Search

A new computational analysis of tilt behaviour in perovskites is presented. This includes the development of a computational program – PALAMEDES – to extract tilt angles and the tilt phase from molecular dynamics simulations. The results are used to generate simulated selected-area electron and neutron diffraction patterns which are compared with experimental patterns for CaTiO3. The simulations not only reproduced all symmetrically allowed superlattice reflections associated with tilt but also showed local correlations that give rise to symmetrically forbidden reflections and the kinematic origin of diffuse scattering.

Bismuth-deficient sodium bismuth titanate (nominal Na0.5Bi0.49TiO2.985, NB0.49T) presents high oxide ion conductivity, which makes it a potential electrolyte material for intermediate-temperature solid oxide fuel cells. Acceptor doping has been proven an effective approach to enhance the bulk conductivity (σb) of NB0.49T. Here, divalent Ca2+ ions were selected to partially replace Bi3+ on the A-site of NB0.49T, and the temperature and composition dependences of σb and permittivity were investigated. Results showed that Ca2+ doping was effective for enhancing σb of NB0.49T by creating oxygen vacancies. The highest σb (0.006 S·cm−1 at 500 °C) was achieved by 2% Ca2+ doping. Further increase in the doping level decreased σb, which was more pronounced at temperatures below ~350 °C. Most importantly, Ca doping increased the temperature at which the activation energy for bulk conduction changed from ~0.80 eV (at low temperatures) to ~0.40 eV (at high temperatures), and reduced the temperature dependence of permittivity of NB0.49T. Results from the average structural parameters and the local defect associates are discussed. The findings of this work are helpful for understanding the defect and conduction mechanisms for acceptor-doped NB0.49T, and are also useful for developing NBT-based dielectrics with temperature-independent permittivity.

The effect of calcining in either air (VSTO-A) or 5% H2/N2 (VSTO-H) on the thermoelectric performance of La and Sm co-doped A-site-deficient Sr1-3x/2Lax/2Smx/2TiO3-δ ceramics is reported. All calcined powders were sintered 6 h in 5% H2/N2 at 1773 K to ≥96% relative density. All peaks in X-ray diffraction patterns indexed as a cubic perovskite phase. Scanning electron microscopy revealed grain sizes ~14 and ~10 μm for VSTO-A and VSTO-H ceramics, respectively. x = 0.30 showed the lowest k (2.99 W/m.K at 973 K) for VSTO-A, whereas x = 0.20 had the lowest (2.67 W/m.K at 973 K) for the VSTO-H ceramics. x = 0.30 VSTO-A showed a thermoelectric figure of merit, ZT = 0.25 (at 973 K), whereas the maximum ZT = 0.30 (at 973 K) was achieved for x = 0.20 VSTO-H ceramics, demonstrating that thermoelectric properties are optimized when all processing is carried out in 5% H2/N2.

The activation energy associated with bulk electrical conduction in functional materials is an important quantity which is often determined by impedance spectroscopy using an Arrhenius-type equation. This is achieved by linear fitting of bulk conductivity obtained from complex (Z*) impedance plots versus T−1 which gives an activation energy Ea(σ) or by linear fitting of the characteristic frequency fmax obtained from the large Debye peak in M′′–log f spectroscopic plots against T−1 which gives an activation energy Ea(fmax). We report an analysis of Ea(σ) and Ea(fmax) values for some typical non-ferroelectric and ferroelectric materials and employ numerical simulations to investigate combinations of different conductivity–temperature and permittivity–temperature profiles on the log fmax–T−1 relationship and Ea(fmax). Results show the log fmax–T−1 relationship and Ea(fmax) are strongly dependent on the permittivity–temperature profile and the temperature range measured relative to Tm (temperature of the permittivity maximum). Ferroelectric materials with a sharp permittivity peak can result in non-linear log fmax–T−1 plots in the vicinity of Tm. In cases where data are obtained either well above or below Tm, linear log fmax–T−1 plots can be obtained but overestimate or underestimate the activation energy for conduction, respectively. It is therefore not recommended to use Ea(fmax) to obtain the activation energy for bulk conduction in ferroelectric materials, instead Ea(σ) should be used.

