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We report measurements of the magnetic-field dependence of the resistive transition in ${\mathrm{Tl}}_{2}$${\mathrm{Ba}}_{2}$${\mathrm{CuO}}_{6+\mathrm{\ensuremath{\delta}}}$ for several crystals with transition temperatures in the range 15 K\ensuremath{\le}${\mathit{T}}_{\mathit{c}}$\ensuremath{\le}62 K. Transition of the most overdoped sample (with the lowest transition temperature) shifts monotonically to lower temperature with increasing field with no appreciable change in shape. As reported previously [A. P. Mackenzie et al., Phys. Rev. Lett. 71, 1238 (1993)], the behavior of the upper critical field (${\mathit{H}}_{\mathit{c}2}$) inferred from these measurements does not, however, have the classical BCS form. The samples with higher transition temperatures show a gradual and systematic crossover to field-broadened transitions, with a kneelike structure developing in sufficiently high fields. Some possible interpretations of these observations are discussed.

A combined powder neutron and resonant X-ray diffraction study of a post-annealed Tl-2223 (“Tl 2 Ba 2 Ca 2 Cu 3 O 10 ”) superconductor with T c (0)=124.5 K shows that Cu substitutes for Tl in the TlO layers and Tl substitutes at the Ca sites. The resulting composition is (Tl 1.72 Cu 0.28 )Ba 2 (Ca 1.86 Tl 0.14 )Cu 3 O 10 , and some indications of how the stoichiometry of Tl2223 varies with starting composition have been obtained. Using excess Cu in the synthesis of Tl2223 gives more phase-pure material than other starting compositions, resulting in a T c =127 K sample after only 3.5 h synthesis time.

We have measured the resistive upper critical field of overdoped single crystals of Tl2Ba2CuO6 from the zero-field transition temperatureTc (approximately 20 K) to temperatures as low as 12 mK, corresponding to less than 0.001Tc. In sharp contrast to the predictions of standard theories of superconductivity, the critical field is found to rise steeply with positive curvature as the temperature is reduced, and no sign of saturation is observed down to the lowest temperatures reached.

The cation distribution in a polycrystalline sample of nominal composition “Tl0.5Pb0.5Sr2Ca2Cu3O9” with Tc = 118 K has been determined from resonant synchrotron X-ray diffraction data collected at the Cu K, Tl LIII, Pl LIII and Sr K edges. Cation disorder is found at all the non-copper sites according to the formula (Tl1−xPbx) (Sr2−yCay) (Ca2−zTlz)Cu3O9 where x = 0.53(2), y = 0.42(2) and z = 0.064(4) for our sample. The overall composition Tl0.53Pb0.53Sr1.58Ca2.36Cu3O9 confirms that an excess of Ca and a deficiency of Sr can stabilise the Tl, Pb-1223 structure.

There has been much recent interest in a so-called “giant-dielectric phenomenon” displayed by an unusual cubic perovskite-type material, CaCu3Ti4O12; however, the origin of the high permittivity has been unclear [M. A. Subramanian, L. Dong, N. Duan, B. A. Reisner, and A. W. Sleight, J. Solid State Chem. 151, 323 (2000); C. C. Homes, T. Vogt, S. M. Shapiro, S. Wakimoto, and A. P. Ramirez, Science 293, 673 (2001); A. P. Ramirez, M. A. Subramanian, M. Gardel, G. Blumberg, D. Li, T. Vogt, and S. M. Shapiro, Solid State Commun. 115, 217 (2000)]. Impedance spectroscopy on CaCu3Ti4O12 ceramics demonstrates that they are electrically heterogeneous and consist of semiconducting grains with insulating grain boundaries. The giant-dielectric phenomenon is therefore attributed to a grain boundary (internal) barrier layer capacitance (IBLC) instead of an intrinsic property associated with the crystal structure. This barrier layer electrical microstructure with effective permittivity values in excess of 10 000 can be fa...

Subsolidus phase relations in the CuO{sub x}-TiO{sub 2}-Nb{sub 2}O{sub 5} system were determined at 935 deg. C. The phase diagram contains one new phase, Cu{sub 3.21}Ti{sub 1.16}Nb{sub 2.63}O{sub 12} (CTNO) and one rutile-structured solid solution series, Ti{sub 1-3x}Cu{sub x}Nb{sub 2x}O{sub 2}: 0 Subsolidus phase relations in the CuO{sub x}-TiO{sub 2}-Nb{sub 2}O{sub 5} system were determined at 935 deg. C. > A new phase, Cu{sub 3.21}Ti{sub 1.16}Nb{sub 2.63}O{sub 12} (CTNO), was found with a CaCu{sub 3}Ti{sub 4}O{sub 12}-like crystal structure. > We discovered one rutile-structured solid solution series, Ti{sub 1-3x}Cu{submore » x}Nb{sub 2x}O{sub 2}: 0 CTNO has square planar Cu{sup 2+}, A site vacancies and Cu{sup +}, Ti{sup 4+} and Nb{sup 5+} disordered on octahedral sites. > CTNO is a modest semiconductor with relative permittivity {approx}63.« less

The title solid solutions are prepared from stoichiometric mixtures of BaCO3, TiO2, and Co3O4 (Pt crucible, 1648 K for 2 d (y = 0.1), 1473 K for 3 d (y = 0.2), 1373 K for 3 d (y = 0.3), 1398 K for 6 d (y = 0.4)).

