[en] The structural origin of fast copper conduction in Cu₆PS₅Cl single crystals is discussed. Cu₆PS₅Cl crystallises in the same cubic F4-bar3m symmetry as the other Cu₆PS₅X's, X=Cl,I,Br. However, this phase is stabilised at much lower temperatures (down to 160 K) and is characterised by higher level of disorder in copper substructure. Complex domain structure developing at low temperatures proves that Cu₆PS₅Cl undergoes ferroelastic phase transition (PT) associated with cubic F4-bar3m to monoclinic Cc symmetry change. The structural distortion connected with this transformation is very subtle. X-ray diffraction measurements show strong precursor phenomena below 205 K which correspond to the low temperature phase and point out to copper ordering in nano- or microscopic scale above the temperature of ferroelastic phase transition (PT). The pseudopotential based on copper joint probability density function is calculated along diffusion paths at 175 and 435 K. - Coordination of copper mobile ions in superionic phase of Cu₆PS₅Cl, T=435K