© 2019 The Royal Society of Chemistry. High electrostrain and breakdown strength (1 - x)BiFeO3-0.3BaTiO3-xNd(Li0.5Nb0.5)O3 (BF-BT-xNLN) ceramics were studied by in situ synchrotron X-ray diffraction (XRD) in combination with Rietveld refinement and conventional transmission electron microscopy. At zero field, compositions transformed from majority ferroelectric rhombohedral to pseudocubic as the NLN concentration increased, with 0.27% strain achieved at 60 kV cm-1 for x = 0.01. The large measured macroscopic strain was commensurate with peak shifting in XRD peak profiles, yielding 0.6% total strain at 150 kV cm-1. Strain anisotropy of ϵ200 > ϵ220 > ϵ111 was observed but despite the large applied field, no peak splitting was detected. We therefore concluded that the large electrostrain is not achieved through a conventional relaxor to field induced long-range ferroelectric transition. Instead, the data supports a model where local polar regions distort in the direction of the applied field within multiple local symmetries (pseudosymmetry) without long range correlation. We proposed that pseudosymmetry is maintained in BF-BT-xNLN even at high field (150 kV cm-1) due to the large ion radii mismatch and competing ionic/covalent bonding between Ba2+ and Bi3+ ions.

Barium titanate is the dielectric material of choice in most multilayer ceramic capacitors (MLCCs) and thus in the production of ≈3 trillion devices every year, with an estimated global market of ≈$8330 million per year. Rare earth dopants are regularly used to reduce leakage currents and improve the MLCC lifetime. Simulations are used to investigate the ability of yttrium, dysprosium, and gadolinium to reduce leakage currents by trapping mobile oxygen defects. All the rare earths investigated trap oxygen vacancies, however, dopant pairs are more effective traps than isolated dopants. The number of trapping sites increases with the ion size of the dopant, suggesting that gadolinium should be more effective than dysprosium, which contradicts experimental data. Additional simulations on diffusion of rare earths through the lattice during sintering show that dysprosium diffuses significantly faster than the other rare earths considered. As a consequence, its greater ability to reduce oxygen migration is a combination of thermodynamics (a strong ability to trap oxygen vacancies) and kinetics (sufficient distribution of the rare earth in the lattice to intercept the migrating defects).

Combining the efforts of physicists, materials scientists, economists and resource-strategy researchers opens up an interdisciplinary route enabling the substitution of rare elements by more abundant ones, serving as a guideline for the development of novel materials.

In this work, American Iron and Steel Institute (AISI) O1 steel was pack borided in the temperature range of 1123–1273 K for treatment times between 2 and 8 hours. A kinetic model was proposed for estimating the boron diffusion coefficients through the Fe2B layers. As a result, the boron activation energy for the AISI O1 steel was estimated as 197·2 kJ mol− 1. This value of energy was compared to the literature data. In addtion, to extend the validity of the present model, two additional boriding conditions were done. The Fe2B layers grown on AISI O1 steel were characterised by use of the following experimental techniques: scanning electron microscopy, X-ray diffraction analysis and Daimler-Benz Rockwell-C indentation technique. Finally, the scratch and pin on disc tests for wear resistance were respectively performed using an LG Motion Ltd and a CSM tribometer under dry sliding conditions.

The impedance response of a core–shell microstructure with 80% core volume fraction has been simulated using finite‐element modeling and compared to two equivalent circuits for a wide range of shell permittivity and conductivity values. Different equivalent circuits, corresponding to different variants of the well‐known brick layer model, are applicable for different combinations of material properties in the microstructure. When the shell has a similar conductivity or permittivity to the core, adding a parallel pathway increases the accuracy of the fit by ≈±10%. When both the conductivity and permittivity values of the core and shell regions are different the series circuit is a better fit. This is confirmed by multiformalism impedance analysis, which reveals features in the data that are not apparent using a single formalism. Finally, the conductivity and permittivity values for both the shell and core are extracted from the simulated spectra using all formalisms and compared to the original input values. The accuracy of the extracted values often depends on the impedance formalism used. It is concluded that impedance spectroscopy data must be analyzed using multiple formalisms when considering core–shell microstructures.

Oxide ion conductors find important technical applications in electrochemical devices such as solid-oxide fuel cells (SOFCs), oxygen separation membranes and sensors1, 2, 3, 4, 5, 6, 7, 8, 9. Na0.5Bi0.5TiO3 (NBT) is a well-known lead-free piezoelectric material; however, it is often reported to possess high leakage conductivity that is problematic for its piezo- and ferroelectric applications10, 11, 12, 13, 14, 15. Here we report this high leakage to be oxide ion conduction due to Bi-deficiency and oxygen vacancies induced during materials processing. Mg-doping on the Ti-site increases the ionic conductivity to ~0.01 S cm−1 at 600 °C, improves the electrolyte stability in reducing atmospheres and lowers the sintering temperature. This study not only demonstrates how to adjust the nominal NBT composition for dielectric-based applications, but also, more importantly, gives NBT-based materials an unexpected role as a completely new family of oxide ion conductors with potential applications in intermediate-temperature SOFCs and opens up a new direction to design oxide ion conductors in perovskite oxides.

