I do not have idea about mechanical properties, but the variation in any physical properties mainly governed by the crystal defects. It will be nice if you put any specific query. Thank you..

Dear Abdalla

please see for example the following links;

https://www.tf.uni-kiel.de/matwis/amat/def_en/kap_1/backbone/r1_3_2.html

https://link.springer.com/article/10.1007/BF02327310

https://www.researchgate.net/post/what_are_the_effect_of_point_defect_on_mechanical_properties_of_a_material

In case of optics, it can lead to shift in resonances, PL quenching, localization of carriers and so on. It will be better, if you can explain what is your application purpose.

https://www.nature.com/articles/srep02657

Article Temperature dependence of defect-related photoluminescence i...

The presence of crystal defects in the material can change the mechanical, physical as well as the optical properties of material. Defect can affect the strength of the material. Different types of defects produce different effects. Defects also may be intrinsic due to external doping of the material or extrinsic due irradiation of material by thee high energetic ionizing radiation. Even the electrical and optical properties also change due to the presence of defects. The band gap of material changes along with observation of new absorption in visible spectra. These are due to presence of defect or color center in the material. Some time effect is positive and some time it is negative. Required change can be obtained by systematic study of changes in the material properties by creating different kinds of defect in the material.

Thank you very much for all