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The influence of A-site Gd3+ doping on the c/a ratio, cell volume and Curie temperature (Tc) of ferroelectric BaTiO3 according to the titanium vacancy mechanism, viz., Ba1−xGdxTi1−x/4O3 has been investigated and compared to that of La3+-doping. The c/a ratio and cell volume of Gd-doped samples are larger than the equivalent La-doped samples and Tc decreases at a rate of only ∼8 °C/at. % for Gd3+ substitution compared to ∼24 °C/at. % for La3+ substitution. These trends are opposite to that expected based on ionic-radii and tolerance factor arguments. This anomalous behavior is attributed to an ion-size mismatch (cation variance) effect on the A-site between the Ba2+ and Gd3+ ions that induces local strain in the lattice that suppresses the reduction in the c/a ratio and cell volume, and as a consequence in the rate of decrease in Tc.

We present a new set of interatomic potentials for modelling the BaTiO3 perovskite system. The potential model is fitted using multiple parameters to a range of experimental and ab initio data including the cohesive energy and lattice parameters of BaTiO3, BaO and rutile TiO2. This procedure provides internal consistency to the potential model for studying the energetics of the defect chemistry of BaTiO3. This is tested by examining rare-earth cation doping in BaTiO3 and considering all five possible compensation schemes. Our simulations are in agreement with experiment and predict small rare-earth cations to dope exclusively on the Ti site; medium sized rare-earth cations to dope on both the Ti and Ba sites and large rare-earth cation doping exclusively on the Ba-site. For Ba-site substitution the simulations predict electron compensation to be energetically unfavourable compared to the formation of Ti vacancies.

Ba1−xDyxTi1−xScxO3 ceramics, 0 ≤ x ≤ 0.075, were prepared and the influence of Dy and Sc co-doping on the ferroelectric, electrical and structural properties of BaTiO3 (BT) investigated. A solid solution limit of x ≈ 0.05 was established using X-ray diffraction, and all compositions x ≤ 0.05 were tetragonal at room temperature. Fixed-frequency capacitance measurements showed the temperature of the cubic to tetragonal phase transition, TC, decreased non-linearly with increasing x and the associated permittivity peak broadened, with frequency-dependent ‘relaxor’-type behaviour observed for cubic x = 0.075. In addition, the magnitude of the relative permittivity at TC showed a negative dependence on x, decreasing from ∼11700 for x = 0 to ∼6950 for x = 0.05. In contrast to the non-linear decrease in TC, the tetragonal to orthorhombic and orthorhombic to rhombohedral phase transition temperatures decreased linearly at rates of ∼40 and ∼28 °C mol%−1 of DyScO3, respectively. The effect on the ferroelectric properties of BT was to quadruple the coercive field, from ∼0.9 kV cm−1 for x = 0 to ∼4.4 kV cm−1 for x = 0.025, without affecting the remnant polarisation, ∼10 to 11 µC cm−2 for x = 0 to 0.025. Finally, the non-linear decrease in TC is discussed in terms of the contributions of the mean size and statistical variance of the cations on the A and B sites of the perovskite, ABO3, lattice and the replacement of Ti4+ with Sc3+ cations.

Al can be doped into the oxide ion conductor La9.33(SiO4)6O2 according to the formula La9.33 + x/3(SiO4)6 − x(AlO4)xO2 where 0 ≤ x ≤ 2; a conductivity enhancement of 1–2 orders of magnitude is observed for intermediate x values and illustrates the important role that La vacancies have in optimising the conductivity.