Interatomic potentials recently developed for the modelling of BaTiO3 have been used to explore the stabilisation of the hexagonal polymorph of BaTiO3 by doping with transition metals (namely Mn, Co, Fe and Ni) at the Ti-site. Classical simulations have been completed on both the cubic and hexagonal polymorphs to investigate the energetic consequences of transition metal doping on each polymorph. Ti-site charge compensation mechanisms have been used for the multi-valent transition metal ions and cluster binding energies have been considered. Simulations show a significant energetic gain when doping occurs at Ti sites in the face sharing dimers (Ti2 sites) of the hexagonal polymorph compared with the doping of the cubic polymorph. This energetic difference between the two polymorphs is true for all transition metals tested and all charge states and in the case of tri- and tetra-valent dopants negative solution energies are found for the hexagonal polymorph suggesting actual polymorph stabilisation occurs with the incorporation of these ions as observed experimentally. Oxidation during incorporation of Ni2+ and Fe3+ ions has also been considered.

The energetics of La-doping in BaTiO3 are reported for both (electronic) donor-doping with the creation of Ti3+ cations and ionic doping with the creation of Ti vacancies. The experiments (for samples prepared in air) and simulations demonstrate that ionic doping is the preferred mechanism for all concentrations of La-doping. The apparent disagreement with electrical conduction of these ionic doped samples is explained by subsequent oxygen-loss, which leads to the creation of Ti3+ cations. Simulations show that oxygen-loss is much more favorable in the ionic-doped system than undoped BaTiO3 due to the unique local structure created around the defect site. These findings resolve the so-called “donor-doping” anomaly in BaTiO3 and explain the source of semiconductivity in positive temperature coefficient of resistance (PTCR) BaTiO3 thermistors. Postprint

The effect of sulphur dioxide (SO2) present in the cathode feed stream on the performance of an 8 mol% yttria-stabilized zirconia/Platinum (8YSZ/Pt) cell was studied. SO2 concentration of 30% in oxygen (5 ml/min) and helium (30 ml/min) was exposed to the cell applying an external DC voltage of 1.5 V at 850 °C. Experiments showed that the cell performance, inferred from current and oxygen permeate flux changes, had two types of degradation. The first was rapid, giving a sharp fall (type I degradation) and the second much more gradual (type II degradation). The presence of Platinum sulphide (PtS) was observed after the SO2 exposure period using X-ray diffraction (XRD), scanning electron microscope with energy dispersive X-ray spectroscopy (SEM-EDX) and Raman spectroscopy. The cell performance was partially recovered after a 2 min DC voltage pulse treatment. The results of XRD, EDX and Raman analysis indicated that the recovery may be attributed to the removal of the sulphide layer from the cathode surface after the voltage pulse treatment. The greatest recovery of cell performance was obtained after a 2 min pulse of 4.5 V. Preliminary investigations on the effect of a much shorter, 1 s voltage pulse on performance recovery were conducted and recovery was found to increase up to the highest voltage tested, 7 V.

A combination of fixed frequency dielectric measurements (103 to 106 Hz) and impedance spectroscopy reveal evidence of (nonferroelectric) relaxor-like behavior in the temperature range ∼600–750 K for the stoichiometric sillenite phase Bi12SiO20 (space group I23, a = 10.1034(5) A). Variable temperature Raman Spectroscopy reveals significant changes associated with the Bi–O framework vibrations, and the relaxor-type behavior is attributed to noncorrelated local dipole moments associated with the BiO5E (E = 6s2 lone pair) units in the framework. Sillenites, therefore, represent another family of Bi-based oxides to exhibit relaxor-like dielectric behavior.

The defect structures for Gd- and La-doping on the A-site of BaTiO3 with the creation of Ti vacancies are reported. The rare-earth cations cluster together around the vacancy. The local relaxations caused by the defect cluster lead to distortion and tilting of nearby TiO6 octahedra and this is responsible for the lowering of the Curie temperature with increasing dopant concentration. Larger distortions to the octahedra are observed for La-doping and this is the proposed origin of the greater decrease in Tc as observed by experimental results associated with La- compared to Gd-doped samples.

