  In a joint experimental and theoretical study, we investigate the
isostructural collapse from the ambient pressure tetragonal phase to a
collapsed tetragonal phase for non-superconducting metallic SrFe2As2 and
SrFe_1.8Ru_0.2As_2. The crystallographic details have been studied using X-ray
powder diffraction up to 20 GPa pressure in a diamond anvil cell. The
structural phase transition occurs at 10 GPa and 9 GPa for SrFe2As2 and
SrFe_1.8Ru_0.2As_2, respectively. The changes in the unit cell dimensions are
highly anisotropic with a continuous decrease of the c lattice parameter with
pressure, while the a-axis length increases until the transition to a collapsed
tetragonal phase and then continues to decrease. Across the phase transition,
we observe a volume reduction of 5% and 4% for SrFe2As2 and SrFe_1.8Ru_0.2As_2,
respectively. We are able to discern that Ru substitution on the Fe-site acts
like `chemical pressure' to the system. Density-functional theory-based
calculations of the electronic structure and electron localizability indicator
are consistent with the experimental observations. Detailed analysis of the
electronic structure in k-space and real space reveals As 4pz interlayer bond
formation as the driving force of the c/a collapse with a change in the As-As
bond length of about 0.35ang.
