Jung Nam Kim

Ionic conduction associated with polaronic hopping in KTiOPO4 single crystal

Abstract The frequency dependent a.c. conductivity in KTiOPO 4 single crystal, in the range from 10 Hz to 100 kHz, has been measured as a function of temperature between 200°C and 700°C. The behavior of σ ac in the low frequency range well obeys the universal power law, σ ac = σ dc + Aω S . The temperature dependence of exponent s is described by the OLP model. It is evidently shown that ionic transport behavior in KTP may well be explained by the contribution of polaronic hopping between localized sites.

Antiferroelectric phase transitions in NH4H2AsO4 and TlH2AsO4

Abstract The phase transitions of NH4H2AsO4 single crystal and TlH2AsO4 powder pellet have been investigated by dielectric measurements and D-E hysteresis loops studies. The dielectric anomaly of TlH2AsO4 near 250.5K was very similar to that of TlH2PO4 undergoing the antiferroelectric transition at 230 K (the lower phase transition point, TL ). This result suggests a weak first-order antiferroelectric phase transition at TL = 250.5 K in TlH2AsO4. Although the dielectric susceptibility above TL does not follow a Curie-Weiss law, the antipolar characteristic below TL was confirmed by hysteresis loop experiment.

Growth of high-quality βII-Li3VO4 single crystals by the Czochralski method

We have successfully grown colorless and crack-free βII-Li3VO4 single crystals by the Czochralski method using a self-flux. To improve the quantity of crystals, we installed a heat reservoir in the growing furnace, used the melt composition as Li2CO3:V2O5=58:42 (Li/58) to avoid the unnecessary phase transition and set the growth temperature below 700°C to avoid the cracking. The physical properties of the crystals were studied by the powder X-ray diffraction, impedance/gain phase, differential thermal analysis (DTA), and UV–VIS transmittance spectroscopy measurements. We found that the physical properties of Li/58 crystals were almost identical to those of Li/70 crystals except the UV–VIS transmittance data.

Electrical properties of Li2B4O7 single crystals in the [001] direction: Comparison between crystals grown from Li2CO3 and B2O3 mixed powder and from Li2B4O7 powder

The two different Li2B4O7 single crystals prepared from Li2CO3 and B2O3 mixed powder (LTBm) and Li2B4O7 powder (LTBp) were compared in the ac impedance study for the [001] direction in the frequency range from 100 Hz to 10 MHz at temperatures up to 350 °C. The thickness dependence of the complex resistivities exhibits that LTBm has distinct grain boundary-like effect while LTBp does not. This boundary effect may be due to the charge-depleted region constructed temporarily by the ion blocking of the microcracks in the applied ac electric field. The activation energies of volume conductivities are almost the same in LTBm and LTBp, but the absolute value of volume conductivity in LTBm is about 3 to 4 times larger than that in LTBp. The conductions in the electrode interface and boundary region are ascribed to the identical charge carriers activated by the applied electric field and result in the same activation energies. A relatively large blocking factor in LTBm below 100 °C is observed. It may be due to a ...

ac conductance of surface layer in lithium tetraborate single crystals

ac conductance for the electrode effect in Li2B4O7 single crystal was investigated by use of a coplanar electrode applied on the surface of a (001) plate. A coplanar electrode in this material more clearly shows conduction of the electrode effect than a conventional parallel planar electrode. The electrode effect in ac conductance is likely to be controlled by the surface layer, which is a poorly conductive depletion layer possibly filled with vacancies of lithium ions. We found that the surface layer is not locally distributed near the electrodes, but, rather, on the broad area of the surface (001) plane of the material. So we conclude that the electrode effect in ac conduction of Li2B4O7 single crystal is mainly due to the poor conductive surface layer distributed over the whole surface of the (001) plane and is not a secondary phase formed by reaction with the electrode material.

Dielectric properties of Li0.4K0.6NbO3 crystal

Abstract Dielectric constants and spontaneous polarization of Li0.4K0.6NbO3-LKN are measured, including the ferroelectric phase transition temperature. The shape of the dielectric spectra looks more like a second order but it shows a thermal hysteresis in the heating and cooling cycles. The Curie range of LKN is wider than that of the relaxors PMN and SBN. Dielectric constants do not obey Devonshires law. Complex impedance and dielectric constants have been studied in the frequency domain at several temperatures. A strong low frequency dielectric dispersion below 1kHz is revealed in the high temperature range. The normalized behavior of the complex dielectric constants satisfies the universal dynamic response with its power exponent of n=0.04. In the high frequency region, the complex impedance is described by a modified Cole-Cole arc.

Dielectric Non-Linearity in the Tetragonal Direction of Li2B4O7 Single Crystals

We studied the polarization and the conduction of Li 2 B 4 O 7 single crystals in the [001] direction prepared from Li 2 CO 3 and B 2 O 3 mixed powder and observed that shapes of the P – E hysteresis loop and the J – E curve changed at different frequencies. We believe that this phenomena was caused by a nonlinear polarization innate in the crystal rather than by a static remnant polarization or a spontaneous polarization. We successfully reproduced the above phenomena by using an equivalent circuit consisting of a nonlinear capacitor and a nonlinear resistor in a parallel connection. Our model was also confirmed by an ac impedance measurement. The observed nonlinearities seem to originate both from a disordered sublattice of mobile ions and from a hindered motion of ions due to microcracks produced by an imperfect crystallization. We observed changes in the amplitude of hysteresis loops over time, which were presumed to be due to mobile charge creation in a high electric field.

Low Frequency Impedance Study of Li2B4O7 Single Crystals

For the study on the electrical characteristics in the electrode effect in lithium tetraborate (Li 2 B 4 O 7 ) single crystal, temperature dependence of dielectric constant and complex impedance in the low frequency (0.1 Hz–10 MHz) along the [001] axis was reported. Strong low frequency dispersion was observed in the complex dielectric constant scheme. Warburg type diffusion process was found in impedance study in the low frequency region, which maybe resulted from enhancement of chemical potential by activation of charge carriers in the deep potential well in the electrode region.

EPR study of Fe3+ in KTiOPO4 single crystals

Abstract EPR measurement of Fe3+ in as-grown KTiOPO4 single crystals has been carried out at room temperature by employing a Bruker Q-band spectrometer. In addition to the centers ST2 and ST4 reported by Gaite et al, we have identified two extra Fe3+ centers having a mirror symmetry of ST2 and ST4, respectively. For all Fe3+ centers, parameters of the general spin Hamiltonian are calculated: the diagonal values of the second-order tensors and orientations of the principal axes are determined. The possible origin of the additional centers is discussed in terms of the local site symmetry at Fe3+ in KTiOPO4.