Physico-Chemical nanomaterials science
The influence of heat treatment on optical properties of Na0.5Bi0.5TiO3 crystals
V.M. Sidak1, A.Yu. Tuluk2, M.P. Trubitsyn2, T.V. Panchenko2, 
T.V. Kruzina2, J. Suchanicz3
1 Dnipropetrovsk Medical Academy of Health Ministry of Ukraine, Vernadsky str. 9, Dnipro-49044, Ukraine. 
E-mail: vsidak@3g.ua
2 Oles Honchar Dnipro National University, prosp.Gagarina 72, Dnipro-49010, Ukraine.
3 Institute of Physics, Pedagogical University, ul. Podchorazych 2, Krakow-30-84, Poland.
Relaxor ferroelectric Na0.5Bi0.5TiO3 (NBT) is considered as promising material for microelectromechanical systems, pyroelectric sensors, multilayer capacitors [1]. At moderate temperatures the physical properties of NBT are determined by polar nanoclasters and strongly influenced by the structural defects. In the abstract we study permittivity and optical transmission spectra of as-grown and heat treated NBT single crystals [2].
The measurements were performed for the as-grown samples and for the samples annealed in air at 1070 K for 1 hour and then in vacuum at 1070 K for 2 hours. Dielectric permittivity ε was measured in the frequency range 0.5-100 kHz and in the temperature interval 300-800 K. The optical transmission spectra were investigated in the energy range hν = 3.0-1.6 eV at room temperature.
The temperature dependence of ε for the as-grown NBT sample demonstrates existence of low-frequency relaxation maximum near 670K and nonlinear ε growth at T> 50K. Low frequency anomaly of ε disappeared after heat treatment in air and could be partially recovered by heat treatment in vacuum. Optical absorption decreased after heat treatment in air and significantly increased after annealing in vacuum. 
The results obtained are discussed under assumption that heat treatment in air reduces the concentration of oxygen vacancies VO, whereas annealing in vacuum generates an additional amount of VO. It is supposed that changes in dielectric and optical properties are determined by their structural defects, such as F+ and O- centers, concentration of which can be controlled by heat treatment in different atmospheres.

1.	S. Nahm, S. Priya eds. Lead-Free Piezoelectrics, Sodium Bismuth Titanate-Based Ceramics //New York: Springer.-2012.
2.	Kruzina T.V., Sidak V.M., Trubitsyn M.P. Popov S.A., Suchanicz J. Thermal treatment and dielectric properties of Na0.5Bi0.5TiO3 single crystal // Ferroelectrics. -2014.-462.-P. 140-144.