Scanning Probe Microscopy with conductive tips has been used to image and study the dielectric properties of giant permittivity CaCu3Ti4O12 ceramics at the nanoscale. Since measurements are generally carried out on sections of a sample, particular attention has been devoted to possible artefacts due to surface imperfections, such as substantial surface roughness and/or contamination that can result in controversial interpretation, particularly at nanometric spatial dimensions. A reliable surface investigation has been carried out after the definition of both the physical and geometrical unbiased criteria to avoid any artefacts due to surface roughness and/or anomalous tip–sample contact variations. The presence of insulating grain boundaries and the measurement of a depletion layer at the grain–grain boundary interfaces unambiguously demonstrate the relevance of the Internal Barrier Layer Capacitor effect, among all the proposed physical mechanisms, to explain the giant dielectric behaviour. Such imaging provided a clear correlation between the macroscopic dielectric properties and the nanometric structure at the interfaces. Moreover, the ''general criteria'' for reliable nanoelectrical characterization as well as the related measurement resolution have been defined.

Polycrystalline phase pure BV1.025O4+x ceramics were prepared by solid state synthesis. These materials were found to exhibit mixed oxide ion/electronic conduction from ac impedance measurements recorded between 150–800°C in various atmospheres. The results indicate predominantly electronic conduction at both low and high temperatures i.e. 600°C. Oxide ion conduction was found to dominate the temperature region between 400 and 600°C, with an activation energy of 0.80±0.01 eV. The bulk conductivity below 400°C was found to be metastable and influenced by the presence of water. The possibility of protonic conduction at low temperatures is therefore identified.

The phase reported in the literature as Bi4V2O11 is a solid solution phase whose composition covers the range ∼66.7 to 70.4% Bi2O3. The phase diagram of the Bi2O3V2O5 system in the region of this phase has been determined and electrical properties of the Bi4V2O11 solid solutions measured. The high temperature γ polymorph is an excellent oxide ion conductor, whose conductivity decreases gradually with increasing Bi2O3 content. The low temperature α polymorph is a ferroelectric whose permittivity appears to increase to a maximum at the α→β transition temperature.

La 3+ substitution into the Bi 2201 structure, of the “ideal” composition Bi 2 Sr 2 CuO δ , has been investigated by means of an extensive phase diagram study into the solid solution formation at 885°C in air. Single phase solid solutions may be presented by the formula Bi 2+ x Sr 2- x - y La y CuO δ , where y has a maximum value of 0.6. With increasing La content, the crystal symmetry changes from orthorhombic to tetragonal and the incommensurate supercell becomes less pronounced, as indicated by a decrease in intensity of the supercell X-ray powder reflections. Superconducting T conset values vary systematically with cation contents x,y and oxygen content δ; a maximum T c of 36.5 K is observed for composition x =-0.05, y =0.4.

The new phase Li2NaTa7O19 has been prepared by high-temperature reaction of Na2CO3, Li2CO3 and Ta2O5. Its structure has been determined and refined, from Cu-Kα1, X-ray powder diffractometer data, in space group Pbam with unit cell: a= 15.2308(6)A, b= 23.573(1)A, c= 3.8366(2)A, V= 1377.5 A3 to RF= 3.1%. The structure exhibits a layered Ta7O193– framework, containing TaO6 octahedra and TaO7 pentagonal bipyramids. The Na atoms are located in pentagonal tunnels along the c axis; the Li atom positions were not determined. High-resolution electron microscopy images confirmed the basic features of the structure. Conductivity measurements showed essentially insulating behaviour with a limited amount of ionic conductivity at high temperatures, ca. 2 × 10–7Ω–1 cm–1 at 300 °C.