In 2009, Karimi et al. reported that Bi1-xNdxFeO3 0.15 ≤ x ≤ 0.25 exhibited a PbZrO3 (PZ)-like structure. These authors presented some preliminary electrical data for the PZ-like composition but noted that the conductivity was too high to obtain radio-frequency measurements representative of the intrinsic properties. In this study, Bi0.85Nd0.15Fe1-yTiyO3 (0 ≤ y ≤ 0.1) were investigated, in which Ti acted as a donor dopant on the B-site. In contrast to the original study of Karimi et al., X-ray diffraction (XRD) of Bi0.85Nd0.15FeO3 revealed peaks which were attributed to a mixture of PZ-like and rhombohedral structures. However, as the Ti (0 0.3 to 1 eV. Plots of permittivity (ϵr) versus temperature for 0.01 ≤ y ≤ 0.05 revealed a step rather than a peak in ϵr on heating at the same temperature determined for the antiferroelectric–paraelectric phase transition by differential scanning calorimetry. Finally, large electric fields were applied to all doped samples which resulted in a linear dependence of polarisation on the electric field similar to that obtained for PbZrO3 ceramics under equivalent experimental conditions.

This work investigates the feasibility of applying high temperature oxygen separation to the sulphuric acid decomposition process, a common step in the Sulphur Iodine and Hybrid Sulphur thermochemical cycles for massive scale hydrogen production. Process simulations indicate that the decomposition yield can be increased from 62.3% to 90.1% by using a catalytic membrane reactor. Equilibrium calculations show yttria-stabilised zirconia (YSZ), combined with platinum electrodes, to be a candidate membrane for the separation process. Bespoke experimental apparatus was designed and commissioned in order to investigate the oxygen flux through YSZ membranes in the presence of sulphur dioxide, during external voltage application. Results from an experiment at 900 °C show that oxygen permeation through YSZ membranes occurs under these conditions. X-ray diffraction and impedance spectroscopy are used to investigate the reason for the oxygen flux decrease with time.

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 microwave (MW) dielectric loss (or quality factor, Q) of BaCo1/3Nb2/3O3 (BCN) ceramics exhibits a strong dependence on sample-processing conditions and is not always consistent with the degree of 1:2 B-site cation ordering. Impedance spectroscopy results reveal that these BCN ceramics exhibit remarkably different electrical properties due to low levels of nonstoichiometry induced during ceramic processing. These compositional changes are difficult to detect by common chemical techniques but can have a significant influence on the electrical properties, and in particular on the extrinsic electrical conduction and Q. The influence of extrinsic conduction induced by ceramic processing should therefore be considered when optimizing Q in MW dielectric ceramics.

BaCo1/3Nb2/3O3 ceramics, with a high density and a similar, high degree of 1:2 B-site cation ordering, exhibit very different quality factors, Q. The ceramics exhibit p-type behavior with higher conductivity and lower Q for samples processed in O2 as compared with those processed in air. It is proposed that unavoidable Co loss during high-temperature ceramic processing leads to p-type doping that must be compensated by oxygen vacancies to impede hole formation. The composition exhibiting only intrinsic conduction and optimized Q is not achieved with processing in atmospheric oxygen due to filling of oxygen vacancies and hole formation during cooling.

Compositons in the pseudoquaternary system, 1-×(0.35Bi(Mg1/2Ti1/2)O3-0.30BiFeO3-0.35BiScO3)—×PbTiO3 were fabricated and characterised at the morphotropic phase boundary (MPB) between ferroelectric rhombohedral and tetragonal phases. The MPB occurred at × ≈ 0.48 at which composition the ferreoelectric to paraelectric phase transition, TC = 450°C, the piezoelectric constant, d33 = 328 pC/N and electromechanical coupling factor, kp = 0.44. The piezoelectric properties are viable for actuator applications but lower than equivalent high TC piezoelectrics such as 0.36BiScO3–0.64PbTiO3 (d33 = 450 pC/N, TC = 450°C). However, the relative reduction in the Sc2O3 content gives a significant cost saving which may prove a commercial advantage.

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...

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.

Bulk crystallized PbTiO3 (PT) was the major crystalline phase in borate-based glass ceramics with starting composition 39PbO–1BaO–25TiO2–9.8Al2O3–1SiO2–24.2B2O3 (mol%). Crystallization of PT starts at ≥520 °C and the crystal size and c/a ratio of PT were approximately the same for all samples heat treated between 520 and 700 °C. Two transient unidentified phases were induced when crystallized between 520 and 650 °C. The first at 520 °C followed by a second at 550 °C. At >650 °C, PbTi3O7 was present in addition to PbTiO3. The electrical properties of glasses heat treated between 550–700 °C were investigated by fixed frequency capacitance measurements and impedance spectroscopy, IS. No permittivity maximum or Curie temperature, T c, for the PT phase was observed from fixed frequency capacitance measurements for glasses heat treated below 700 °C, however IS data analysis using electric modulus (M″) spectroscopy revealed the PT phase to have T c ∼480 and 440 °C for samples heat treated at ∼550 and 700 °C, respectively. The conductivity of the PT phase and the dc conductivity of the glass heat treated at 700 °C were significantly higher than those of glasses heat treated at lower temperatures suggesting there is significant compositional differences between the glass heat treated at 700 °C compared to those treated at ≤650 °C.

