Many transition-metal oxides show very large ("colossal") magnitudes of the dielectric constant and thus have immense potential for applications in modern microelectronics and for the development of new capacitance-based energystorage devices. In the present work, we thoroughly discuss the mechanisms that can lead to colossal values of the dielectric constant, especially emphasising effects generated by external and internal interfaces, including electronic phase separation. In addition, we provide a detailed overview and discussion of the dielectric properties of CaCu3Ti4O12 and related systems, which is today's most investigated material with colossal dielectric constant. Also a variety of further transition-metal oxides with large dielectric constants are treated in detail, among them the system La2−xSrxNiO4 where electronic phase separation may play a role in the generation of a colossal dielectric constant. a

The dielectric properties of CaCu 3 Ti 4 O 12 , a material showing colossal values of the dielectric constant, were investigated over a broad temperature and frequency range extending up to 1.3 GHz. A detailed equivalentcircuit analysis of the results and two crucial experiments, employing different types of contacts and varying the sample thickness were performed. The results provide clear evidence that the apparently high values of the dielectric constant in CaCu 3 Ti 4 O 12 are nonintrinsic and due to electrode polarization effects. The intrinsic properties of CaCu 3 Ti 4 O 12 are characterized by charge transport via hopping of localized charge carriers and a relatively high dielectric constant of the order of 100.Recent reports on the observation of colossal dielectric constants (CDCs) in CaCu 3 Ti 4 O 12 (CCTO) reaching values up to 10 5 (Refs. 1-3) have generated considerable interest in this material and related compounds. 4-14 The stunning observation in CCTO was a high and almost temperatureindependent dielectric constant Ј at elevated temperatures and a steep decrease by almost three orders of magnitude at low temperatures. The steplike decrease of Ј as function of temperature and an associated peak in the dielectric loss Љ, strongly depend on the measuring frequency and roughly follow an Arrhenius behavior. This relaxational behavior was ascribed to the slowing down of highly polarizable relaxational modes 2 or to the slowing down of dipolar fluctuations in nanosize domains. 3 Similar effects were obtained in thin films of CCTO making this system a good candidate for many applications. 7 However, shortly after the reports of the CDCs, their intrinsic nature has been questioned and arguments have been put forth that extrinsic effects as contributions from the electrode/sample interface, from grain boundaries in polycrystalline materials, or from twin boundaries in single crystals may be the sources of the giant dielectric constant. 4,10,12,14,15 Indeed, as it is known since decades, 16,17 the above outlined characteristics of the dielectric permittivity exactly corresponds to what is expected in case of interfacial polarization. Similar behavior, termed MaxwellWagner relaxation, has been observed in numerous materials (see, e.g., Ref. 15). It was pointed out that, even if the CDCs in CCTO are not intrinsic, this material could be a possible candidate for commercial applications as internal barrierlayer capacitor (IBLC). In IBLCs, internal barriers at grain boundaries lead to the observed high values of Ј (Ref. 5). Thus, clearly there is an urgent need for clarification of the true origin of the observed CDCs, which in case of an intrinsic nature would have important theoretical implications and in case of an IBLC scenario still would be of high technical relevance.As has been pointed out in Ref. 15, intrinsic (or IBLC) and electrode effects can well be separated using different contacts and sample geometries. Consequently, Ramirez et al. 8 remeasured CCTO using a different type of contact preparation. ...

Since the first investigations of perovskite type oxynitrides with the generalised composition ABO 3Àx N x about twenty years ago, these compounds have become of growing interest. The incorporation of nitride ions in the perovskite lattice results in distinct changes in the electronic structure leading to unusual physical properties. In this article we report on new synthesis techniques, different analytical methods, progress in the structural characterisation by comprehensive diffraction techniques and local spectroscopic methods like XAS and NMR as well as state of the art theoretical investigations. Various physical characteristics like electrical and thermal transport parameters and dielectric properties are described. The thermal and chemical stability of oxynitride perovskites are investigated and their applications in different photocatalytic reactions are discussed.

In the present work the authors report results of broadband dielectric spectroscopy on various samples of CaCu 3 Ti 4 O 12 , 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.

We present dielectric measurements of the colossal dielectric constant material CaCu 3 Ti 4 O 12 extending up to 1.3 GHz also covering so far only rarely investigated single crystalline samples. Special emphasis is put on the second relaxation reported in several works on polycrystals, which we detect also in single crystals. For polycrystalline samples we provide a recipe to achieve values of the dielectric constant as high as in single crystals.Among the vast number of papers on the extremely high ("colossal") dielectric constants (ε') found in CaCu 3 Ti 4 O 12 (CCTO) there are at least ten so-called "highly-cited" papers 1,2,3,4,5,6,7,8,9,10 . This demonstrates the tremendous interest in new high-ε' materials, which are prerequisite for further advances in the development of capacitive electronic elements. It soon became clear that the colossal ε' in CCTO must have a non-intrinsic origin 4,5,6,7,8,9,11,12,13 . Nowadays usually the results are interpreted within an "internal barrier layer capacitor" (IBLC) picture: Polarization effects at insulating grain boundaries between semiconducting grains or other internal barriers generate non-intrinsic colossal values of ε', accompanied by a strong Maxwell-Wagner (MW) relaxation mode. As an alternative, a "surface barrier layer capacitor" (SBLC) picture was proposed, assuming, e.g., the formation of Schottky diodes at the contact-bulk interfaces 8,9,14 . Nearly all experimental evidence for a non-intrinsic mechanism so far is based on measurements of ceramic samples (e.g., refs. 4,5,6,7,9,11,12,13). However, extremely high values of ε' of the order of 10 5 were observed particularly in CCTO single crystals (SCs) 3 . With only few exceptions 11 , measurements of polycrystals (PCs) reveal much lower values in the range of 10 3 -10 4 . There are no grain boundaries in SCs and thus there were speculations about other internal boundaries as, e.g., twinboundaries 4,6,15,16 . However, if there is any kind of planar defects in SCs generating strong relaxations, should they not contribute to a separate relaxational response in PCs, where grains can reach sizes up to 100 µm, too? Interestingly, it seems clear now that indeed there is a second relaxation in CCTO PCs 9,12,14 and it even was already observed in one of the earliest reports on CCTO 2 .It leads to even larger ε' values than the well-known main relaxation. Thus one could speculate that one of the two relaxations is due to grain boundaries and the other due to planar defects within the grains. Alternatively, one relaxation could be due to an IBLC and the other to a SBLC mechanism 9 . In the present work we aim to elucidate differences and similarities in SC and PC behavior, address the question of the second relaxation, and try to help solving the IBLC/SBLC controversy. For this purpose, we performed measurements on various samples including single-crystals, which to our knowledge so far only were investigated in two works 3,16 . The spectra cover up to nine frequency decades and extend up to the technically relev...