The correlation between crystal structure and intrinsic dielectric properties of the unusual cubic perovskite-related family of compounds $A{\text{Cu}}_{3}{\text{Ti}}_{4}{\text{O}}_{12}$, where $A=\text{Ca}$, Cd, ${\text{La}}_{2/3}$, ${\text{Bi}}_{2/3}$, ${\text{Na}}_{1/2}{\text{La}}_{1/2}$, ${\text{Na}}_{1/2}{\text{Bi}}_{1/2}$, etc., (space group $Im\overline{3}$ ) has been a controversial topic for several years, especially for the most studied member of the family ${\text{CaCu}}_{3}{\text{Ti}}_{4}{\text{O}}_{12}$ (CCTO). ${\text{Na}}_{1/2}{\text{Bi}}_{1/2}{\text{Cu}}_{3}{\text{Ti}}_{4}{\text{O}}_{12}$ (NBCTO) is isostructural with CCTO and retains centrosymmetric and cubic symmetry (space group $Im\overline{3}$) down to 4 K. NBCTO ceramics exhibit high intrinsic relative permittivity of $\ensuremath{\sim}145$ at 300 K that rises on cooling and levels off below $\ensuremath{\sim}50\text{ }\text{K}$ with a value of $\ensuremath{\sim}260$ at 10 K without any signature of a structural transition. Such behavior is a typical feature of incipient ferroelectrics such as the perovskite ${\text{CaTiO}}_{3}$. Infrared and terahertz spectroscopy on NBCTO ceramics are presented and reveal a room-temperature soft polar optic mode at $\ensuremath{\sim}34\text{ }{\text{cm}}^{\ensuremath{-}1}$ that softens on cooling and is responsible for the temperature dependence of the relative permittivity. The temperature dependence of the soft-mode frequency obeys the Barrett formula with the onset of quantum fluctuations ${T}_{1}$ occurring near 85 K and a hypothetical Curie temperature ${T}_{0}$ of $\ensuremath{\sim}\ensuremath{-}155\text{ }\text{K}$. A fitted vibrational zero-point energy $({k}_{\text{B}}{T}_{1})/2$ is in good agreement with the measured soft-mode frequency. The crystal structure-intrinsic dielectric properties of NBCTO and possibly all cubic $A{\text{Cu}}_{3}{\text{Ti}}_{4}{\text{O}}_{12}$ phases are, therefore, consistent with that of other untilted/tilted ${\text{TiO}}_{3}$-based centrosymmetric perovskites such as ${\text{CaTiO}}_{3}$ and ${\text{SrTiO}}_{3}$. The ``so-called'' giant permittivity values of $g1000$ reported for CCTO, NBCTO, and related phases from radio-frequency capacitance measurements near room temperature are an extrinsic effect associated with the semiconducting nature of these materials, as opposed to an intrinsic effect associated with their rather unusual perovskite-type crystal structure.

The electrical properties of Ca-doped BaTiO3 are very different when Ca substitutes onto Ba or Ti sites. The p-type semiconductivity of Ti-substituted ceramics increases reversibly by one to two orders of magnitude under a dc-bias voltage of 􏰅100 V cm−1, whereas Ba-substituted ceramics show little sensitivity to a dc bias. This increase in BaTi1−xCaxO3−x, studied over the temperature range 150–600 °C, is independent of electrode material and atmosphere and is attributed to ionization of underbonded O2− ions adjacent to acceptor-doped Ca2+ ions.

A phase diagram study has been made to determine the range of compositions of single phase, Pb-doped R solid solutions in the Bi–Sr–Pb–Cu–O system. These have the formula Bi2+x−zPbzSr2−xCuO6+δ, with 0.15 < x < 0.40 and 0 < z < 0.55 at 750 °C. With increasing z, the unit cell symmetry changes from tetragonal to orthorhombic; cell parameters are given as a function of x and z. From the compositional extent of the solid solutions, in terms of x and z, it is possible to infer that Pb enters predominantly “Bi” sites but also “Sr” sites, in the crystal structure of the R phase, in contrast to Ca which appears to enter exclusively the “Sr” sites. The change from tetragonal to orthorhombic symmetry may be interpreted as indicating the onset of Pb occupation of the “Bi” sites.

An investigation of Ca substitution in the multiferroic material BiFeO3 shows that a wide range of perovskites BixCa1−xFeO3 (0.4 ≤ x ≤ 1.0) can be prepared by sintering in air at 810−960 °C. 0.4 ≤ x < 0.8 samples are cubic Pm3m, whereas x = 0.8 and 0.9 show a coexistence of the cubic and the rhombohedral R3c BiFeO3-type structure. Considerable disorder arising from Bi3+ lone-pair distortions is evidenced by synchrotron X-ray and neutron studies of the average structures of the cubic phases, and electron microscopy reveals commensurate and incommensurate local superstructures. The BixCa1−xFeO3 (0.4 ≤ x ≤ 1.0) materials show a remarkably robust antiferromagnetic order with TN = 623−643 K and ordered moments of 3.6−4.1 μB. They are “leaky” dielectrics with relative permittivities of ∼30−100 and bulk resistivities ∼50−500 kΩ cm at room temperature. The activation energy for bulk conduction increases from 0.27 eV for x = 0.4 to 0.5 eV for x = 1, but with a discontinuity at the cubic-rhombohedral boundary. Fur...