Scanning probe microscopy with conductive tips has been used to image the dielectric properties of ceramics with giant permittivity. In particular, measurements in impedance mode and of local resistivity allowed to image the permittivity map on polycrystalline materials. Such imaging allows to correlate the dielectric properties with the local sample structure and with defects inside the single grains of the polycrystalline ceramics. However, artifacts due to surface imperfections should be distinguished from bulk properties and eliminated.

Borate- and silicate-based glasses that devitrify on heating to form ferroelectric lead titanate have been developed. Their potential for use as sintering aids in PbO-based electroceramics has been evaluated by studying the densification of PbTiO3 (PT) with increasing wt.% addition of glass powder. When ≥3 wt.% of a borate-based glass was added, ≥94% theoretical density was achieved at 1,100 °C but for a silicate-based glass system 10 wt.% glass addition was required to achieve the same density. As the wt.% glass content increased the maximum and room temperature permittivity decreased and there was also a decrease in T c and the c/a ratio. The decrease in T c is believed to be due to increased clamping from neighbouring grains as the density of the PT increases. The decrease in maximum and room temperature permittivity is related to the presence of secondary phases.

(1 − x)BaTiO3–xBiGdO3 ceramics were prepared by the solid state reaction method. X-ray diffraction and Raman spectroscopy indicate a maximum co-solubility of Bi/Gd in BaTiO3 at x = 0.10 with a change of symmetry from tetragonal to pseudo-cubic at x = 0.08. Backscattered electron images, however, reveal the presence of a secondary phase in x ≥ 0.06. The dielectric behaviour evolves continuously with x from a classical ferroelectric to a typical relaxor and this transition is accompanied by a shift in the permittivity maxima towards lower temperatures. The presence of two dielectric anomalies for x ≥ 0.06 is associated with residual core–shell structures, as revealed by transmission electron microscopy. The dielectric anomaly associated with the core regions remains at ∼120 °C, whereas the other anomaly decreases continuously towards lower temperature with x. This study shows that chemical equilibrium is much more difficult to achieve than in other (1 − x)BaTiO3–xBi[Me]O3 systems, where Me is Yb or Sc.

BaTi1-2yGayNbyO3 (BTGN) (0 0.08 there is no evidence of core-shell structures, however, some grains have a high density of dislocations, consistent with chemical inhomogeneity. BTGN ceramics exhibit a diverse range of dielectric behavior in the temperature range 120-450 K and can be subdivided into two groups. 0.025 300 at 10 kHz), whereas 0.25

La germanate apatite-based materials, La 9.33 + x (GeO 4 ) 6 O 2 + 1.5x , are good oxide ion conductors but there are uncertainties over the solid solution limits, the defect crystal structure and conductivity variations with composition and temperature. Volatilisation of GeO 2 during synthesis is increasingly important for temperatures above ∼1300 °C. The solid solution range at 1100 °C is ∼0.17 ≤ x ≤ ∼0.50 at which temperature GeO 2 loss appears not to be significant. With increasing x , the symmetry changes from hexagonal to triclinic; for high x compositions, high temperature X-ray diffraction shows a gradual triclinic to hexagonal transition over the range 600–800 °C. Conductivity data of the high x solid solutions also show evidence of the phase transition region, with lower activation energy and higher conductivity in the high temperature hexagonal structure. The extent of, and structural changes within, the apatite domain in the LaO 1.5 –GeO 2 –SrO ternary system at 1100 °C have been studied and single-phase samples are obtained for La 9.33 + x − 2y / 3 Sr y (GeO 4 ) 6 O 2 + 1.5x y = 1.0 with x = 0.17 and 0.34. The hexagonal to triclinic transition is clearly associated with increasing oxygen content rather than filling the La sites by addition/substitution of Sr into the structure.

Glass composition 39PbO–1BaO–25TiO2–9.8Al2O3–24.2SiO2–1B2O3 (mol%) crystallized on heat treatment at ≥600 °C to form glass-ceramics whose majority phase was PbTiO3 (PT). At 600 °C, nano-size PT crystals (∼20–50 nm) with a c/a ratio of 1.000(1) were observed, but as heat-treatment temperature increased to 1000 °C, the crystal size and c/a ratio increased to ∼1.2 μm and 1.056(4), respectively. Permittivity measurements as a function of temperature revealed a broad peak at ∼400 °C associated with the nanocrystalline PT crystals, but it sharpened and increased in temperature as heat treatment temperature increased to 1000 °C. The causes of peak broadening and shift of Tc are believed to be due to either clamping of PT crystals by glass matrix, finite size effects due to their intrinsically small size, or the incorporation of dopant impurities such as Al, Si, or Ba ions in the PT phase.

