We determined pressure-volume (P-V) data for (Mg0.6Fe0.4)SiO3 post-perovskite (ppv) upon decompression from 140 GPa. The data can be divided into four regions: above 95 GPa, diffraction peaks are sharp and follow a smooth and tight P-V curve; below 90 GPa, peak widths increase, and a kink develops in the P-V curve; below 60 GPa, the broadening plateaus; below 4.6 GPa, the ppv pattern disappears. This suggests that defects and stacking faults start to develop in ppv at 90 GPa and saturate at 60 GPa, and the structure disintegrates below 4.6 GPa. The volume and density of the (Mg0.6Fe0.4)SiO3 ppv is well constrained by our data at the pressures relevant to the Earth's D″ layer. Our bulk modulus in this region is much higher than theoretical calculations for pure MgSiO3 ppv.