International audience; The crystal and magnetic structures and electrical properties of two new Mn-rich 6H′-type hexagonal perovskites in the BaMn1−xFexO3−δ system have been investigated. Structural characterization performed by X-ray, electron, and neutron diffraction and high resolution electron microscopy indicates that both BaMn0.85Fe0.15O2.87 and BaMn0.6Fe0.4O2.72 crystallize in the 6H′ hexagonal polytype (P6m2 space group). The structure is formed by tetramers and dimers of face-sharing octahedra that are linked by corners. The anion deficiency is located at random through the hexagonal layers and increases with the Fe-content. In both phases, the central position of the tetramers is fully occupied by Mn, the remaining Mn and Fe cations being randomly distributed over different polyhedra. The Mssbauer spectroscopy data show Fe to be present only as FeIII in octahedral and tetrahedral coordination. The magnetic structure is formed by ferromagnetic sheets with the magnetic moments aligned along the x-axis and stacked antiferromagnetically perpendicular to the c-axis. The electrical properties have been characterized by impedance spectroscopy and reveal both compounds behave as leaky insulators at room temperature with bulk permittivity values

A combination of X-ray, neutron and electron diffraction, and high-resolution electron microscopy have been used to establish the 5H structural type of a new hexagonal-type perovskite BaMn0.2Co0.8O2.80. The structure can be described as a (cc′chh) 5H hexagonal polytype with ordered oxygen vacancies where the cubic c′ layer corresponds to a composition of [BaO2] as opposed to [BaO3]. The resulting layer structure consists of [MnCo2O12] blocks of three sharing faces octahedra linked by corners to two unconnected [CoO4] tetrahedra. Electron Energy Switch order Loss Spectroscopy shows Mn to be present only as Mn(+IV) and therefore Co is present as mixed +III/+IV. Mn(+IV) and Co(+III) ions are distributed over the face sharing octahedral sites whereas Co(+IV) ions are located on the tetrahedral sites. The magnetic behavior is more complex than is observed for BaCoO2.80 (a ferromagnet with Tc = 47 K) and can be described by a Stoner–Wohlfarth model of random-anisotropic, noninteracting monodomain ferromagnetic clusters. The ferromagnetic clusters occur below 35 K and are assigned to groups of Co ions in octahedral and/or tetrahedral sites; however, incorporation of Mn ions in the octahedral sites disrupts the transition into long-range three-dimensional ferromagnetic order. Impedance Spectroscopy data reveals semiconducting grain conductivity at room temperature (1 × 10−2 S cm−1); however, subambient data reveal an unusual temperature dependence with a smooth changeover from a thermally activated process (0.07 eV) in the range 40–300 K to a low-temperature state below 40 K with a near-zero activation energy. The data cannot be described by conventional Arrhenius or variable-range hopping conduction models and the conduction mechanism(s) remain unresolved. Several possible suggestions for the conductivity behavior are made, including Anderson localization, anisotropic conduction associated with the 5H crystal structure or some complex correlated mechanism between the magnetic and electronic transport properties. The electrical microstructure of BaMn0.2Co0.8O2.8 ceramics consist of semiconducting grains and constrictive grain boundaries and therefore exhibit internal barrier layer capacitor (IBLC) behavior, with a high and temperature-stable apparent permittivity of 10 000 (at 10 kHz) above 100 K.

Nanoscale imaging of the electrical properties of CaCu3Ti4O12 ceramics has been performed using scanning impedance microscopy. Two kinds of electrical inhomogeneity are detected, namely, depletion layers at grain boundaries and calcium titanate inclusions, both of which are more resistive than the bulk of the grains. Energy filtered transmission electron microscopy was used to estimate the chemical composition of the inclusions. (c) 2008 American Institute of Physics.

A working subsolidus phase diagram for the system BaTiO 3 -Ba 5 Nb 4 O 15 has been determined by firing sol-gel-synthesized samples over a range of temperatures. The main difference from previous diagrams is the greater extent of the Nb-doped BaTiO 3 cubic solid solutions, BaTi 1-5x Nb 4x O 3 , at lower temperatures with x extending to 0.09 at 900°C, but only 0.05 at 1400°C. Electrical property measurements show that compositions with large x (x > 0.0025) are highly insulating for pellets sintered at 1300°C in air, followed by a slow cool. Compositions with low x, however, exhibit a residual semiconducting grain core and are not fully reoxidized readily. Composition dependence of the dielectric properties shows a continuous and smooth transition from classic ferroelectric behavior with pure BaTiO 3 to normal dielectric response with a temperature-independent relative permittivity of approximately 22-24 for x > ∼ 0.08. At intermediate compositions, ranges of both relaxor ferroelectric and quasi-ferroelectric behavior are observed. Possible reasons for an observed anomalous increase in value of the permittivity at the ferroelectric transition temperature at low x, which is superposed on an overall decrease in permittivity with increasing x, are discussed.

Liquid In/Ga alloys are easily applied to the surfaces of glasses and ceramics for use as electrodes in ac impedance measurements. They operate as blocking electrodes over a wide temperature range, -100 to 300°C at least. Results are shown for beta alumina ceramics and lithium silicate glasses and compared with those obtained using gold electrodes.