Scanning probe microscopy (SPM) with conductive tips has been used to image the dielectric properties of ceramics with giant permittivity. In particular, measurements in impedance mode of local resistivity allowed to image the permittivity map on polycrystalline materials. Such imaging provides correlation between the dielectric properties and the sample structure, in particular focusing on defects inside the single grains. Great attention has been devoted to the possible artefacts due to surface imperfections, such as huge roughness and/or contamination. A reliable surface investigation has been obtained after the definition of both the physical and geometrical criteria to avoid the artefacts due to both the surface or anomalous tip-sample contact area variation (for instance, in grain boundaries, holes and cracks in the ceramic pills). In particular, the power spectral density (PSD) allows to get access to the different periodic components of the surface roughness. The PSD demonstrated to be a sensitive tool to check the surface conditions after the polishing procedures aimed to the progressive decreasing of surface roughness, in order to reach the SPM limits and to avoid artefacts inducing wrong data interpretation.

The Ba(Ti1−xZrx)O3 (BTZ) system with x≤0.35 has been studied using a combination of x-ray diffraction, dielectric measurements, Raman spectroscopy, and electron diffraction with a view to better understand the changes in structure and correlation length of polar order as a function of composition and temperature. Careful fitting of Raman spectra has confirmed stabilization of the orthorhombic phase at room temperature in x=0.05 with mixed phase (orthorhombic and rhombohedral) present at 80 K. Raman data suggest that the rhombohedral phase is most likely to be present at low temperature for x=0.15 and 0.25 and that only short range polar order evolves in x=0.35. Interpretation of the Raman spectra is in broad agreement with the dielectric data as a function of composition and temperature. Diffuse scatter at room temperature in high order zone axis electron diffraction remains essentially identical for all compositions irrespective of structure and correlation length of polar order. The diffuse maxima are t...

CaCu3Ti4O12 ceramics with a range of resistivities have been prepared using both conventional sintering and spark plasma sintering. For all cases, the high effective permittivity is associated primarily with an internal barrier layer capacitor mechanism. Additional polarization associated with the electrode-sample interface may appear but its visibility depends on the grain boundary resistivity (Rgb) of the ceramic. If the Rgb is large, the electrode polarization is obscured by sample-related effects; if the Rgb is small, a separate impedance associated with the electrode polarization may be seen. Discrepancies in the literature regarding the magnitude and origin of the high effective permittivity are attributed to a combination of differences in processing conditions, electrode contact material and measuring frequency range.

Ceramics around the morphotropic phase boundary (MPB) in the (1−x)BiSc1−yFeyO3–xPbTiO3 solid solution were fabricated. For y=0.5, ceramics were single phase, and piezoelectric coefficients (d33) and electromechanical coupling coefficients (kp) for MPB compositions were 300 pC/N and 0.49, respectively; a level of piezoelectric activity similar to that of hard, lead zirconate titanate compositions but with TC∼60 °C higher at ∼440 °C. For ceramics with y≥0.7, dielectric measurements in combination with diffraction contrast transmission electron microscopy revealed the existence of two ferroelectric phases for most PbTiO3 contents studied. The presence of two ferroelectric phases was associated with a decrease in piezoelectric activity and although raw materials costs for y=0.7 and 0.8 with respect to y=0 were significantly lower (less Sc2O3) and TC greater (∼500 °C), d33 (∼100 pC/N) and kp (0.18) were too low to be commercially useful for actuator applications.

The electrical behavior of copper oxide (CuO) ceramics sintered at 920 °C has been characterized by a combination of fixed, radio frequency (rf) capacitance measurements, and impedance spectroscopy (IS). Fixed rf capacitance measurements on ceramics with sputtered Au electrodes revealed a temperature- and frequency-dependent high effective permittivity of ∼104 in the temperature range of 150–320 K. The response is similar to that observed for relaxor-ferroelectrics, however, the magnitude of the effect can be suppressed by thermal annealing of the ceramics with Au electrodes in air at 300 °C or by changing the work function of the electrode material by using In–Ga as opposed to Au. IS data analysis revealed the ceramics to be electrically heterogeneous semiconductors with a room temperature dc resistivity

The grain semiconductivity of Na1/2Bi1/2Cu3Ti4O12 ceramics is suppressed by several orders of magnitude by Mn-doping and Na1/2Bi1/2Cu2.82Mn0.18Ti4O12 ceramics exhibit high room temperature relative permittivity, er, ∼140. er data at 100 kHz fit the Barrett equation over the range ∼10–320 K and the magnitude and temperature dependence of er are consistent with incipient ferroelectricity in this compound.

