Herein, we report a study on the structural and thermodynamic effects that cation size disparity may have in NASICONtype solid solutions. A sol−gel procedure was used to synthesize two new NASICON-type lithium-ion conductors with nominal compositions LiGe 2−y Sn y (PO 4 ) 3 and Li 1+x Al x Ge 2−y−(1/2)x Sn y−(1/2)x (PO 4 ) 3 . The effect of tin substitution on structure and lithium-ion conductivity was studied with powder X-ray diffraction, Raman spectroscopy, and dielectric spectroscopy. It is found that, although increased unit-cell dimensions derived from X-ray data suggest that tin incorporation should open the conduction bottleneck regions and improve conductivity, a decrease in conductivity is observed. Analysis of the electrical data shows that the conduction activation energy is comprised of contributions from carrier motion and generation, the latter accounting for up to 20% of the total activation energy. This result, currently unreported for NASICON-type materials, is correlated with local structural distortions observed in Raman spectra. It is deduced that the bottleneck regions suffer distortions due to the large ionic radius disparity among cationic constituents, which results in the "trapping" of charge carriers. Data estimated for the entropy of motion are also presented and discussed, considering the most probable thermodynamic equilibrium states.

No abstract

This paper reports an experimental and theoretical investigation of the effects of adding Fe to the perovskite strontium titanate SrTiO 3 . The effects include changes in the short-order range structure as well as in the electronic and electrical properties. X-ray diffraction analysis reveals that the

The need to improve the sensitivity, selectivity and stability of ozone gas sensors capable of monitoring the environment to prevent hazard to humans has sparked research on binary metal oxides. Here we report on a novel ozone gas sensor made with ca. 0.5 m yolk-shelled ZnCo 2 O 4 microstructures synthesized via an eco-friendly, co-precipitation method and subsequent annealing. With these ZnCo 2 O 4 microspheres, ozone concentrations down to 80 parts per billion (ppb) could be detected with a.c. and d.c. electrical measurements. The sensor worked within a wide range of ozone concentrations, from 80 to 890 ppb, being also selective to ozone compared to CO, NH 3 and NO 2. The high performance could be attributed to the large surface area to volume ratio inherent in yolk-shell structures. Indeed, ozone molecules adsorbed on the ZnCo 2 O 4 surface create a layer of holes that affect the conductivity, as in a p-type semiconductor. Since this mechanism of detection is generic, ZnCo 2 O 4 microspheres can be further used in other environment monitoring devices.

Herein we report a study on the energetics of ion transport in NASICON-type solid electrolytes. A sol−gel procedure was used to synthesize NASICON-type lithium-ion conductors with nominal compositions Li 1+X Al X Ge 2−X (PO 4 ) 3 where 0 ≤ X ≤ 0.6. Trends in the conductivity and activation energy, including both enthalpic and entropic contributions, were examined with electrochemical impedance spectroscopy. Physical interpretations of these results are drawn from structural characterizations performed by synchrotron powder X-ray diffraction and Raman spectroscopy. Considering X = 0 → 0.6, we conclude that initial drops in activation energy are driven by a growing Li + population on M2 sites, while later increases in activation energy are driven by changes in average bottleneck size caused by the Al-for-Ge substitution. Values of the entropy of motion are rationalized physically by considering the changing configurational potential of the mobile Li + population with changes in X. We conclude that entropic contributions to the free energy of activation amount to ≤22% of the enthalpic contributions at room temperature. These insights suggest that while entropic contributions are not insignificant, more attention should be paid to lowering the activation energy when designing a new NASICON-type conductor. ■ INTRODUCTIONWith the demand for high-performing rechargeable lithium-ion batteries continually on the rise, much research effort over the past decade has been spent on the development of materials with enhanced electrochemical properties. As all-solid-state batteries show great promise to meet many performance needs, a targeted research effort to develop high-conducting solid-state separator materials has developed. Materials of the sodium superionic conductor (NASICON) family are promising candidates, as many have demonstrated high conductivity, electrochemical stability, and mechanical integrity. 1−4 With these and other classes of ion conductor showing promise for use in next-generation energy storage technologies, it is helpful to have a clear picture of the various factors that contribute to the measured conductivities.In the case of ion-conducting solids such as the NASICONtype materials, the conductivity (σ) is governed by the relation σ = cμq, where c is the density of charge carriers, μ is the mobility of the charge carriers, and q is the charge carried by each carrier. Here we can see two obvious approaches to enhance the conductivity of such a material: increasing the concentration of charge carriers or increasing carrier mobility. Indeed, a common method of enhancing the conductivity of the established NASICON-type conductor lithium germanium phosphate is with a heterovalent doping scheme: 5,6 LiGe 2 (PO 4 ) 3 → Li 1+X Al X Ge 2−X (PO 4 ) 3 . Since the radii of Ge 4+ (0.53 Å) and Al 3+ (0.535 Å) are nearly the same and there are many unfilled sites available for the additional lithium ions in the structure, the substitution is easily accomplished. This doping has been reported to increase the conductiv...

The defect chemistry‐modulated dielectric properties of dense yttria‐doped zirconia ceramics prepared by conventional sintering (at 1350°C–1500°C) and electric field‐assisted flash sintering (55 V/cm at 900°C) were studied by impedance spectroscopy. While the bulk dielectric properties from both sets of samples showed only small and insignificant changes in conductivity and permittivity, respectively, a huge increase of these properties was measured for the grain boundaries in the flash sintered specimens. A close analysis of these results suggests that flash sintering reduced grain‐boundary thickness (by about 30%), while increasing the concentration of oxygen vacancies near these interfaces (by about 49%). The underlying mechanism proposed is electric field‐assisted generation and accommodation of defects in the space‐charge layers adjacent to the grain surface. The changes in measured permittivity are attributed to the boundary thickness effect on capacitance, while conductivity involved variations in its defect density‐dependent intrinsic value, accounting for changes also observed in grain‐boundary relaxation frequencies. Therefore, in terms of modifications to the specific dielectric properties of these materials, the overall consequence of flash sintering was to considerably lower the semi‐blocking character of the grain boundaries.

The sensitivity of ZnO-SnO 2 heterojunctions to ozone gas was investigated in this work, the two-phase materials of which were prepared via a hydrothermal route, resulting in nanocomposites in which the formation of heterojunctions was confirmed by microscopy analyses. While the sensing effectiveness of these materials is currently verified for application above 150 • C, these temperatures are here drastically reduced to room temperature by considering sensing activity under continuous UV irradiation, even for ozone concentrations as low as 20 ppb. This approach resulted in a fast sensing response, a short recovery time and a good selectivity compared to other gases, demonstrating a great potential of such heterojunctions for applications in environmental monitoring devices.

No abstract