Electroceramics are advanced materials whose properties and applications depend on the close control of structure, composition, ceramic texture, dopants and dopant (or defect) distribution. Impedance spectroscopy is a powerful technique for unravelling the complexities of such materials, which functions by utilizing the different frequency dependences of the constituent components for their separation. Thus, electrical inhomogeneities in ceramic electrolytes, electrode/electrolyte interfaces, surface layers on glasses, ferroelectricity, positive temperature coefficient of resistance behavior and even ferrimagnetism can all be probed, successfully, using this technique.

Solid oxide cells (SOCs) are highly efficient electrochemical energy conversion devices capable of operating in both fuel cell and electrolysis modes. The operating temperatures of SOCs (500 – 800 oC, for IT-SOCs) create opportunities for direct utilisation of a wide variety of reactants, including low-grade fuels such as biogas, however, in-situ characterisation of evolving electrode morphologies and compositions is challenging. Conventional in-situ characterisation techniques, including impedance spectroscopy, can provide valuable information about electrochemical processes, but are unable to resolve specific chemical and physical changes during cell operation which can adversely affect SOC performance. Raman spectroscopy is a valuable analytical tool for in-operando monitoring of SOCs. Molecular species adsorbed on the surface can be identified, thus providing direct insight into reaction intermediates, material phase transformations and the presence of contaminants and poisons, such as carbon, chromium, sulfur and silica. Raman spectroscopy is a versatile technique that can be used for temperature sensing, semi-quantitative estimation of infiltrated material and many other applications. The design of optically accessible experimental setup will be presented alongside methodology for obtaining and interpreting in-situ Raman spectroscopic measurements. Raman spectroscopic evidence of carbon deposition during CO2 electrolysis will be discussed.

High temperature solid oxide cells (SOCs) can be used for co-electrolysis of carbon dioxide and steam to produce carbon monoxide and hydrogen: key reactants for the production of synthetic fuels. During co-electrolysis, two electrochemical reactions occur at the fuel electrode, in addition to the reverse water gas shift reaction (rWGSR). It is currently not well understood which processes relating to these reactions are rate-limiting. Here we propose a method that enables rate-limiting processes to be identified using electrochemical impedance spectroscopy (EIS). SOCs were operated at 850oC under varied fuel concentrations and current densities. Inductance effects were accounted for and the resulting EIS data was de-convoluted using the Distribution of Relaxation Times (DRT) method, giving the characteristic frequencies of rate-limiting processes occurring during cell operation. The resistances and capacitances associated with these processes were quantified using the impedance and electric modulus formalisms, respectively. DRT results in combination with the analysis of differences in spectra (ADIS) associated with the EIS data, plotted in both impedance and electric modulus formalisms, were used to justify the number of processes associated with the cell polarization resistance and to develop a meaningful equivalent circuit to model cell performance.

Low levels of so-called “donor-doping” in titanate-based perovskite oxides such as La for Ba, Sr, and Nb for Ti in (Ba, Sr)TiO3 can significantly reduce the resistivity of these typical (d0) dielectric materials and expand application areas to positive temperature coefficient resistors, thermoelectrics, conductive wafers as thin film substrates, and solid oxide fuel cell anode materials. Here, we show low levels of Nb-doping (≤1 at. %) on the Ti-site in the well-known lead-free piezoelectric perovskite oxide Na 0.5Bi0.5TiO3 (NBT) produces completely different behaviours whereby much higher resistivity is obtained, therefore indicating a different donor-doping (substitution) mechanism. There is a switch in conduction mechanism from oxygen-ions in undoped NBT with an activation energy (Ea) of

The influence of electrode material, dc bias, and pellet thickness on the electrical properties of fine- and coarse-grained $\mathrm{Ca}{\mathrm{Cu}}_{3}{\mathrm{Ti}}_{4}{\mathrm{O}}_{12}$ (CCTO) ceramics has been investigated using impedance spectroscopy. The low frequency arc observed in ${\mathrm{Z}}^{*}$ plots near room temperature is independent of the electron work function of the metal electrode. It shows significant variation with dc bias and pellet thickness for coarse-grained ceramics, but no such variations for fine-grained ceramics. The results demonstrate the importance of ceramic microstructure in controlling the electrical properties of CCTO ceramics and support the internal barrier layer capacitor (IBLC) model of Schottky barriers at the grain boundaries between semiconducting grains. The IBLC model explains the high permittivity and nonlinear current-voltage characteristics commonly reported for CCTO ceramics.

