The Absence of Superconductivity in Single Phase CaFe2As2 under   Hydrostatic Pressure
  Recent high-pressure studies found that superconductivity can be achieved under very low pressure in the parent iron arsenide compound CaFe2As2, although details of the sharpness and temperature of transitions vary between liquid medium and gas medium measurements. To better understand this issue, we performed high-pressure susceptibility and transport studies on CaFe2As2, using helium as the pressure medium. The signatures of the transitions to the low-temperature orthorhombic and collapsed tetragonal phase remained exceptionally sharp and no signature of bulk superconductivity was found under our hydrostatic conditions. Our results suggest that phase separation and superconductivity in CaFe2As2 are induced by non-hydrostatic conditions associated with the frozen liquid media. 
W. Yu, A.A. Aczel, T.J. Williams, S.L. Bud'ko, N. Ni, P.C. Canfield,   and G.M. Luke