The Electrical Properties of Some Composite Materials Based on Sodium and Tantalum Oxides
D. Malaescu a, I. Grozescua,b, P. Sfirloaga b, P. Vlazan b and C.N. Marin c
a"Politehnica" University, Victoriei No. 2, RO-300006 Timisoara, Romania
bNational Institute for Research and Development in Electrochemistry and Condensed Matter, Plautius Andronescu No. 1, RO-300224 Timisoara, Romania
cWest University of Timisoara, Faculty of Physics, Bd. V. Parvan no. 4, 300223 Timisoara, Romania
Received: March 30, 2015; Revised version: August 20, 2015; In final form: September 28, 2015
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Two samples of Na-Ta oxides were synthesized by the hydrothermal method at reaction temperatures of 160°C (sample A) and 200°C (sample B). For reference, a third sample of pure NaTaO3 was prepared by the sol-gel method (sample C). Using X-ray diffraction, scanning electron microscopy, UV-vis diffuse reflectance spectra and electric measurements, structural, morphologic, spectroscopic and electric properties of samples were investigated. The structural characterization by X-ray diffraction revealed that samples A and B are mixtures of Na-Ta oxides (including NaTaO3 and other compounds), whilst sample C is pure NaTaO3. UV-vis diffuse reflectance spectra allowed evaluation of the band gap energy (Eg), resulting in 3.88 eV for sample A, 3.93 eV for sample B and 4.1 eV for sample C. Electrical resistivity measurements, over the temperature range 300-450 K, showed a typical semiconductor behavior of the investigated samples, with the effective activation energy, Ea of 0.47 eV (sample A), 0.45 eV (sample B) and 0.82 eV (sample C). Based on the Mott variable range hopping model, the conductivity mechanism in the investigated samples was analyzed. The results shown that the density of states at the Fermi-level, N(EF) is constant in the investigated temperature range and the typical values of N(EF) are 0.713× 1018 eV-1 cm-3 (sample A), 0.621× 1018 eV-1 cm-3 (sample B) and 0.855× 1017 eV-1 cm-3 (sample C). Other parameters of VRH model such as the hopping distance R and the hopping energy W have also been computed and the following values at the room temperature were obtained: R=15.7 nm and W=86 meV (for sample A); R=16.3 nm and W=89 meV (for sample B) and R=26.7 nm and W=147 meV (for sample C).

DOI: 10.12693/APhysPolA.129.133
PACS numbers: 81.20.Ev, 81.20.Fw, 72.20.Ee, 72.80.Tm