The new phase Li2NaTa7O19 has been prepared by high-temperature reaction of Na2CO3, Li2CO3 and Ta2O5. Its structure has been determined and refined, from Cu-Kα1, X-ray powder diffractometer data, in space group Pbam with unit cell: a= 15.2308(6)A, b= 23.573(1)A, c= 3.8366(2)A, V= 1377.5 A3 to RF= 3.1%. The structure exhibits a layered Ta7O193– framework, containing TaO6 octahedra and TaO7 pentagonal bipyramids. The Na atoms are located in pentagonal tunnels along the c axis; the Li atom positions were not determined. High-resolution electron microscopy images confirmed the basic features of the structure. Conductivity measurements showed essentially insulating behaviour with a limited amount of ionic conductivity at high temperatures, ca. 2 × 10–7Ω–1 cm–1 at 300 °C.

Solid oxide electrolysis cell (SOEC) technology is a promising route for sustainable recycling of carbon dioxide and steam into synthetic fuels and commodity chemicals via syngas (H2+ CO) production. This technology is based on the reverse polarity of the prominent solid oxide fuel cell (SOFC) and commonly consists of a porous Ni/YSZ cathode, dense YSZ electrolyte and porous LSM/YSZ anode. High temperature co-electrolysis of CO2 and H2O is a complicated process and has not yet been fully understood. This is due to a number of reactions that take place simultaneously: water electrolysis, carbon dioxide electrolysis and the reverse water-gas shift reaction (rWGS CO2 + H2 ↔ CO + H2O). Other possible reactions driven by the equilibrium may include the Bosch reaction (CO + H2 ↔ C(s) + H2O) and Boudouard reaction (2CO ↔ CO2 + C(s)). Carbon deposition on the nickel-cermet surface of SOECs can severely degrade the performance of the cell by deactivating the reaction sites. Recent work by Tao et al. has shown that, at high current densities and conversion efficiencies of CO2 and H2O to CO and H2, carbon formation on the surface of Ni/YSZ fuel electrode is likely to occur [1]. SOECs are typically analyzed in-situ using electrochemical impedance spectroscopy and current-voltage characteristics and ex-situ using scanning electron microscopy to obtain information about cell’s surface topography and composition. However, these techniques are not able to provide conclusive information about the processes occurring during operation of the cell. In this work we aim to use in-situ Raman spectroscopy alongside traditional electrochemical techniques to probe the surface species of the SOEC electrodes for carbon impurities deposition and provide insight into the fundamental processes and reaction pathways that occur during high temperature (700 – 850 oC) co-electrolysis of CO2 and steam. In-situ Raman spectroscopy is also applied to characterize structural changes and material transformations during SOEC operation. The particular area of interest is the triple phase boundary (TPB), where the electrochemical reactions take place. Accessing the TPB is challenging due to high operation temperatures and the depth of laser penetration into the sample. The experimental setup with optical access for the Raman microscope will be discussed in detail as well as the correlation of in-situ spectroscopic data with the electrochemical information obtained from the SOECs. References [1] Tao Y., S. D. Ebbesen, and M. B. Mogensen, 2014, J. Electrochem. Soc. 161 (3), F337 – F343.

The hydrothermal crystallisation of barium titanate, BaTiO3, has been studied for the first time by in situ time-resolved neutron powder diffraction; we deduce that BaTiO3 is formed from solution by reaction between [Ti- (OH)x](4−x)+(aq) and [Ba(OH)x](2−x)+(aq) species, rather than by a heterogeneous reaction between solid TiO2 and Ba2+(aq) ions.

A double doping mechanism of Na+ + ½ Ti4+ → Ca2+ + ½ Cu2+ on the general formula Ca1−xNax(Cu3−x/2Tix/2)Ti4O12 has been used to prepare a series of isostructural CaCu3Ti4O12 (CCTO)-type perovskites. A complete solid solution exists for 0 ≤ x ≤ 1 and all compositions exhibit incipient ferroelectric behaviour with higher than expected intrinsic relative permittivity. Although CCTO ceramics typically exhibit n-type semiconductivity (room temperature, RT, resistivity of ∼10–100 Ω cm), Na(Cu5/2Ti1/2)Ti4O12 (NCTO) ceramics sintered at 950 °C consist of two insulating bulk phases (RT resistivity > 1 GΩ cm), one p-type and the other n-type. With increasing sintering temperature/period, the p-type phase transforms into the n-type phase. During the transformation, the resistivity and activation energy for electrical conduction (Ea ∼ 1.0 eV) of the p-type phase remain unchanged, whereas the n-type phase becomes increasingly conductive with Ea decreasing from ∼ 0.71 to 0.11 eV with increasing sintering temperature. Th...

A combined powder X-ray lattice parameter and ceramic impedance spectroscopy study is presented on\ud materials within the CaO–CuO–TiO2 ternary phase diagram. Several compositions containing CaCu3Ti4O12\ud (CCTO) and small amounts of secondary phases such as TiO2, CaTiO3 and CuO are analysed and two\ud different defect mechanisms are identified as the cause of the non-stoichiometry in CCTO. The first\ud mechanism involves a variation in the Cu content, which explains the large differences in the intrinsic bulk\ud and extrinsic grain boundary (GB) resistance, and the formation of the ceramic internal barrier layer\ud capacitor (IBLC) structure. The second mechanism is associated with Ca–Cu anti-site disorder causing an\ud unusually high intrinsic bulk permittivity above that predicted from Clausius–Mossotti calculations.

