Over the last 15 years significant advancements in induced polarization (IP) research have taken place, particularly with respect to spectral IP (SIP), concerning the understanding of the mechanisms of the IP phenomenon, the conduction of accurate and broadband laboratory measurements, the modelling and inversion of IP data for imaging purposes, and the increasing application of the method in near-surface investigations. We here summarized the current state of the science of the SIP method for near-surface applications and describe which aspects still represent open issues and should be the focus of future research efforts.Significant progress has been made over the last decade in the understanding of the microscopic mechanisms of IP; however, integrated mechanistic models involving the different possible polarization processes at the grain/pore scale are still lacking. A prerequisite for the advances in the mechanistic understanding of IP was the development of improved laboratory instrumentation, which has led to a continuously growing database of SIP measurements on various soil and rock samples. We summarize the experience of numerous experimental studies by formulating key recommendations for reliable SIP laboratory measurements. To make use of the established theoretical and empirical relationships between SIP characteristics and target petrophysical properties at the field scale, sophisticated forward modelling and inversion algorithms are needed. Considerable progress has been made also in this field, in particular with the development of complex resistivity algorithms allowing the modelling and inversion of IP data in the frequency domain. The ultimate goal for the future are algorithms and codes for the integral inversion of 3-D, time-3 lapse and multi-frequency IP data, which defines a 5-D inversion problem involving the dimensions space (for imaging), time (for monitoring) and frequency (for spectroscopy). We also offer guidelines for reliable and accurate measurements of IP spectra, which are essential for improved understanding of IP mechanisms and their links to physical, chemical and biological properties of interest. We believe that the SIP method offers potential for subsurface structure and process characterization, in particular in hydrogeophysical and biogeophysical studies.

NaNbO(3) powders with various particle sizes (ranging from 30 nm to several microns) and well-controlled stoichiometry were obtained through microemulsion-mediated synthesis. The effect of particle size on the phase transformation of the prepared NaNbO(3) powders was studied using X-ray powder diffraction, Raman spectroscopy, and nuclear site group analysis based on these spectroscopic data. Coarsened particles exhibit an orthorhombic Pbcm (D(2h)(11), no. 57) structure corresponding to the bulk structure, as observed for single crystals or powders prepared by conventional solid-state reaction. The crystal symmetry of submicron powders was refined with the space group Pmc2(1) (C(2v)(2), no. 26). The reduced perovskite cell volumes of these submicron powders were most expanded compared to all the other structures. Fine particles with a diameter of less than 70 nm as measured from SEM observations showed an orthorhombic Pmma (D(2h)(5), no. 51) crystal symmetry. The perovskite formula cell of this structure was pseudocubic and was the most compact one. A possible mechanism of the phase transformation is suggested.

The concept of specific polarizability [Formula: see text], being the ratio between imaginary conductivity and specific surface area, can be used to represent the polarization of the mineral-fluid interface per unit pore-volume-normalized surface area [Formula: see text] and to account for the control of the fluid chemistry and/or mineralogy on induced polarization (IP) measurements. We used a database of IP measurements on sands and sand-clay mixtures to investigate the variation in [Formula: see text] as a function of clay content and/or mineralogy. We found an apparent variation in [Formula: see text] between sands and sand-clay mixtures when [Formula: see text] was calculated using the nitrogen adsorption (Brunauer-Emmett-Teller — BET) method, with clays having an apparently higher [Formula: see text] than sands. However, this variation was considerably reduced when [Formula: see text] was calculated using a wet-state methylene blue (MB) method that also sensed the surface area associated with internal layers of clay minerals inaccessible with the dry-state BET method. Furthermore, the imaginary conductivity was significantly better correlated with [Formula: see text] determined from the MB method relative to [Formula: see text] determined from the BET method. We found no evidence for a strong difference in the specific polarizability of quartz and clay minerals. This finding contradicted predictions from recent mechanistic formulations of the IP response of the Stern layer. Our findings have significant implications for improving and simplifying the interpretation of IP measurements in near-surface materials.

Undoped BaTiO 3 ceramic samples with an average grain size of ϳ35 nm were prepared and the electrical properties investigated. The defect structure is dominated by acceptor impurities; therefore, the conductivity of nanocrystalline BaTiO 3 is of p-type. Comparing with microcrystalline BaTiO 3 , the conductivity of nanocrystalline BaTiO 3 is about 1 to 2 orders of magnitude higher and the activation energy remarkably lower, which is ascribed to a greatly reduced oxidation enthalpy in nanocrystalline BaTiO 3 ͑ϳ0.3 versus ϳ0.92 eV for microcrystalline BaTiO 3 ͒. respectively, either in bulk form or thin-film form, have been prepared and the electrical properties investigated. Among these nanocrystalline oxides, the most dramatic property changes were observed for nanocrystalline CeO 2 . Comparing with microcrystalline counterparts, nanocrystalline CeO 2 is characterized by an orders-of-magnitude enhanced n-type conductivity and a significantly reduced activation energy. This phenomenon was explained by a greatly reduced reduction enthalpy of nanocrystalline CeO 2 , and the grain-boundary sites of lower vacancy formation enthalpy were proposed to be the atomic-level origin of this behavior.1,2 However, Kim and Maier 5 proposed that the higher conductivity of nanocrystalline CeO 2 is due to the electron accumulation at the grain boundaries. With its comparatively high electronic bulk contribution and high density of grain boundaries, the grain boundaries in nanocrystalline CeO 2 can become electronically conducting and dominate the overall behavior.Nanocrystalline BaTiO 3 materials of high density have also been prepared in thin-film form with an average grain size of ϳ25 nm, 10 and in bulk form with an average grain size of ϳ70 nm, 11 respectively, and the ferroelectric properties characterized. It is found that the ferroelectricity is weakened and the dielectric constant is remarkably lower for nanocrystalline BaTiO 3 ceramics.11 The ferroelectric response is even absent when the grain size is 25 nm.10 In neutral to oxidizing atmospheres, acceptor-doped BaTiO 3 shows a p-type conductivity. 12,13 In this letter, the defect and transport properties of nanocrystalline BaTiO 3 ceramics, nominally undoped but actually doped with acceptor impurities, are characterized, featuring an enhanced p-type conductivity.BaTiO 3 powder with an average particle diameter of ϳ10 nm was synthesized through the hydrolytic decomposition of a barium-titanium-isopropoxide solution in a waterin-oil microemulsion, consisting of 10.47 wt. % of Tergitol NP35, 80.70 wt. % of cyclohexane, 6.04 wt. % of 1-octanol and 2.79 wt. % of ultrapure and degassed water. Since the reaction is confined to the ultrasmall space of individual aqueous micelles, this approach allows the formation of nanopowders.14 The Ti/ Ba ratio of the powder was determined to be 1.0035± 0.0004 by x-ray fluorescence spectroscopy. According to inductively coupled plasma mass spectroscopy analyses, the nominally undoped BaTiO 3 powder was actually doped with acceptor impurities ͑...