Investigation of ion mobility by the lines width of the L 03N 97T 4N 6O3 solid solution raman spectrum icon

Investigation of ion mobility by the lines width of the L 03N 97T 4N 6O3 solid solution raman spectrum




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НазваInvestigation of ion mobility by the lines width of the L 03N 97T 4N 6O3 solid solution raman spectrum
Дата30.06.2013
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INVESTIGATION OF ION MOBILITY BY THE LINES WIDTH OF THE Li0.03Na0.97Ta0.4Nb0.6O3 SOLID SOLUTION RAMAN SPECTRUM


N. V. Sidorov, M. N. Palatnikov, N. A.Teplyakova, E.Yu. Obryadina, I.V.Tananaev

Institute of Chemistry and Technology of Rare Elements and Mineral Raw Materials of the Kola Science Center of the Russian Academy of Sciences

e-mail: sidorov@chemy.kolasc.net.ru


B
Figure 1 − Raman spectra of the Li0.03Na0.97Ta0.4Nb0.6O3 solid solution in the region of totally symmetrical oxygen octahedra vibrations as a whole and in the region of Li+ and Na+ cations vibrations in the cuboctahedra at different temperatures.
y temperature changes of the low-frequency region of the Raman spectrum Li
+ cations mobility increase in the Li0.03Na0.97Ta0.4Nb0.6O3 solid solution structure was investigated (Fig. 1). LixNa1-xТауNb1-уO3 solid solutions with perovskite structure relate to a new class of compounds – ferroelectric ion conductors, where the ionic mobility is carried out by Li+ cations and ferroelectric properties manifestation is determined by Nb5+ (Ta5+) shift from the center of the oxygen octahedra. We have previously detected [1, 2] phase transitions to the high Li+ conductivity states in some of the LixNa1-xТауNb1-уO3 solid solutions when increasing temperature up to 400÷460 °С. In the Raman spectra, disappearance of the lines in the region 100÷160 сm-1, which presumably were attributed by us to the translational vibrations of the Li+ and Na+ cations in the cuboctahedral cavities of the structure, was observed [3]. It is difficult to hold the estimated interpretation of these compounds spectra, because of significant static disordering of the structural units even at room temperature. In this paper the lines width dependence on temperature in the Raman spectrum in 100÷160 сm-1 region for the Li0.03Na0.97Ta0.4Nb0.6O3 solid solution was investigated.

It is known, that the width and shape parameter of lines dependence on temperature in the Raman spectrum is responsible for changing of the structural units ordering and also depends significantly on ions mobility [4]. When ion mobility increases with the increasing of temperature, particles hops by their equilibrium positions will cause a violation in the

correlated vibrational motion of particles in the crystal structure. In the Raman spectra this will lead to broadening and relative decrease in intensity of phonon lines and in the limit, where the orientation correlation becomes negligibly small – to a complete blurring of the phonon spectrum of the disordered crystal modification. But the presence of the reorientational motion may not lead to a complete blurring of the spectrum if some degree of anisotropy is manifested in the character of the motion [5].

In the Raman spectra of the solid solutions, investigated with the increasing of temperature, a significant preferential intensity decrease of the line at 77 cm-1, corresponding to the librational vibrations of the oxygen octahedra as a whole, and also of the lines at 118 and 142 cm-1 was observed (Fig. 1). Rest of the lines remained the same. The width of the line at 118 cm-1 has an exponential dependence on temperature (Fig. 2). This fact indicates a significant increase in the transport of the relevant structural units with the temperature increasing, which is confirmed by the conduction studies in the Li0.03Na0.97TaуNb1-уO3 solid solution. On this basis, the line at 118 cm-1 was attributed by us to the Li+ vibrations in the cuboctahedral cavities of the structure. Accordingly, the line at 142 cm-1, apparently, can be attributed to the Na+ cations vibrations. The width of this line with the increasing of the temperature increases slightly and linearly, which is associated with an increase in the anharmonicity of the structural units vibrations in the equilibrium positions with the increasing temperature.




Figure 2 − The width dependence on temperature of the Li0.03Na0.97Ta0.4Nb0.6O3 solid solution spectrum lines: 1 –the line at 118 cm-1, 2 – 142 cm-1


REFERENCES

  1. M. N. Palatnikov, N. V. Sidorov, V. T. Kalinnikov, Ferroelectric solid solutions based on oxide compounds of niobium and tantalum: synthesis, investigation of structural ordering and physical characteristics, Nauka, St Petersburg , 2001. – P. 350.

  2. M. N. Palatnikov. Materials for electronic engineering based on ferroelectric single crystals and ceramic solid solutions of niobates – tantalates of alkali metals with micro- and nanostructures. Doct. Thesis (IHTREMS KSC RAS, Apatity, 2011).

  3. N. A. Teplyakova. Structural phase transitions in the ferroelctric Li0.12Na0.88TayNb1-yO3 solid solutions and their manifestation in the Raman spectra. Cand. Thesis (Petrozavodsk, 2012).

  4. N. V.Sidorov, T. R. Volk, B. N. Mavrin, V. T. Kalinnikov. Lithium niobate: Lithium niobate: defects, photorefraction, vibrational spectrum, polaritons, Nauka, Moscow, 2003. – P.250.

  5. V. N. Rogovoy. Phonon spectra and phase transitions in plastic cyclohexane, cyclopentane and some substituted crystals. Cand. Thesis (Troizk, 1984).

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