Compounds in the Bi 2 O 3 –P 2 O 5 binary system with Bi/P ratios of 13:1 to 16:1 crystallize with the sillenite structure (space group I23 ) and form a solid solution with the cubic lattice parameter increasing from 10.1660(4) to 10.1765(1) A with increasing Bi/P ratio. Differential thermal analysis (DTA) shows these compounds to exhibit complex thermal behaviour on thermal cycling with the presence of at least two endotherms above 800 °C on heating and at least three exotherms on cooling (all >630 °C). Impedance spectroscopy measurements as a function of temperature and oxygen partial pressure on the 14:1 composition show it to be a mixed n-type electronic and oxide-ion conductor at temperatures below 750 °C. Substitution of P by V and As in Bi/P=14:1 results in the formation of partial (30 at.%) and complete solid solutions, respectively. An enhancement in conductivity is observed in these solid solutions with V-doped materials exhibiting the highest conductivity with σ ∼27 mS cm −1 at 800 °C for Bi 14 P 0.7 V 0.3 O δ . Substitution of P by elements such as Pb, Sr, Al, Ga, Fe, Si, Ge and Ti leads to the formation of limited solid solutions. Most of these materials have similar or slightly higher total conductivity compared to the parent compound.

Hexagonal BaTiO3 materials have been stabilised at room temperature according to the formula BaTi0.95M0.05 O3−δ where M = Mn, Fe, Co and Ni. Dense ceramics (> 96% of the theoretical X-ray density) were sintered at 1450∘C in flowing O2 gas from calcined powders prepared by the mixed oxide route at 1300∘C. All samples were single-phase and the bulk conductivity, σb, measured by Impedance Spectroscopy and Q.f measured by microwave dielectric resonance methods showed a strong dependence on the type of dopant. σb at 300∘C was 10−7, 10−5.5, 10−5.5 and 10−4 Scm−1 for M = Mn, Fe, Ni and Co, respectively and Q.f at ∼ 5 GHz was 7790, 6670, 2442 and 1291 GHz, for M = Mn, Fe, Ni and Co, respectively. The correlation between σb and Q.f is attributed to the presence of oxygen vacancies and/or mixed valency of the dopant ions.

PbBi6V2O15, PbBi6P2O15, PbBi6As2O15 and their solid solutions have been prepared by the mixed oxide route at 800–850 °C. Complete solid solutions were formed in the systems PbBi6V2O15–PbBi6P2O15, PbBi6V2O15–PbBi6As2O15 and PbBi6P2O15–PbBi6As2O15. In addition, substitution of Pb by Sr in PbBi6V2O15 resulted in a complete solid solution series. The composition and phase purity of PbBi6V2O15 and the hitherto unreported SrBi6V2O15 were confirmed by electron probe microanalysis (EPMA). Although their crystal structures remain unknown, all compositions appeared to be isostructural from X-ray diffraction (XRD) results and the data could be indexed on an orthorhombic cell derived from the cubic δ-Bi2O3 structure, with a∼23.9, b∼17.1 and c∼11.1 A. Impedance spectroscopy and transference number measurements show these materials to be moderate oxide ion conductors with bulk conductivities of 5×10−5–10–7 S cm−1 at 500 °C with transference numbers ∼0.8 at 600–700 °C. Conductivity decreased in the order SrBi6V2O15≫PbBi6V2O15>PbBi6As2O15>PbBi6P2O15 indicating that lone pair electron effects associated with Pb2+ and cation size effects associated with V, As and P play an important role in controlling the anion transport in these materials.

Modelling of impedance spectroscopy (IS) data of electroceramics depends critically on the correct choice of equivalent electrical circuit so that the extracted parameters have physical significance. The strategy proposed to choose the correct circuit involves analysis of IS data in several of the four complex formalisms: impedance, electric modulus, admittance and permittivity together with consideration of the frequency and temperature dependence of data and the magnitude and temperature dependence of extracted resistance and capacitance values. This is demonstrated using IS data from oxide-ion conducting La0.80Sr0.20Ga0.83Mg0.17O2.82 ceramics over the range 182 to 280°C. Low temperature data are fitted first, to allow a full characterisation of the bulk response; some of the bulk parameters may then be fixed to enable fitting of the higher temperature data which increasingly feature lower frequency phenomena such as grain boundary impedances. The most appropriate circuit in this case is found to consist of a parallel combination of a resistor, capacitor and constant phase element (CPE) for the bulk response in series with a resistor and capacitor in parallel for a constriction resistance. The origin of the constriction resistance may be associated with the presence of plate-like LaSrGaO4 secondary phase within the grains and/or with the presence of pores at the grain boundaries. The importance of choosing (a) the correct equivalent circuit and (b) approximately correct input values for the various circuit parameters to be fitted and refined are demonstrated.

