Dr.S.(Phys.-Math.) Sidorov N.V., Teplyakova N.A., Obryadina E.Y.,

Dr.S.(Eng.) Palatnikov M.N.

The laboratory of materials for electronic engineering, Institute of Chemistry and Technology of Rare Elements and Mineral im. I. V. Tananaeva of Kola Scientific Center of Russia Academy of Sciences.184209, Apatity, Russia.

e-mail:obryadina@chemy.kolasc.net.ru, www.chemy.ksc.ru

Structural phase transitions and processes of “order-disorder” in ferroelectric solid solution of Li_0.12Na_0.88Ta_yNb_1-yO₃

The system of solid solutions of the Li_0.12Na_0.88Ta_yNb_1-yO₃ (LNTN) with oxygen-octahedral type of structure is of great interest because the change of the components concentration and the temperature is accompanied by a variety of structural phase transitions. Thus, on the basis of this system one can obtain the materials with ferroelectric, superionic and semiconducting properties and also the one with cross-effects.

Particularly, solid solutions of the Li_0.12Na_0.88Ta_yNb_1-yO₃ are promising as materials with high superionic conductivity. When õ=0.12 the structure orders in such a way as to make the phase transition in a superionic state possible. We may vary the temperature of transition and its degree of fuzziness in wide scope by ranging the order of structural units in niobium and tantalum sublattice. Besides, when the temperatures equal to 315–350^îÑ in the Li_0.12Na_0.88Ta_yNb_1-yO₃ solid solution we may observe the phase transition from ferroelectric to antiferroelectric taking place before the superionic phase transition. Temperature dependence of conductivity and Raman spectra of the Li_0.12Na_0.88Ta_yNb_1-yO₃ solid solutions with ó=0, 0.2, 0.4 è 0.5 and of the Li_õNa_1-õTa_0.1Nb_0.9O₃ solid solution have been investigated (fig 1). Phase transitions in the superionic state were found at the temperature ~400÷460^îÑ (fig 2). It was found that the superionic phase transition in the Li_õNa_1-õTa_yNb_1-yO₃ solid solution was observed with õ=0.125 only.

Fig.1. Raman spectra of Li_0.12Na_0.88Ta_yNb_1-yO₃ solid solution at different temperatures: (à) - Li_0.12Na_0.88NbO₃; (b) - Li_0.12Na_0.88Ta_0.2Nb_0.8O₃; (c) -Li_0.12Na_0.88Ta_0.4Nb_0.6O₃

Fig.2. The temperature dependence of the conductivity of Li_0.12Na_0.88Ta_yNb_1-yO₃ solid solution with ó =0, 0.2, 0.4, 0.5

In the Li_õNa_1-õTa_0.1Nb_0.9O₃ solid solution with õ~0.12 by Raman spectra the increasing in structural ordering was discovered. It is explained by the proximity of this composition to a particular concentration point (Li:Na=1:7) where an increasing of degree of a short- and long-range order in the sublattice of an alkali metal is likely to be. Since the degree of short-range order determines significantly the physical properties of a solid solution then the anomalies of the physical properties may correspond to peculiar concentration points. In this case it is the superionic conductivity.

While studying at a room temperature the concentration phase transitions by Raman spectra of Li_0.12Na_0.88Ta_yNb_1-yO₃ (ó=0÷1) solid solution it was found that the substitution of niobium ions by tantalum ions with the same radius resulted in structure distortion and the change of the geometry of the oxygen octahedral with tantalum ≥0.5. It is evidenced by the appearance of new lines as well as gaps in the frequencies of certain bands corresponding to oscillations of oxygen octahedra (500÷700 cm^-1). In addition, when ó~0.55 the band at 80 cm^-1 accorded to the totally symmetric libration of the oxygen octahedra as a whole disappears and this shows a complete disorientation of the oxygen octahedra in the solid solution structure. Materials with a disordered structure are usually characterized by lower points in the structural phase transition as compared with the materials of an ordered structure. Thus, by reducing a degree of the long-range order in the sublattice of niobium when the latter is substituted by isomorphous tantalum cations a decrease of the phase transition point in antiferroelectric and superionic states is possible. We can supported it by our investigation of structural arrangements at high temperature in the Li_0.12Na_0.88Ta_yNb_1-yO₃ (ó=0.2, 0.4) solid solutions.

In the Raman spectra from the temperature dependence of the band intensity with frequency 875-877 cm^-1 corresponding to the stretching vibrations of oxygen atoms, there discovered morphotropic phase transitions between phases with different symmetry of the unit cell at temperatures 100-120^îÑ. At ~315-350^îÑ the phase transition from ferroelectric to antiferroelectric was observed. This transition manifests itself in the Raman spectra mainly in disappearance of the band corresponding to the stretching modes of oxygen atoms of the polar axis which is forbidden by the selection rules for centrosymmetric octahedron (fig 3). It was shown that the phase transition given refers to the "order-disorder» type and is caused by the predominant increase in the anharmonicity of cation’s vibrations in the octahedral cavities of the structure.

Fig.3. Raman spectra of ceramic Li_0.12Na_0.88Ta_0.2Nb_0.8O₃ solid solution in the region of stretching modes of oxygen atoms Â-Î-Â in the ÂÎ₆ octahedral anion at different temperatures

It was registered by the Raman spectra that the increase in the vibrational anharmonicity of all the cations and the translational mobility of Li⁺ cations with temperature substantionally simplify misorientation of the oxygen octahedra. It reveals itself in the Raman spectra in the disappearance of the line at 80 cm^-1 corresponding to the librations of the oxygen octahedra as a whole. In addition, the Raman spectra with the increase of temperature have a significant, preferential in comparison with other lines of the spectrum, broadening and a decrease in intensity of lines in the region of 100-160 cm^-1 corresponding to the vibrations of the Li⁺ and Na⁺ cations in the cuboctahedron and their complete disappearance near the point of the phase transition in the superionic state. This fact, in our opinion, corresponds to the "melting" of the alkali metal sublattice at the phase transition to the superionic state.