We have performed a computational study on the rare-earth (RE) doping of the perovskite structured materials; SrTiO3 and CaTiO3. The calculations have been completed using new Sr–O and Ca–O potentials in combination with a recently developed set of interatomic potentials, previously fitted and tested on polymorphs of BaTiO3. Particular attention has been given to the energetic and structural consequences of rare-earth doping via the five major dopant incorporation schemes. For SrTiO3, large RE ions dope at the Sr-site via a Sr vacancy mechanism, whereas smaller RE ions prefer doping via self-compensation due to the size of the ions being approximately half way between the size of the larger Sr-site and smaller Ti-site. Our simulations show that for CaTiO3, large to mid-sized RE ions (La to Eu) energetically favour Ca-site doping with Ca vacancy charge compensation and smaller ions dope via self-compensation. On comparison with previous calculations for BaTiO3, our results show the effect of the A-site size decrease from Ba to Ca on the favoured incorporation mechanism. The results for both materials are in good agreement with experiment. An overall assessment of the RE-doping in this perovskite series (ATiO3, where A = Ba, Sr or Ca) is given.

Density functional theory (DFT) calculations have been used to study the nature of intrinsic defects in the hexagonal polymorph of barium titanate. Defect formation energies are derived for multiple charge states, and due consideration is given to finite-size effects (elastic and electrostatic) and the band gap error in defective cells. Correct treatment of the chemical potential of atomic oxygen means that it is possible to circumvent the usual errors associated with the inaccuracy of DFT calculations on the oxygen dimer. Results confirm that both mono- and di-vacancies exist in their nominal charge states over the majority of the band gap. Oxygen vacancies are found to dominate the system in metal-rich conditions with face sharing oxygen vacancies being preferred over corner sharing oxygen vacancies. In oxygen-rich conditions, the dominant vacancy found depends on the Fermi level. Binding energies also show the preference for metal-oxygen di-vacancy formation. Calculated equilibrium concentrations of va...

Aberration corrected scanning transmission electron microscopy revealed that Bi0.85Nd0.15Fe0.9Ti0.1O3 ceramics contain coherent Nd-rich precipitates distributed throughout the perovskite lattice, implying charge compensation is obtained by the creation of VNd/// and not VBi///. At low concentrations, therefore, Ti4+ replace Fe2+ with the creation of 2/3VNd///, and at higher concentrations (when Fe2+ have been eliminated and the conductivity suppressed), Fe3+ with the creation of 1/3VNd///. The switch in ionic compensation mechanism from 2/3VNd/// at low Ti concentrations (∼1%) to 1/3VNd/// at higher concentrations (>1%) results in a decrease in the magnitude of ΔTC/Δx, as the disruption of long range anti-polar coupling declines.

Recently, it was demonstrated that donor doping with Ti on the B-site significantly reduces the conductivity in Bi0.85Nd0.15FeO3 ceramics [Kalantari et al., Adv. Funct. Mater. 21, 3737 (2011)]. In this contribution, the phase transitions as a function of Nd concentration are investigated in 3% Ti doped Bi1-xNdxFeO3 ceramics. Paraelectric (PE) to ferroelectric (FE) transitions were observed for compositions with x ≤ 0.125 which manifested themselves as peaks in permittivity. In contrast, PE to antiferroelectric (AFE) transitions for 0.15 ≤ x ≤ 0.20 gave rise to a step-like change in the permittivity with x = 0.25 exhibiting no sharp anomalies and remaining PE until room temperature. The large volume change at the PE to FE/AFE transitions, reported by Levin and co-workers [Phys. Rev. B 81, 020103 (2011)] and observed here by dilatometry, coupled with their first-order character constrain the transitions to occur uniformly throughout the material in an avalanche-like manner. Hence, the anomalies in DSC, permittivity and thermal expansion occur over a commensurately narrow temperature interval. However, despite the large volume change and eye-catching anomalies in DSC, the latent heats for the transitions in Ti-doped Bi1-xNdxFeO3 are similar to Pb(Zr,Ti)O3 (1–3 kJ/mol) with each an order of magnitude greater than BaTiO3 (∼0.2 kJ/mol). A broad frequency dependent dielectric anomaly of unknown origin in the temperature range 250–450 °C was also observed in all samples.

