Nanocomposites and nanomaterials

Nanocrystalline Li2-xNaxGe4O9 solid solutions devitrified from glass 

Diachenko A.O., Trubitsyn M.P., Skrypnik Ye.V.

Institute for Energy Efficient Technologies and Materials Sciences, Oles Honchar Dnipro National University, prosp. Gagarina 72, Dnipro-49005, Ukraine.

E-mail: annadiachenko459@gmail.com

Lithium-sodium tetragermanate LiNaGe4O9 crystal undergoes a phase transition from the nonpolar phase (space symmetry group D2h8) to the ferroelectric one (group C2v5) on cooling below TC = 113 K [1]. Changing x from 1 to 0.2 in the series of Li2-xNaxGe4O9 crystalline solutions is accompanied by an increase in the phase transition temperature TC from 113 K (x = 1) to 330 K (x = 0.2). In addition, the typical values of electrical conductivity and the character of its anisotropy in Li2-xNaxGe4O9 change significantly [2]. In accord with the data in [2], charge transfer in Li2-xNaxGe4O9 crystals can be mainly associated with Li ions interstitials moving along the channels within the structural framework. In amorphous and nanocrystalline states, electrical conductivity of LiNaGe4O9 increases by approximately two and three orders of magnitude correspondingly as compared to the single crystal [3]. It is of interest to study the crystallization processes and ionic conductivity in glasses and nanocrystalline state of lithium-sodium tetragermanate solid solutions. Here we report the thermal analysis data on Li2-xNaxGe4O9 glasses (x = 1, 0.6, 0.45, 0.2) devitrification performed on heating. Also, we discuss electrical properties of Li2-xNaxGe4O9 in amorphous and partially crystallized nanocrystalline states.
The samples of Li2-xNaxGe4O9 glasses were obtained by rapid quenching the melt. The glasses were crystallized by heating the samples in the temperature range of 300 ÷ 1200 K. The heating and cooling rates were 10 and 20 K/min. Thermal analysis of devitrification processes was performed by differential scanning calorimetry (DSC). The phase states of amorphous and polycrystalline Li2-xNaxGe4O9 were controlled by X-ray diffraction analysis. Electrical properties were measured in the temperature interval 300 ÷ 930 K and frequency diapason 1 ÷ 500 kHz and 1 MHz.
For all studied compositions the DSC curves demonstrated presence of exothermic anomalies, which indicated softening and subsequent crystallization of the amorphous phase. The concentration dependences of the glass transition temperature Tg(x), the crystallization temperatures TC(x), as well as the heat released on crystallization, were determined. The presence of a single intense exothermic peak on the DSC curves indicated a polymorphic mechanism of crystallization of the amorphous phase for all studied compositions x = 1, 0.6, 0.45, 0.2. A comprehensive analysis of DSC data and X-ray diffraction analysis was performed. The changes of ionic conductivity accompanying transitions from the glass to the nanometer and micrometer sized crystalline states were discussed.

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