Rietveld refinement of room temperature (RT) neutron diffraction (ND) data reveals 12R-type hexagonal perovskites Ba3LaNb3O12 (BLN) and Sr3LaNb3O12 (SLN) to adopt space group R3¯ with tilted NbO6 octahedra. The presence of an octahedral tilt transition (Ttilt) at 465 K in BLN from R3¯ to R3¯m is proposed from a combination of high temperature ND data and fixed frequency permittivity measurements. Ttilt is estimated to be much higher at ∼720 K for SLN. The large difference in the RT temperature coefficient of the resonant frequency (τf), −100 ppm/K for BLN compared to −5 ppm/K for SLN, is attributed to the closer proximity of Ttilt to RT for BLN. τf in these 12R-type hexagonal perovskites can therefore be tuned by controlling the tolerance factor and therefore Ttilt in a manner similar to that used for many Ba- and Sr-based 3C-type ABO3 perovskites.

The crystal structure of hexagonal (h)-BaTi0.85Ho0.15O2.925 has been determined using neutron powder diffraction data. The structure is derived from that of h-BaTiO3 and contains Ho3+, the largest cation known to be accommodated by the B site in h-BaTiO3. Ti and Ho are disordered over the B1 octahedral sites and the structure may be regarded as intermediate between those of h-BaTiO3 and Ba3Sr(Nb,Ta)2O9. h-BaTi0.85Ho0.15O2.925 forms as a long-lived but metastable, intermediate phase before transforming, slowly, to the thermodynamically stable cubic polymorph of the same composition; its formation is an example of Ostwald's rule of successive reactions. It is an electrical insulator with relative permittivity of ∼50.

Impedance spectroscopy has been performed on CaCu3Ti4O12 (CCTO) single crystals and ceramics using Au and InGa alloy electrodes. The extrinsic effect responsible for the giant permittivity observed at radio-frequencies near room temperature is shown to be different for single crystals and ceramics. For single crystals it is attributed to a non-ohmic electrode contact to semiconducting CCTO. For ceramics it is attributed to an internal barrier layer capacitor mechanism associated with semiconducting CCTO grains and insulating grain boundaries.

The ferroelectric Curie temperature TC in ATiO3 perovskites is shown to increase linearly with the size variance of the A-site cation mixture [A = (Ba, Sr, Ca)] and this effect accounts for the unusual TC variation in the Ba1–xCaxTiO3 system.

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.

Mn-doped CaCu3Ti4O12 ceramics with nominal composition CaCu2.85Mn0.15Ti4O12 (CCMTO) have been prepared and their radio- and micro-wave dielectric properties established. At 300 K, dense CCMTO ceramics have appreciable grain resistivity, ∼20MΩcm, and exhibit microwave dielectric resonance with er∼93, Qf∼3950GHz (at 3.95GHz) and temperature coefficient of resonant frequency ∼+657ppm∕K. er decreases smoothly from ∼150 to 90 in the range of 10–300K, obeys the Curie-Weiss law in the range of ∼60–180K, and is consistent with incipient ferroelectricity. The source of the incipient ferroelectricity and potential applications of CCMTO ceramics are discussed.

Raman spectroscopy has been used to study the ferroelectric tetragonal to paraelectric cubic phase transition Tc in undoped BaTiO3 (BT) and (Ba0.92A0.08)TiO3 [where A=Ca (BCT), Sr (BST), and Pb (BPT)]. In BT and BPT, mode characteristic of the tetragonal phase at 303 and 710cm−1 persisted until >50°C above Tc, indicating the presence of local dynamic polar clusters well above the displacive phase transitions, ∼120 and ∼149°C, respectively. However, in BCT and BST, the Raman spectra exhibited only broad second order bands above Tc, suggesting that Ca and Sr, A-site dopants supress local dynamic polar clusters.

