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Superconductivity in MFe2As2 (M = Ba, Ca) compounds appears either at a critical doping level at ambient pressure or in the parent compound itself by application of pressure above a critical value. We report high pressure powder x-ray diffractions studies for these compounds at 300 K up to about 56 GPa using membrane diamond anvil cells. The measurements for BaFe2As2 show a new tetragonal to collapsed tetragonal phase transition at about 22 GPa that remains stable upto 56 GPa. CaFe2As2 is already known to transform to collapsed phase at 1.7 GPa at 300 K. Our measurements on CaFe2As2 do not show any post collapsed phase transition on increase of pressure 50 GPa at 300 K. It is important to note that the transition in both compounds occurs when they are compressed to almost the same value of the unit cell volume and attain similar ct/at ratios. We present a detailed analysis of the pressure dependence and structure phase transitions as well as equation of state in these important FeAs compounds that should be useful in the context of possible superconductivity in the collapsed phase. [ABSTRACT FROM AUTHOR]

We report high pressure powder x-ray diffractions studies for AFe2As2 (A = Ba and Ca) at high pressures up to 56 GPa at 300 K and low temperatures down to 33 K. A phase transition to a collapsed tetragonal phase is observed in AFe2As2. However, at 300 (33) K in Ba-compound the transition occurs at 26 (29) GPa, which is much higher pressure than 1.7 (0.3) GPa at 300 (40) K in the Ca-compound, due to its larger volume. The transition to collapsed phase occurs in the two compounds at nearly the same values of unit cell volume and ct/at ratio. We present a detailed analysis of the pressure dependence structure phase transitions and equation of state in these important FeAs compounds. [ABSTRACT FROM AUTHOR]