133Cs and deuteron NMR study of the superionic transition in CsDSO4

The temperature dependence of the ¹³³Cs and deuteron NMR spectra of a CsDSO₄ single crystal has been investigated between 20°C and 170°C. In the room temperature phase II' there are two physically non-equivalent Cs⁺ and deuteron sites per unit cell. On heating from room temperature to 110°C the doublet structure of the ¹³³Cs and deuteron lines collapses due to random Cs and deuteron exchange between the two non-equivalent sites. At the transition to the superionic phase III there is a further drastic decrease in the deuteron and a somewhat smaller decrease in the ¹³³Cs quadrupole coupling. The fact that the quadrupole coupling of these two nuclei remains non-zero in the superionic phase demonstrates that the Cs⁺ and deuteron translational diffusion involves well defined lattice sites and directions and is not completely “random” or “liquid”-like.