BaTi1−xCaxO3−x ceramics, where 0⩽x⩽0.04, prepared by the mixed oxide route at 1500°C display a strong suppression of the Curie temperature and a significant increase in grain conductivity with increasing x. The ramifications of these results on the applicability of using Ca2+ as a dopant to achieve X8R specification in BaTiO3-based formulations are discussed.

The crystal structure and electrical properties of CeAlO3 ceramics prepared by the mixed oxide route have been investigated. X-ray and electron diffraction data show the room temperature structure to be described by the tetragonal I4∕mcm space group [a=5.3253(5)A and c=7.5860(9)A; Z=4, and theoretical density, Dx=6.64g∕cm3]. CeAlO3 ceramics sintered at 1500°C under flowing 5%H2–95%N2 gas for 4h reach ∼94% of Dx with a microstructure of randomly distributed equiaxed grains of average grain size ∼2μm. Conventional transmission electron microscopy reveals needle-shaped ferroelastic domains in some grains. The relative permittivity er of CeAlO3 ceramics is ∼18–20 in the range of ∼10–300K. At room temperature and microwave frequencies, CeAlO3 ceramics exhibit er∼20.7, Qfr∼40117GHz (at 9.5GHz), and τf∼−57ppm∕K. The ion polarizability for Ce3+ is calculated to be 4.46A3. Revised dielectric polarizabilities for rare-earth lanthanides are also presented.

The electrical properties of Na1∕2Bi1∕2Cu3Ti4O12 (NBCTO) ceramics between ∼8 and 400K are electrically inhomogeneous, consisting of semiconducting grains and insulating grain boundaries, similar to isostructural CaCu3Ti4O12 (CCTO); however, the intrinsic permittivity of NBCTO of ∼250 at 10K is much higher compared to ∼110 for CCTO. Possible explanations for this high intrinsic permittivity are discussed.

After this article appeared in the September 2005 issue of Journal of Materials Research, an error was identified in Fig. 5(d). The correct figure is below.

CaCu3Ti4O12 (CCTO) ceramics sintered in air at 1115°C for 3 and 24 h have been heat treated in N2 at 1000°C. Surface layers develop on the outer regions of the ceramics, and a combination of X-ray diffraction and analytical electron microscopy has been used to establish the phase content of the layers. A model to explain the formation of the surface layers is proposed based on decomposition of CCTO into a mixture of CaTiO3, TiO2, and Cu2O. The role of limited decomposition in the development of electrically inhomogeneous CCTO ceramics prepared at elevated temperatures in air is discussed.

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Comparison of impedance spectroscopy data for undoped CaCu3Ti4O12 (CCTO) and CaCu2.94Mn0.06Ti4O12 CCMTO ceramics shows Mn doping to suppress the room temperature bulk semiconductivity of ∼0.01Scm−1 for CCTO by six orders of magnitude. The magnitude of and activation energy for bulk conduction in CCMTO are similar to those of the insulating grain boundaries in CCTO ceramics. A model to explain the suppression of the bulk conductivity in CCMTO ceramics is proposed; the grain and grain boundary regions in CCTO ceramics may consist of the same phase but with slightly different compositions.