The mechanism of doping BaTiO3 with La has been investigated by a combination of X-ray diffraction, electron probe microanalysis, scanning and transmission electron microscopy and impedance measurements. Phase diagram results confirm that the principal doping mechanism involves ionic compensation through the creation of titanium vacancies. All samples heated in oxygen at 1350–1400°C are electrical insulators, consistent with an ionic compensation mechanism. Samples heated in air or atmospheres of low oxygen partial pressure, at similar temperatures, lose a small amount of oxygen and this gives rise to a second, electronic compensation mechanism in addition to the main, ionic compensation mechanism; as a result, samples are dark-coloured and semiconducting. The change from insulating to semiconducting behaviour is reversible, by changing the atmosphere on heating at 1350–1400°C. We find no evidence for any changes in cationic composition of the BaTiO3 solid solutions arising from changes in oxygen content.

Impedance spectra of La0.8Sr0.2Ga0.83Mg0.17O2.815 ceramics were measured using gold and platinum paste, and sputtered gold electrodes. Impedance complex plane plots, Z*, for samples with sputtered gold electrodes consist of a single arc associated predominantly with the bulk response and a low frequency 'spike' associated with electrode-polarisation and diffusion-related phenomena. An additional arc at intermediate frequencies with an associated capacitance of the order of 1 nFcm−1 and a resistance with a similar temperature dependence to that of the bulk resistance is present in Z* plots for samples measured using gold paste electrodes hardened at temperatures between 500 and 850°C. Scanning electron microscopy and impedance spectroscopy show the additional response to be a 'spreading resistance' caused by the electrodes making well-separated discrete contacts with the ceramic due to partial sintering of gold particles in the paste. Z* plots for samples measured using platinum paste electrodes hardened between 725 and 1000°C exhibit a much smaller additional response with similar behaviour to that observed with gold paste electrodes but also showed an apparent increase in sample resistance due to significant loss of contact between sample and electrode in some regions of the interface.

Partial substitution of Ca with Sr in high permittivity Ca1−xSrxCu3Ti4O12 ceramics can lead to an anomalous increase in intrinsic bulk dielectric permittivity. The similar electron configuration of Ca and Sr excludes electronic structure variations as a source of this permittivity increase, and thus an alternative mechanism must exist.

Densification of Na4Zr2Si3O12 (NZS) solid electrolytes was performed by dispersing TiO2 (0.8–5.9 wt. %, corresponding to 5–30 mol %) in NZS powders prior to sintering at 1200°C. Increases in pellet density, from ca. 65 to 94% of the theoretical (X-ray density) value, and in electrical conductivity from 10−7 to 10−6 S/cm at 50°C were observed for small additions of TiO2, which acts as a sintering aid. AC impedance spectroscopy reveals that the enhancement is not a bulk effect but instead is associated with a reduction in inter-granular constriction resistances within porous NZS ceramics. The presence of adsorbed water species in NZS powders prepared via a sol-gel route is found to have a dramatic effect on the conductivity enhancement.

The syntheses of La 0.95 Sr 0.05 GaO 3− δ (LSG), La 0.95 Sr 0.05 AlO 3− δ (LSA) and Y 0.95 Sr 0.05 AlO 3− δ (YSA), and their electrical properties using constant frequency (10 kHz) a.c. conductivity measurements as a function of oxygen partial pressure ( P O 2 ranging from air to 10 −15 Pa) and temperature (800–1000°C) are described. Ion-blocking measurements were also performed on some compositions. All materials showed dominant ionic conductivity within limited ranges of working conditions, although their ionic domains differed considerably in magnitude. LSG samples had the highest ionic conductivity, with only a minor p-type contribution in air. LSA had intermediate conductivity, with p-type conductivity dominant in air. YSA samples had the lowest conductivity of all those measured, again with dominant p-type conductivity in air. All materials were stable within large P O 2 domains, and exhibited negligible n-type conductivities even under reducing conditions typical of fuel-rich atmospheres.

A review is given of some of the problems encountered in the analysis and interpretation of impedance data. The importance of choosing the correct equivalent circuit to represent the data is emphasized and it is shown how ferroelectric materials, with their characteristic temperature-dependent capacitance, are particularly suited to discriminating between plausible equivalent circuits. Results are discussed for two materials, LiTaO3 single crystal and BaTiO3 ceramics.