Recently, many charge-ordered compounds containing mixed-valence transition metal ions have been considered as possible multiferroics. Theoretically it has been proposed that charge (orbital) ordering could generate “electronic ferroelectricity” in some of these compounds. Experimentally, temperature- and frequency-dependent high permittivity is commonly presented as evidence of ferroelectricity in these materials. Here, systematic impedance spectroscopy studies on Y0.5Ca0.5MnO3 and La1.5Sr0.5NiO4 ceramics reveal, however, the high permittivity and its temperature- and frequency-dependence over the ranges of 10–300 K and 10 Hz–1 MHz, respectively, are mainly associated with an extrinsic non-ohmic sample-electrode contact effect, rather than an intrinsic response. A general strategy to distinguish the intrinsic dielectric properties of these charge-ordered compounds from extrinsic contributions using impedance spectroscopy is also demonstrated.

The polytypism and physical properties of BaTi1−yCoyO3−δ for y = 0.8 and 0.9 prepared with different δ values are reported. Samples quenched from 1293 K are strongly reduced, 3C-type perovskites (space group Pmm) with δ ∼ 0.4 and an average oxidation state of ∼Co3+ whereas slow cooled (4 °C min−1) samples are 5H-type perovskites (space group Pm1 and stacking sequence (ccchh)) with δ = 0.2 and an average oxidation state of ∼Co3.5+. An unusual feature of the 5H polymorph is the presence of ∼15 to 20% Ti(IV) on the tetrahedral (M3) sites. The 3C-polytypes are antiferromagnetic semiconductors with Neel temperatures 285 K. The reasons for the changes in polymorphism and physical properties with cation and anion content are rationalised on the basis of the different chemical nature and concentration of the Co(III), Co(IV) and Ti(IV) ions on the B-sites of these perovskites. The results are discussed in context to behaviour observed in related mixed-valent B-site perovskites to obtain a better understanding of the structure–composition–property relationships in these materials.

The microwave dielectric resonance properties of Na1/2Bi1/2Cu2.82Mn0.18Ti4O12 (NBCMTO) ceramics are reported. Single-phase NBCMTO powder were obtained from the mixed oxide route at 975°C and dense ceramics (~ 97% of the theoretical X-ray density) with a grain size distribution between ~ 2 and 10 μm by sintering in air at 1060°C. Ceramics exhibit room temperature microwave dielectric resonance with er ~ 130, Qf0 ~ 1750 GHz and a temperature co-efficient of resonance, τf, ~ +1000 ppm/K. The high permittivity and the large magnitude and sign of τf are attributed to incipient ferroelectricity.

Chemical doping is one of the major methods by which the properties of BaTiO3 are modified to induce various device characteristics. Doping strategies are generally separated into iso- and alio-valent (donors and acceptors) and into A- and B-site species within the perovskite structure. The A1g octahedral breathing mode at ∼800 cm−1 in BaTiO3 is Raman inactive for compositions with single B-site species. However, this mode becomes Raman active if more than one B-site species is present, including titanium vacancies, VT′′′′. Moreover, the relative intensity of the A1g breathing mode is qualitatively related to the concentration of the species replacing Ti in the B-site. This article illustrates some clear cases where the A1g octahedral breathing mode can be utilized to verify the site occupancy of dopants in BaTiO3 when used in conjunction with more conventional indirect methods such as x-ray diffraction (Vegard’s Law) and dielectric measurements as a function of temperature.

Using a recently established BaTiO3 potential model specifically designed for the calculation of defect energetics, atomistic simulations have been carried out on the intrinsic defect chemistry and Rare Earth (RE3+) doping of hexagonal barium titanate (h-BaTiO3). Five charge compensation schemes have been considered as well as potential cluster binding energies. The results show that ion size arguments are obeyed. In the dilute concentration limit, large RE3+ cations dope at the Ba-site via a titanium vacancy mechanism and mid sized RE3+ cations dope at the Ba and Ti sites simultaneously via a self compensation mechanism. In contrast, small RE3+ cations dope exclusively on the Ti-site via an oxygen vacancy compensation scheme. Comparisons between the hexagonal and cubic phases of BaTiO3 have also been drawn. It is suggested that Ba-site doping is less favorable and that Ti-site doping is considerably more favorable in h-BaTiO3 and that different defect configurations have a significant effect on the binding energies between such defects, leading to some mechanisms becoming more or less energetically favorable as a result.

Although the origins of the high effective permittivity observed in CaCu3Ti4O12 (CCTO) ceramics and single crystals at ∼100–400 K have been resolved, the relaxorlike temperature- and frequency-dependence of permittivity obtained from fixed frequency capacitance measurements at higher temperatures reported in the literature remains unexplained, especially as CCTO adopts a centrosymmetric cubic crystal structure in the range of ∼35–1273 K. Impedance spectroscopy studies reveal that this type of relaxorlike behavior is an artifact induced mainly by a nonohmic sample-electrode contact impedance. In addition, an instrument-related parasitic series inductance and resistance effect modifies the measured capacitance values as the sample resistance decreases with increasing temperature. This can lead to an underestimation of the sample capacitance and, in extreme cases, to so-called ‘negative capacitance.’ Such a relaxorlike artifact and negative capacitance behavior are not unique to CCTO and may be expected in o...