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Thermo-Raman spectroscopy study of phase transitions in barium and strontium titanate nanostructures

The phase transition from ferroelectric to paraelectric phase in the case of perovskite materials is usually caused by structural phase transition. Typical example is BaTiO3 (BTO) and SrTiO3 (STO) where phase transition from tetragonal to cubic phase induces the transition from ferroelectric to paraelectric state. Expected phase transition temperature is between 380 and 420 K for BTO, and between 100 and 120 K for STO. These phase transitions depends on the different factors such as doping [1], grain and particle sizes [2] phase purity [3] etc. In this work the influence of the phase purity and morphology of the barium and strontium titanate nanostructures to the phase transition temperatures are investigated using Raman spectroscopy as the main method. For the synthesis of potentially ferroelectric barium and strontium titanate nanostructures we apply two different approaches. In the first method we synthesize potentially ferroelectric titanate nanostructures by ion exchange barium and strontium ion with titanium ions in the crystal structure of titanate nanotubes and nanowires (H2Ti3O7). For this synthesis we used different aqueous solution of barium and strontium salts and hydrothermally prepared titania nanostructures. In the second method we mixed strontium and barium hydroxide with titanium dioxide nanoparticles and hydrothermally prepared barium/strontium titanate nanostructures. Obtained nanostructures were characterized by micro-Raman spectroscopy (RS), X- ray powder diffraction and high resolution transmission electron microscopy (HRTEM). Temperature dependent micro-RS measurements were done in situ at different temperature using Linkam heating stage. Tetragonal phase of BFO, having 4mm simetry with c/a axis ratio close to one, is hard or impossible to distinguish from cubic phase by using diffraction techniques, so the phase purity and the crystal structure of the samples were studied by the combination of XRD and RS. The purity of the samples depends on the synthesis procedure. In situ low and high temperature RS was used for study of ferroelectric phase transitions from tetragonal to cubic structure in BTO and STO related nanostructures having different phase purity. The influence of the morphology and the phase purity to the temperature of phase transitions will be discussed. References [1] P. S. Dobal, A. Dixit, R. S. Katiyar, D. Garcia, R. Guo, A. S. Bhalla, J. Raman Spectrosc, 32, 147 (2001). [2] Y. Shiratori, C. Pithan, J. Dornseiffer, R. Waser, J. Raman Spectrosc, 38, 1288 and 1300 (2007). [3] A. Gajović, J. Vukajlović Pleština, K. Zagar, M. Plodinec, S. Šturm, M. Čeh, J. Raman Spectrosc (2012), in press, doi: 10.1002/jrs.4206.

Thermo-Raman spectroscopy; Barium titanate; Strontium titanate; phase transition

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