Structural phase transitions and conduction properties of superionic, ferroelastic Cu6PS5Br1-xIx single crystals (x=1.0.75, 0.5, 0.25)

A new concentration-temperature phase diagram of superionic Cu6PS5Br1-xIx mixed crystals with compositions x = 1, 0.75, 0.5, 0.25 is proposed on the basis of calorimetric, optical and single-crystal x-ray diffraction measurements. In the high temperature form, above the first phase transition, all compounds crystallize in cubic symmetry F (4) over bar 3m. In iodine rich crystals (for x = 1, 0.75 and 0.5) below 270 K intermediate cubic F (4) over bar 3c superstructure is stabilized down to 143-169 K for x = 1 and around 220 K for x = 0.75 and 0.5. Below these temperatures ferroelastic phase transition to monoclinic Cc phase takes place. On the other hand, in bromine rich compounds (x = 0.25) a direct F (4) over bar 3m-Cc phase transition is observed without any indications of intermediate cubic superstructure. The influence of compositional disorder on superionic properties is discussed on the basis of conductivity measurements for temperatures between 150 and 290 K. Since conduction properties are coupled with the structure changes the different conductivity behaviour is observed for different I/Br concentration. Ionic conductivity decreases with the increase of bromine concentration. Even in the high temperature phase conductivity is lower and activation energy is higher in bromine rich compounds.