2As2 are synthesized via the self-flux
method. Ba1-xSrxFe2As2 display spin-density wave features (TN) at temperatures
intermediate to the parent materials, x = 0 and 1, with TN(x) following an
approximately linear trend. Cobalt doping of the 1 to 1 Ba:Sr mixture,
Ba0.5Sr0.5(Fe1-yCoy)2As2, results in a superconducting dome with maximum
transition temperature of TC = 19 K at y = 0.092, close to the maximum
transition temperatures observed in unmixed A(Fe1-yCoy)2As2; however, an
annealed crystal with y = 0.141 showed a TC increase from 11 to 16 K with a
decrease in Sommerfeld coefficient from 2.58(2) to 0.63(2) mJ/(K2 mol atom).
For the underdoped y = 0.053, neutron diffraction results give evidence that TN
and structural transition (To) are linked at 78 K, with anomalies observed in
magnetization, resistivity and heat capacity data, while a superconducting
transition at TC ~ 6 K is seen in resistivity and heat capacity data. Scanning
tunneling microscopy measurements for y = 0.073 give Dynes broadening factor of
1.15 and a superconducting gap of 2.37 meV with evidence of surface
inhomogeneity.