The crystal structure type, ceramic microstructures, and dielectric properties of BaTi1-xGaxO3-delta (BTG) (0 0.12, a secondary phase isostructural with Ba2TiO4 is detected. Scanning electron microscopy (SEM) reveals three distinctive ceramic microstructures dependent on the firing temperature. BTG compacts fired at 1350 degrees C consist of fine (2-3 mu m) equiaxed grains within a narrow grain size distribution, whereas either large anisotropic or large (100-300 mu m) rounded grains embedded in a fine-grained matrix are observed for ceramics fired at 1400 degrees and 1450 degrees C, respectively. Transmission electron microscopy (TEM) of BTG (x=0.04) ceramics reveals some grains exhibiting both ferroelectric domains and stacking faults, which appear absent in ceramics with x >= 0.06. Ga-enriched grain boundaries were also revealed by energy dispersive spectroscopy (EDS) analysis. Dielectric measurements show the room-temperature relative permittivity, epsilon(r), for x = 0.06 decreases continuously with x from similar to 80 to similar to 65. h-BTG ceramics show a strong decrease in epsilon(r) with increasing temperature in the range 125-425 K, suggesting the possibility of a phase transition below 125 K.

Perovskite-structured ceramics in the (1−x)BiSc1∕2Fe1∕2O3-xPbTiO3 (BSF-PT) system were fabricated for x⩾0.45 in which a morphotropic phase boundary (MPB) occurred between rhombohedral and tetragonal phases at x≈0.50(TC=440°C). Samples close to the MPB gave piezoelectric and electromechanical coupling coefficients of ∼300pC∕N and 0.5, respectively. The low cost of BSF-PT in comparison to (1−x)BiScO3-xPbTiO3 (BS-PT) coupled with its high TC and usable piezoelectric properties suggests future commercial exploitation.

Stoichiometric BaTiO3 ceramics fabricated from sol-gel-derived powders and sintered at temperatures ⩽1100°C are highly insulating and electrically homogeneous. At higher sintering temperatures, samples gradually lose oxygen and the conductivity increases as a consequence. The latter phenomena are very sensitive to the furnace atmosphere and are partially reversible during cooling when partial reoxidation can occur. This results in ceramics that are often electrically heterogeneous with insulating surfaces or grain boundaries but semiconducting grain cores. In samples that were heated at 1450°C in N2 and quenched, a positive temperature coefficient of resistance (PTCR) effect was observed, associated with an additional impedance arising from space-charge effects. These results demonstrate that, depending on sample processing, insulating cation-stoichiometric BaTiO3 can instead be semiconducting and under certain circumstances, exhibit a PTCR effect, without the need for donor dopant additives.

Single-phase cubic (La0.4Ba0.4Ca0.2)(Mn0.4Ti0.6)O3 (LBCMT) was reported by Jha et al. [J. Solid State Chem. 177, 2881 (2004)] to exhibit an extraordinarily high effective permittivity (eeff∼6980 and 590 at 1 and 100kHz, respectively) at room temperature. According to x-ray-diffraction analysis, the structure of this perovskite can be fully described on the centrosymmetric space-group Pm3¯m, which precludes the occurrence of ferroelectricity. In the present work, impedance spectroscopy has shown that LBCMT exhibits a bulk permittivity, er∼40–45, in accordance with that expected for a cubic, centrosymmetric perovskite. LBCMT is a semiconductor with a bulk resistivity of ∼3kΩcm at 300K and an activation energy for conduction of ∼0.20eV which is consistent with an electron hopping conduction mechanism. Finally, it is demonstrated that the high effective permittivity eeff reported previously is not an intrinsic bulk effect arising from hopping conduction between Mn+3 and Mn+4, but is ultimately associated with...

The cation distribution in a polycrystalline sample of starting composition ‘Tl 0.5 Pb 0.5 Sr 2 Ca 2 Cu 3 O 9 ‘ with Tc(zero resistivity) = 118.2 K and Δ Tc = 1 K has been determined from resonant synchrotron X-ray diffraction data collected at the Cu K, Tl L III , Pb L III and Sr K edges. Cation disorder is found at all the non-Cu sites according to the formula (Tl 0.47 Pb 0.53 )(Sr 1.58 Ca 0.42 )(Ca 1.94 Tl 0.06 )Cu 3 O 9 . Assuming the presence of Tl 3+ and Pb 4+ , this formula gives a mean oxidation state of +2.14 for the three copper atoms, consistent with the high Tc of this material. Time-of-flight powder neutron diffraction data between 100 and 150 K have been collected to discover any structural anomalies around Tc.

