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Register a new userDefect Structure of the High-Dielectric-Constant Perovskite Cacu3ti4o12
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Using transmission electron microscopy (TEM) we studied CaCu3Ti4O12, an intriguing material that exhibits a huge dielectric response, up to kilohertz frequencies, over a wide range of temperature. Neither in single crystals, nor in polycrystalline samples, including sintered bulk- and thin-films, did we observe the twin domains suggested in the literature. Nevertheless, in the single crystals, we saw a very high density of dislocations with a Burger vector of [110], as well as regions with cation disorder and planar defects with a displacement vector 1/4[110]. In the polycrystalline samples, we observed many grain boundaries with oxygen deficiency, in comparison with the grain interior. The defect-related structural disorders and inhomogeneity, serving as an internal barrier layer capacitance (IBLC) in a semiconducting matrix, might explain the very large dielectric response of the material. Our TEM study of the structure defects in CaCu3Ti4O12 supports a recently proposed morphological model with percolating conducting regions and blocking regions.
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In the present work the authors report results of broadband dielectric spectroscopy on various samples of CaCu3Ti4O12, including so far only rarely investigated single crystalline material. The measurements extend up to 1.3 GHz, covering more than nine frequency decades. We address the question of the origin of the colossal dielectric constants and of the relaxational behavior in this material, including the second relaxation reported in several recent works. For this purpose, the dependence of the temperature- and frequency-dependent dielectric properties on different tempering and surface treatments of the samples and on ac-field amplitude are investigated. Broadband spectra of a single crystal are analyzed by an equivalent circuit description, assuming two highly resistive layers in series to the bulk. Good fits could be achieved, including the second relaxation, which also shows up in single crystals. The temperature- and frequency-dependent intrinsic conductivity of CCTO is consistent with the Variable Range Hopping model. The second relaxation is sensitive to surface treatment and, in contrast to the main relaxation, also is strongly affected by the applied ac voltage. Concerning the origin of the two insulating layers, we discuss a completely surface-related mechanism assuming the formation of a metal-insulator diode and a combination of surface and internal barriers.
In the present work we demonstrate that in addition to the well-known colossal-dielectric-constant material CaCu3Ti4O12 also various members of the series Ln2/3Cu3Ti4O12 with Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er and Tm, exhibit giant values of the dielectric constant. Just as CaCu3Ti4O12, all these materials show a Maxwell-Wagner type relaxation process. For the best material, Pr2/3Cu3Ti4O12, we provide a detailed investigation of its dielectric properties in a broad frequency range up to 1 GHz. Polarization at internal barriers, most likely grain boundaries, seems to be the reason for the observed very high values of the dielectric constant. Taking into account the present results and those reported in literature
The present work reports synthesis, as well as a detailed and careful characterization of structural, magnetic, and dielectric properties of differently tempered undoped and doped CaCu3Ti4O12 (CCTO) ceramics. For this purpose, neutron and x-ray powder diffraction, SQUID measurements, and dielectric spectroscopy have been performed. Mn-, Fe-, and Ni-doped CCTO ceramics were investigated in great detail to document the influence of low-level doping with 3d metals on the antiferromagnetic structure and dielectric properties. In the light of possible magnetoelectric coupling in these doped ceramics, the dielectric measurements were also carried out in external magnetic fields up to 7 T, showing a minor but significant dependence of the dielectric constant on the applied magnetic field. Undoped CCTO is well-known for its colossal dielectric constant in a broad frequency and temperature range. With the present extended characterization of doped as well as undoped CCTO, we want to address the question why doping with only 1% Mn or 0.5% Fe decreases the room-temperature dielectric constant of CCTO by a factor of ~100 with a concomitant reduction of the conductivity, whereas 0.5% Ni doping changes the dielectric properties only slightly. In addition, diffraction experiments and magnetic investigations were undertaken to check for possible correlations of the magnitude of the colossal dielectric constants with structural details or with magnetic properties like the magnetic ordering, the Curie-Weiss temperatures, or the paramagnetic moment. It is revealed, that while the magnetic ordering temperature and the effective moment of all investigated CCTO ceramics are rather similar, there is a dramatic influence of doping and tempering time on the Curie-Weiss constant.
The origins of an ultrahigh dielectric constant in polycrystalline CaCu3Ti4O12 (CCTO) was studied using the combination of impedance spectroscopy, electron microscopy, and scanning probe microscopy (SPM). Impedance spectra indicate that the transport properties in the 0.1 Hz .. 1 MHz frequency range are dominated by a single parallel resistive-capacitive (RC) element with a characteristic relaxation frequency of 16 Hz. Dc potential distributions measurements by SPM illustrate that significant potential drops occur at the grain boundaries, which thus can be unambiguously identified as the dominant RC element. High frequency ac amplitude and phase distributions illustrate very weak contrast at the interfaces, which is indicative of strong capacitive coupling. These results demonstrate that the ultrahigh dielectric constant reported for polycrystalline CCTO materials are related to the grain boundary behavior.
By means of in situ synchrotron X-ray diffraction and Raman spectroscopy under hydrostatic pressure, we investigate the stability of the quadruple perovskite LaMn7O12. At 34 GPa, the data unveil a first-order structural phase transition from the monoclinic I2/m symmetry stable at ambient conditions to cubic Im-3 symmetry. Considering that the same structural transition occurs at 653 K upon heating at ambient pressure, we propose a rare scenario of reentrant-type phase transition. In the high-pressure Im-3 phase, the Jahn-Teller distortion of the MnO6 octahedra and the orbital order present in the I2/m phase are suppressed, which is promising to investigate the possibility of pressure-induced Mott insulator-metal transition in the ideal situation of no structural distortions. The observation of a progressive line broadening of almost all Raman modes with pressure suggests that this transition may be incipient above 20 GPa.
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