The main conditions for fast ionic transport are related to the disorder in the positions occupied by the mobile ion and the presence of conduction channels running inside the structure*.

A few examples which confirm the aforementioned statements are reported below. Synthetic analogs of tetragonal Na₂TiSiO₅ natisite, namely Na₂TiGeO₅ and Li₂TiGeO_5, can serve as an illustration of the structural condition of ionic conductivity. Their structures contain the layers (001) formed by tightly connected GeO₄ tetrahedra and TiO₅ semioctahedra. As a result, the alkaline cations can easily move between the layers and the conductivity along a axis is 10³–10⁴ times higher compared with one parallel to c-axis. Another example is the Na₅YSi₄O₁₂ crystal. Its characteristic features are 12-membered rings of silicon-oxygen tetrahedra. According to the symmetry of the cell, it should contain 90 Na atoms, but only 48 atoms were localized. This discrepancy should be associated with the possible movement of Na atoms within the structure. The results obtained using X-ray diffraction methods are extremely important for understanding the structural requirements of ionic conductivity in crystals. The structural refinement of K₃NdSi₆O₁₅ with the silicate layer [Si₆O₁₅], similarly to those found in rare mineral dalyite K₂ZrSi₆O₁₅, allowed us to establish that one of three unequivalent K atoms has the highest thermal displacement U₃₃. Accordingly, these cations are most mobile along the c-axis, and it is confirmed by the values of the principal elements of the specific electrical conductivity tensor. Among the materials exhibiting high ionic conductivity with relatively low transition temperatures to the superionic state, Li-ionic conductors with the general formula Li₃M₂(PO₄)₃ (M = Fe, Sc, Cr, In) and with mixed framework structures are considered in this presentation.

*Pushcharovsky, D.; Ivanov-Schitz, A. Minerals, 2024, 14, 770.