Phase transition and dielectric properties of Li2-x NaxGe4O9 crystals (0.2 ≤ x ≤ 0.3)

The dielectric nonlinearity of ferroelectric Li_2-x Na_xGe₄O₉ crystals (0.2 ≤ x ≤ 0.3) is measured in the region of the phase transition temperatures. The ?(T) dependences for various values of the applied dc electric field E ₌ and the ?(E ₌)_T dependences at a constant temperature are studied in the Li_2-x Na_xGe₄O₉ crystals with T _c > 300 K and T _c < 300 K. It is shown that the Landau theory for second-order phase transitions describes the dielectric properties of the crystals with T _c < 300 K and does not describe the behavior of the crystals with T _c > 300 K. The results obtained lead to the conclusion that the different properties of the crystals with T _c > 300 K are likely to be related to the changed structure of these crystals caused by a change in the ratio between the Li and Na atoms.