Tetragonal lattice collapse in SrFe$_2$As$_2$ - a combined experimental and theoretical study

In a joint experimental and theoretical study we investigate the crystal structure of the Fe pnictide compounds SrFe$_2$As$_2$ under applied hydrostatic pressure. Applying high pressure X-ray diffractyion, for a critical pressure of about 10 GPa we observe a sudden collapse of the tetragonal $c$ axis, accompagnied by a small expansion of the basal plane. This results in a drastic reduction of the $c/a$ ratio and a significant decrease of the unit cell volume. This tetragonal collapse is well described by DFT band structure calculations and can be assigne d to the formation of an additional As-As bond along the tetragonal $c$ axis.