A high pressure-high temperature study of Bi{sub 2}Sr{sub 2}MnO{sub 6+{delta}} samples were subjected to a high pressure-high temperature process and the effects on crystal structure and magnetic properties recorded. The unit cell was found to expand anisotropically with the c axis length increasing at four times the rate of the a and b axes where (da/dP) = (db/dP) = 1.0 {+-} 0.2 {times} 10{sup {minus}3} {angstrom}MPa{sup {minus}1} and (dc/dP) = 3.5 {+-} 0.5 {times} 10{sup {minus}3} {angstrom}MPa{sup {minus}1} with an overall unit cell volume expansion of 1%. The net oxidation state of the manganese decreased from +3 towards +2 with a resultant increase in T{sub N}. These changes can be attributed to the effect of removing interstitial oxygen from the structure which primarily changes the oxidation state of the manganese.

Low-field magnetization M(H) measurements are reported for single crystals of various high-${\mathit{T}}_{\mathit{c}}$ cuprates with different anisotropies (\ensuremath{\gamma}=${\ensuremath{\lambda}}_{\mathit{c}}$/${\ensuremath{\lambda}}_{\mathit{ab}}$, where ${\ensuremath{\lambda}}_{\mathit{c}}$ and ${\ensuremath{\lambda}}_{\mathit{ab}}$ are the c-axis and ab-plane penetration depths). For ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ (\ensuremath{\gamma}\ensuremath{\approxeq}5) and ${\mathrm{La}}_{1.89}$${\mathrm{Sr}}_{0.11}$${\mathrm{CuO}}_{4}$ (\ensuremath{\gamma}\ensuremath{\approxeq}12), qualitatively similar M(H) behavior is observed when the magnetic field is applied both perpendicular (H\ensuremath{\parallel}c) and parallel (H\ensuremath{\parallel}ab) to the ${\mathrm{CuO}}_{2}$ planes, consistent with a description of these compounds as anisotropic three-dimensional superconductors. ${\mathrm{Tl}}_{2}$${\mathrm{Ba}}_{2}$${\mathrm{CuO}}_{6+\mathrm{\ensuremath{\delta}}}$ (\ensuremath{\gamma}\ensuremath{\approxeq}25) and ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$O (\ensuremath{\gamma}\ensuremath{\approxeq}250), on the other hand, show significant differences in M(H) with H\ensuremath{\parallel}ab which can be attributed to the presence of Josephson screening currents across the layers. These observations imply that the nature of the interlayer coupling is determined primarily by the ratio of the out-of-plane coherence length ${\ensuremath{\xi}}_{\mathit{c}}$ to the interlayer spacing s. \textcopyright{} 1996 The American Physical Society.

We report a study of the normal-state Hall effect and magnetoresistance of single crystals of Tl{sub 2}Ba{sub 2}CuO{sub 6+{delta}}. Using samples with {ital T}{sub {ital c}}{lt}15 K, we can suppress the superconductivity down to low temperatures with magnetic fields of 16 T, and can thus study the normal state properties over three decades of temperature, extending into the {ital T}{r_arrow}0 limit where it is possible to make a reliable estimate of {ital k}{sub {ital F}}{approximately}0.7 A{sup {minus}1} from the Hall effect in the elastic-scattering regime. The temperature dependence of the Hall coefficient, {ital R}{sub {ital H}}, below 30 K rules out models in which {ital R}{sub {ital H}}({ital T}) is taken as a measure of a real temperature-dependent change in the carrier concentration. The two scattering rates (probed by the resistivity and the cotangent of the Hall angle) which characterize normal-state transport in the cuprates also appear in this overdoped material for {ital T}{ge}30 K. However, as {ital T}{r_arrow}0, we observe only a {ital single} scattering rate, whose temperature dependence is dominated by low power terms, in contrast to the {ital T}{sup 2} dependence predicted for a Fermi liquid. The relationship between these findings and anomalous behavior previously reported formore » the upper critical field is discussed. {copyright} {ital 1996 The American Physical Society.}« less

Conductivity data on single crystal and cold-pressed polycrystalline samples of Cu 2 P 3 I 2 demonstrate that Cu + ion migration dominates the electrical conductivity over the temperature range 25–300 °C in argon. Both sets of samples exhibit fully reversible Arrhenius-type conductivity behavior with activation energy values of ~0.45eV and typical conductivity values of ca. 1.90 × 10 −3 S.cm −1 at 186 °C. The conductivity appears to be one-dimensional.

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.