BRIEF NOTE
Japanese Journal of Applied Physics 50 (2011) 05FC09 DOI: 10.1143/JJAP.50.05FC09
Superionic Conductivity in One-Dimensional Nanofibrous TlGaTe2 Crystals Rauf Sardarly, Oktay Samedov, Adil Abdullayev, Famin Salmanov, Andzej Urbanovic1 , Fre´de´ric Garet1 , and Jean-Louis Coutaz1 Institute of Radiation Problems, National Academy of Sciences of Azerbaijan, F. Agaev 9, AZ1143 Baku, Azerbaijan 1 Laboratory IMEP-LAHC, UMR CNRS 5130, University of Savoie, 73376 Le Bourget du Lac, France Received November 25, 2010; accepted February 24, 2011; published online May 20, 2011 We study the temperature dependence of the electrical conductivity ðT Þ of TlGaTe2 crystals under an electric field (E ¼ 181 V/cm). A strong sharp increase (1500) in ðT Þ is observed at T ¼ 242 K (along the c-axis) and T ¼ 267 K (perpendicular to it). This increase is attributed to a transition towards a superionic conductivity behavior. Over the superionic transition threshold, disordering of the Tlþ sublattice in TlGaTe2 occurs, and nanoscale topological disordering arises owing to aperiodicity in placing Ga3þ Te2 2 nanofibers. Terahertz time-domain spectroscopy reveals absorption lines at approximately 0.2 THz that may be attributed to the libration oscillations of the nanofibers. # 2011 The Japan Society of Applied Physics
The fit of the experimental data (inset of Fig. 1) gives the activation energy Ea ¼ 0:21 eV for TlGaTe2 , in agreement with published values.4,5) The observed sharp growth of the TlGaTe2 conductivity of crystals results from the variation with temperature of the high-mobility ion density in the crystal. This variation is mainly caused by the diffusion of the Tlþ ions towards vacancies in the Tl sublattice. This
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Nanodimension topologic-disordered materials constitute an important feature in the development of modern electronics. For example, among such materials, TlGaTe2 is a p-type semiconductor with a nanofibrous structure made of Ga3þ Te2 2 groups that form chains extending along the c-axis of the material.1) These layers of negatively charged chains are bonded together by Tlþ ions. The resulting tetragonal lattice is characterized by a D18 4h group symmetry. Much attention has been paid to such crystallographic systems,2,3) which behave as if they have less than three spatial dimensions. Such materials are often called quasi-one-dimensional (1D) nanorods, nanofibers, or nanochains. Superionic conductors represent a special class of these materials, which have recently been the subject of numerous studies. One of the major distinctive features of superionic conductors is their abnormally high ionic conductivity.3) Indeed, superionic crystals with structural disorder exhibit two qualitatively different phases. Below a critical temperature, they behave as usual ionic crystals (dielectric phase), while at higher temperatures, they show superionic conductivity (electrolytic phase). We perform electrical conductivity measurements, but also far infrared (terahertz) spectroscopy studies, to obtain more information on the superionic behavior of TlGaTe2 . We first investigate the temperature dependence in the conductivity ðT Þ of the TlGaTe2 crystal in two different geometries, i.e., in the directions parallel and perpendicular to the tetragonal c-axis of the crystal (Fig. 1). Above T 310 K, for both experimental geometries, a sharp increase of the conductivity is observed. This is a typical behavior of ionic conductivity, for which ln½T ðT Þ varies as 1=T : Ea T ðT Þ ¼ 0 exp : kB T
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Fig. 1. Temperature dependence of the conductivity ðT Þ of TlGaTe2 in two experimental geometries, i.e., parallel and perpendicular to the c-axis of the crystal. Curves labeled with E have been recorded under a static electric field (E ¼ 181 V/cm). The inset shows ln½T ðT Þ versus 1=T for the perpendicular case without applied field.
phenomenon is linked to the thermally induced disordering (fusion) in locating the Tlþ sublattices in the TlGaTe2 crystal. This topological disorder arises on the nanoscale, which does not exceed the characteristic length between two neighboring Tlþ sublattices (1 nm). The disorder degree should change under the influence of external forces. Here, we apply to the crystal an electric DC field (E ¼ 181 V/cm) and we measure again the temperature dependence of ðT Þ under this applied field (Fig. 1). A strong increase (1500)—sharper than without the DC field of ðT Þ is observed above T 250 K. Above room temperature, i.e., where superionic conductivity occurs, TlGaTe2 crystals present interesting nonlinear electrical behaviors, such as a negative-differential-resistance (NDR) region thanks to the S-type shape of the current–voltage characteristics (Fig. 2). In the NDR region, we have also observed self-excited oscillations of the voltage.6) Investigations in the far infrared region could give additional information on the physics of these nanofibrous crystals. Here, the samples have been studied for the first time by THz time-domain spectroscopy,7) which permits recording the entire THz spectral response (0.1–3 THz) in a single measurement. Our classical setup has been modified to focus most of the THz beam onto a small sample, whose sizes are typically on the order 10–20 mm2 . We observe
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several absorption peaks in the THz transmission spectrum of TlGaTe2 (Fig. 3). Most of them could be attributed to the excitation of phonons (A2u and Bu ).2,8) The absorption line at approximately 0.2 THz, seen only when the THz field is aligned along the nanofibers, occurs at a frequency lower than the lowest phonon one (A2u ). Therefore, this frequency is probably related to the libration oscillation of the Ga3þ Te2 2 nanofibers. In conclusion, we have studied in detail the electrical conductivity of TlGaTe2 . The superionic behavior of the conductivity is clearly observed over the phase transition temperature. Moreover, our original records of the THz transmission spectra of TlGaTe2 reveal features that may be attributed to the libration of the nanofibers. This explanation merits definitive proofs, for which we are presently performing additional works.
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Fig. 3. Experimental transmission spectrum of TlGaTe2 for two polarizations of the THz beam: continuous curve, ETHz parallel to the nanofibers (Ga3þ Te2 2 ). For the sake of legibility, this curve has been multiplied by 100; dashed curve, ETHz perpendicular to the nanofibers.
Acknowledgement This study has been supported by a collaboration agreement between CNRS (France) and ANAS (Azerbaijan).
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# 2011 The Japan Society of Applied Physics
