Formation of collapsed tetragonal phase in EuCo$_{2}$As$_{2}$ under high pressure*

The structural properties of EuCo$_{2}$As$_{2}$ have been studied up to 35 GPa, through the use of x-ray diffraction in a diamond anvil cell at a synchrotron source. At ambient conditions, EuCo$_{2}$As$_{2}$ ($I$4/\textit{mmm}) has a tetragonal lattice structure with a bulk modulus of 48 $\pm $ 4 GPa. With the application of pressure, the $a $axis exhibits negative compressibility with a concurrent sharp decrease in $c$-axis length. The anomalous compressibility of the $a $axis continues until 4.7 GPa, at which point the structure undergoes a second-order phase transition to a collapsed tetragonal (CT) state with a bulk modulus of 111 $\pm $ 2 GPa. We found a strong correlation between the ambient pressure volume of 122 parents of superconductors and the corresponding tetragonal to collapsed tetragonal phase transition pressures.