Crystalline matter responses on external stimuli like temperature, humidity or pressure in various ways ranging from simple expansion/contraction to phase transitions. This response materializes in gradual or rapid changes in the physical properties. In real crystalline materials, in particular in layered materials defects like antisite defects, occupational disorder or stacking fault disorder are omnipresent which creates a microstructure that can also have a significant impact on the material’s physical properties. Accordingly the questions on how the microstructure responds on external stimuli and on how microstructural investigation can be carried out in situ are of great interest.

In this talk, we will present in situ studies on microstructural changes of inorganic and organic bulk materials. Cubic, disordered, sphalerite-type Cu₂ZnSnS₄ (CZTS) undergoes a slow and irreversible phase transition towards the tetragonal polymorph upon heating. Annealing of disordered, cubic CZTS leads to a growth of the domains that is associated with a reduction of strain. After the domains exceed a certain size, the ordering of cation substructure starts. This induces a healing of the stacking faults, which the DFT calculations revealed to be energetically less favourable in a structure with cation ordering. Eventually the reduction of the degree of faulting leads to a decrease of long-range strain. Porous, covalent organic framework compounds (COFs) exhibit notoriously faulted layered structures, with layers showing a random staggered stacking order, that is randomized with in direction and in lengths of the layer offset. In dependence of the functionalization of the COF layers, heating leads to an increase in the layer offset as well as to a buckling effect, which has a significant impact on the effective pore size of the material.