Orthorhombic and tetragonal Tl-2201 (‘Tl2Ba2CuO6+δ’) samples have been studied using resonant synchrotron X-ray powder diffraction data at energies just below the Cu K and Tl LIII absorption edges. The resonant contributions enable us to demonstrate that Cu substitutes at the Tl site; the tetragonal structure is stabilised by a higher (5.5%) degree of substitution than the orthorhombic material whereit is

Using a low-field dc SQUID susceptometer, we have measured the zero-field-cooled magnetization M ( H ) of a single crystal of Tl-2201 ( T c = 82.5K) with the field applied prallel to the CuO 2 planes. For fields sweeps carried out below T = 50K, a large jump in magnetization is observed at H = H d , close to the field of first flux penetration. It is thought that the discontinuity in the magnetization arises from the destruction of Josephson screening currents between the CuO 2 planes at a field close to the lower critical field H c 1 .

The ‘R’ phase in the Bi–Sr–Cu–O system, which is actually a solid solution of formula Bi2 +xSr2 –xCuOz, forms an extensive solid solution by partial substitution of Ca for Sr. This has formula, Bi2 +xSr2 –x–yCayCuOz, with 0 < y≲ 0.45 and 0.15 ≲x≲ 0.40 at 800 °C. Lattice parameters of the solid solutions are given; pseudotetragonal symmetry is retained throughout but the incommensurate superstructure becomes less marked with increasing contents of both Ca and Bi. The excess oxygen content of the solid solutions, determined by iodometry, is unchanged by replacement of Sr by Ca, but for a given composition, can be varied by heating at 750 °C in different atmospheres.

New free-solue ferrocene/ferricinium electrochemistry, in liquid electrolyte on Tl-containing oxocuprate superconducting electrodes studied at several temperatures above and below Tc shows no discontinuties or abnormalities at the transition, in contrast with solid-electrolyte behaviour but possible in accord with theoretical predictions regarding virbronic interactions or reroganisation barriers.

Single-phase samples of the ‘90 K’ BiSCCO superconductor can be prepared, by careful attention to the experimental conditions, for the compositions: (a) stoichiometric Bi2Sr2CaCu2Oδ(b) Bi2+X Sr2–X CaCu2Oδ 0 < X ⩽ 0.2; (c) Bi2Sr2–y Ca1–y Cu2 Oδ, 0 < y⩽ 0.5: (d) Bi2+x Sr2–x–y Ca1 +y Cu2Oδ, all of which retain the same (Bi + Sr + Ca) : Cu ratio of 5 : 2 as the ideal ‘2212’ Stoichiometry. The ranges of x and y values for the solid solutions are given in the form of a pseudoternary phase diagram. Lattice parameters were indexed on a tetragonal cell; a was independent of composition; c decreased with both x and y. Formation rates of the 90 K phase vary greatly with composition and generally increase with both x and y. The stoichiometric composition requires typically 5 days at 860 °C for complete reaction, whereas the composition x= 0, y= 0.5 requires only 2 days at 820 °C.

The composition-temperature stability region of the solid solution Bi2+x Sr2-x CuO z , phase R, has been determined. At 800°C, 0.15

Polycrystalline barium titanate that has been doped to give a positive temperature coefficient of resistance (PTCR) effect is an inhomogeneous material electrically. Analysis of ac impedance data using the complex impedance plane representation gives the dc resistance of PTCR ceramics. By additional use of the complex electric modulus formalism to analyze the same data, the inhomogeneous nature of the ceramics may be probed. This reveals the presence of two, sometimes three elements in the equivalent circuit. Grain‐boundary and bulk effects may be distinguished from capacitance data: grain‐boundary effects have temperature‐independent capacitances, whereas bulk effects show a capacitance maximum at the Curie point and Curie–Weiss behavior above the Curie point. Both grain‐boundary and bulk effects appear to contribute to the PTCR effect. These results reveal limitations in current theories of the PTCR effect.

