Mechanism of glass crystallization and ionic conductivity in lithium–sodium tetragermanate LiNaGe4O9

Structural evolution and ionic conductivity were investigated in lithium–sodium tetragermanate (LiNaGe 4 O 9 ) glass subjected to crystallization induced by heating or by isothermal treatment. Crystallization was monitored using differential scanning calorimetry; the resulting phases were identified by X-ray diffraction. AC conductivity (10 2 –10 6 Hz) was measured over the temperature interval of 295–900 K. In the range between ~ 500 K and the crystallization temperature ( T cr = 826 K), the glass exhibited conductivity one order of magnitude higher than that of the corresponding single crystal (5 × 10 −4 S/m vs. 4 × 10 −5 S/m at T = 667 K, f = 787 Hz). Full crystallization above T cr led to a sharp conductivity drop of nearly three orders of magnitude from 3 × 10 −2 S/m to 1 × 10 −5 S/m ( f = 787 Hz). Partial crystallization reduced the conductivity depending on the crystalline volume fraction. Impedance spectroscopy confirmed that structural ordering significantly slows the charge transport. It is supposed that ionic conduction is dominated by Li + ions, while the contribution from Na + is insufficient. Comparison with Li 2 O– x GeO 2 compounds revealed that the effect of partial crystallization on conductivity depends on the devitrification mechanism. Polymorphic crystallization decreases conductivity, whereas sequential crystallization can enhance it via formation of a highly conductive nanocrystalline phase.