We report on high-pressure powder synchrotron x-ray diffraction studies on $M$Fe${}_2$As${}_2$ ($M$$=$Ba, Ca) over a range of temperatures and pressures up to about 56 GPa using a membrane diamond-anvil cell. Our data indicate a phase transition to a collapsed tetragonal phase in both compounds upon compression. The data at 300 K are measured in both pressure-increasing and -decreasing cycles. Our measurements show that at 300 K in the Ba compound, the transition occurs at 27 GPa, which is much higher than the transition pressure of 1.7 GPa in the Ca compound. At low temperature, we could obtain data only in the pressure-increasing cycle, therefore a precise transition pressure is not identified. At a low temperature of 33 K, the transition to the tetragonal phase in the Ba compound starts, upon compression, at about 29 GPa, which is much higher than the transition pressure of 0.3 GPa at 40 K as known in the case of the Ca compound. The much higher transition pressure in the Ba compound may be due to its larger unit-cell volume at ambient pressure. 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 $c_t/a_t$ ratios. We also show that the FeAs${}_4$ tetrahedra are much less compressible and more distorted in the collapsed tetragonal phase than their nearly regular shape in the ambient pressure phase. We present a detailed analysis of the pressure dependence of the structures as well as equations of state in these important BaFe${}_2$As${}_2$ and CaFe${}_2$As${}_2$ compounds.

• Received 14 July 2010

DOI:https://doi.org/10.1103/PhysRevB.83.054503