${\mathrm{LiTaO}}_{3}$ is both a modest conductor of ${\mathrm{Li}}^{+}$ ions and electrons at high temperatures and a ferroelectric. From ac-impedance data recorded over a wide temperature (500--700 \ifmmode^\circ\else\textdegree\fi{}C) and frequency (30 mHz--1 MHz) range, it was possible to measure the following with the crystal c axis oriented parallel to the electric field: the charge polarization associated with the ferroelectric domains, the intrinsic polarization---which is high but not ferroelectric---of the crystal structure, the resistance associated with domain reorientation, and the resistance due to lithium-ion migration. The correct choice of equivalent circuit is crucial to the determination of these parameters. With the crystal c axis oriented perpendicular to the electric field, a much simpler response was seen. The capacitance values are small, have very little temperature dependence, and show no signs of ferroelectricity. Also the crystal is a modest electronic conductor at high temperatures, 500--700 \ifmmode^\circ\else\textdegree\fi{}C, in this orientation. These results indicate the potential of the ac-impedance technique for probing the electrical properties of ferroelectric materials; some of the disadvantages associated with using fixed-frequency dielectric loss measurements are discussed.

Cells containing either LiClO4/poly(ethylene oxide) or LiCF3SO3/poly(ethylene oxide) complexes as the electrolyte with lithium metal and/or V6O13-based composite electrodes have been assembled and their a.c. impedance measured at temperatures in the range of room temperature to 120 °C. Two resistances are identified from the equivalent circuit analysis. One is attributed to the polymer and shows a characteristic, non linear, Arrhenius plot with a change in slope at about 70 °C. The other is attributed to the lithium/polymer interface and gives an essentially linear Arrhenius plot. The magnitude of this second resistance in Li/V6O13 cells decreases during an initial period of open circuit voltage stabilisation from about 3.5 to 3.2 V. Possible origins of this second resistance are discussed.

The use of combined impedance and modulus spectroscopy to analyse a.c. impedance data for doped polycrystalline BaTiO3 shows the presence of two component impedances which are attributed to bulk and grain boundary effects. This assignment is based on the temperature dependence of their associated capacitances: the grain boundary capacitance does not change significantly with temperature, whereas the bulk capacitance passes through a maximum at the Curie point and shows Curie-Weiss behaviour at higher temperatures. The PTCR effect, characterised by a large increase in resistance as the temperature is raised through the Curie point, is shown by both components. The influence of processing conditions on the PTCR effect, especially the rate at which the sample is cooled to room temperature after sintering, is discussed. At the shorter rates, the PTCR response is dominated by the bulk impedance, whereas at longer times, the grain boundary impedance dominates.

Barium titanate (BaTiO 3 ) can be doped with La 3+ ions via partial substitution for Ba 2+ ions; charge balance is maintained by the creation of Ti 4+ vacancies. Samples processed in an atmosphere of 1 bar O 2 and a temperature of 1350°C are insulating and free from electronic defects associated with either O 2 loss or reduction of Ti 4+ to Ti 3+ . The Curie temperature (T c ) decreases approximately linearly as the lanthanum content increases and, at the same time, an increase in the permittivity (e') maximum at T c occurs. For the composition Ba 1-x La x Ti 1-x/4 O 3 , where x = 0.05, e' has a maximum value of 19000 at 18°C, compared with a typical value of 10000 at 130°C in undoped BaTiO 3 ceramics. This value is the highest value reported for A-site-doped BaTiO 3 and is linked to the mechanism of combined A-site doping and Ti-vacancy creation.