Mikhail Zimin An increase in dislocation density generally alters the lattice constants due to local strain, and these distortions may lead to either a slight expansion or contraction of the lattice, depending on the type of dislocation (edge or screw) and the nature of interactions between dislocations.

A crystalline material is a close-packed structure. Meaning that long-range order exists in the material, and it is the most efficient way in which atoms can be arranged in the material. So, intuitively, when dislocations are formed, the material will need to expand, thus reducing its density and increasing the lattice parameter. Most material should follow this trend.

Now, take the example of the two crystalline forms of carbon. The density of diamond is approximately 3.51 g/cm³, while graphite's density is around 2.26 g/cm³. There exist forms of 'defective' carbon, which are characterized by their fraction of sp2/sp3 content. So, the densities of such amorphous carbons will stand between that of graphite and diamond, depending on the sp2/sp3 content.

So, to sum up, if your material can exist in only one stable crystalline structure at a particular temperature, adding dislocations will expand the lattice. If there is more than one stable crystal structure at that temperature, it depends on which structure you have, and which of these structures is the more efficiently packed one.