Peculiar Dielectric Behaviors on Li_2B_4O_7 Single Crystals (Condensed Matter : Electronic Structure, Electrical, Magnetic and Optical Properties)

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Real and imaginary parts, ∈' and ∈", of the dielectric constant of Li_2B_4O_7 were measured at temperatures of 10-300 K and at frequencies of 30Hz-1.8GHz, using two kinds of the single crystals prepared for a surface acoustic wave (SAW) device and for a nonlinear optical (OPT) device. Commonly to the both crystals, two new features were found : Debye-type dielectric dispersion appears near 2 GHz at room temperature, and a broad dielectric maximum occurs at approximately 120K. On the other hand, only on the OPT crystal, ∈' along the c-axis shows a small cusp at 220 K, and both ∈' and ∈" show sharp peaks at 237 K. These anomalies occur mainly on increasing temperature, and the magnitudes strongly depend on samples. In addition, the ∈" along the c-axis increases rapidly with increasing temperature above about 230 K due to ionic conduction of mobile Li-ions as reported previously, but it shows an obvious thermal hysteresis in the temperature range of 230-300 K. We discuss causes of these peculiar dielectric behaviors above 220 K, by assuming an interaction between an incommensurate modulation wave and Li-ions, which are mobile at room temperature and freeze at low temperatures below about 230 K.
社団法人日本物理学会の論文
2004-05-15