Field-enhanced bulk conductivity and resistive-switching in Ca-doped BiFeO3 ceramics

The bulk conductivity at room temperature of Ca-doped BiFeO3 ceramics is p-type and increases reversibly by up to 3 orders of magnitude under the influence of a small dc bias voltage in the range B3 to 20 V mm1 . The effect occurs in both grain and grain boundary regions, is isotropic and does not involve creation of filamentary conduction pathways. It is proposed that, by means of capacitive charging and internal ionisation processes under the action of a dc bias, hole creation leads to a more conductive excited state. This gradually returns to the ground state when the dc bias is removed and the holes recombine with electrons trapped at the sample surface. The holes are believed to be created on oxygen, as O ions

Phase transition hysteresis and anomalous Curie-Weis behavior of ferroelectric tetragonaltungsten bronzes Ba2RETi2Nb3O15:RE=Nd, Sm

The ferroelectric tetragonal tungsten bronze TTB phases, Ba2RETi2Nb3O15 :RE=Nd,Sm, were prepared by low temperature solvothermal synthesis. The permittivity versus temperature data of sintered ceramics show two unusual features: first, a hysteresis of 50–100 °C between values of the Curie temperature Tc on heat-cool cycles and second: a huge depression in the Curie–Weiss temperature T0. Both effects are attributed to the complex nature of their TTB-related crystal structures with different superstructures above and below Tc and the difficulty in nucleating ferroelectric domains on cooling through Tc. Several factors may contribute to the latter difficulty: first, the structures contain two sets of crystallographic sites for the “active” Ti, Nb ions; second, the distribution of Ti and Nb over these two sets of sites is not random but partially ordered; and third, below Tc a weak commensurate superstructure perpendicular to the polar c axis is present, but above Tc a weak incommensurate superstructure in a similar orientation is present. Hence the formation of the ferroelectric structure on cooling requires both nucleation of polar domains involving two sets of cation sites and structural change from an incommensurate to a commensurate supercel

Enhanced conductivity and nonlinear voltage–current characteristics of nonstoichiometric BaTiO3 ceramics

The electrical conductivity of both BaO-deficient and TiO2-deficient BaTiO3 ceramics shows nonohmic, low-field characteristics at temperatures >∼200°C in contrast to stoichiometric BaTiO3 for which the electrical conductivity is independent of applied voltage. The nonlinearity is observed in both bulk and grain-boundary resistances of ceramics that are both porous (∼82%) and nonporous (∼98%) and is not associated with interfacial phenomena such as Schottky barriers and memristors or with charge injection from the electrodes. Results, shown as a function of time over the temperature range 200°–750°C with field gradients in the range ∼0.5–20 V/mm, indicate that an excited state is reached that is time, temperature, and field dependent. This effect appears to be caused by departures from local electroneutrality in the defect structure of nonstoichiometric BaTiO3 which are reduced by electron transfer on application of a dc bias, leading to a more conducting, low-level excited state in which holes associated with underbonded oxygens, presumably as O− ions, are the principal charge carriers. Ceramics gradually return to their ground state in two stages on removal of the dc bias and the conductivity decreases overall by two to three orders of magnitude

Non-Ohmic Phenomena in Mn-doped BaTiO3

We report here a novel effect in which the resistance of a semiconducting oxide ceramic increases on application of a small dc bias. The ceramic conducts at high temperatures by an n-type hopping mechanism. On application of a dc bias, conduction electrons are trapped at surface states and the resistance increases. On removal of the dc bias, the trapped electrons are released and the sample regains its original state. This effect is the mirror image of that seen with similar ceramics that conduct by a p-type mechanism whose resistance decreases reversibly on application of a small dc bias. These two phenomena together offer the possibility of novel switching devices and memristive applications, especially if the switching times can be reduced

Field enhanced bulk conductivity of acceptor-doped BaTi1−xCaxO3−x ceramics

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

Electrical properties of stoichiometric BiFeO3 prepared by mechanosynthesis with either conventional or spark plasma sintering

Phase-pure powders of stoichiometric BiFeO3 have been prepared by mechanosynthesis. Ceramics sintered by either conventional heating in air or spark plasma sintering (SPS) followed by oxidative anneal in air are highly insulating with conductivity e.g. ~10–6 Scm–1 at 300 °C and activation energy 1.15(2) eV, which are comparable to those of a good-quality BiFeO3 single crystal. By contrast, the as-prepared SPS sample without the post-sinter anneal shows higher conductivity e.g. ~10–6 Scm–1 at 225 °C and lower activation energy 0.67(3) eV, indicating some reduction of the sample by the SPS process. The reason for the high conductivity observed in some ceramic samples reported in the literature appears to be unclear at presentGobierno de España CTQ 2011-27626 MAT 2008-06619Junta de Andalucia TEP-0300

Effect of tri- and tetravalent metal doping on the electrochemical properties of lanthanum tungstate proton conductors

Metal doping on lanthanum tungstates at W2 site was confirmed by TOF-NPD. TGA and EIS demonstrated proton conductivity.</p

Baltijas Psiholoģijas žurnāls

Contents: Aleksandrs Koļesovs. Time Perspective of Latvian and Russian (Ethnic Minority)High School Students in Riga and Latgale ; Toshiaki Shirai, Danguole Beresneviciene. Future orientation in culture and socio-economic changes: Lithuanian adolescents in comparison with Belgian and Japanese ; Anita Pipere. Primary and Secondary Teachers: Beliefs and Performance Related Self-Perceptions about Engaged Learning ; Ļubova Černova. Aggression and anxiety of intellectually gifted Russian adolescents in Latvia ; Anda Gaitniece-Putāne. Liverpool Stoicism Scale Adaptation ; Arthur Cropley, Iourii Gribov. Two-dimensional Education: Fostering the «Prepared Mind» for Creativity ; VII International Baltic psychology conference. First Announcemen

Synthesis, structure and dielectric properties of a new family of phases, ABC 3 O 11 : A = La, Pr, Nd, Sm, Gd; B = Zr, Hf; C = Ta, Nb

Eight new phases with the general formula of ABC O with different rare earth, (Zr, Hf), (Nb, Ta) combinations, have been prepared by solid-state reactions at a temperature range of 1200–1500 °C. The new phases—LaHfTa, LaHfNb, LaZrNb, PrHfTa, NdHfTa, NdHfNb, SmHfTa and GdHfTa—are characterised by X-ray and neutron diffraction data at room temperature and variable frequency impedance measurements. They are isostructural with LaZrTa O which consist of alternating single layers of UO pentagonal bipyramids and octahedra as shown by Rietveld refinement of X-ray and neutron powder diffraction data. Lattice parameters decrease with decreasing size of rare earth element substitution at A-site, and of all, Gd is the smallest rare earth that formed LaZrTa O analogues. Detailed attempts of attained and unattainable LaZrTa O analogues with different temperatures are included in this paper. All phases are highly insulating with temperature-independent bulk permittivities in the range 17 to 50; LaHfNb demonstrates the highest permittivity. Arrhenius plot shows that the activation energies are in the range 0.8 to 1.94 eV.We thank EPSRC and the Ministry of Higher Education of Malaysia for their financial support through the research grant of Fundamental Research Grant Scheme (FRGS) no. FRGS/1/ 2017/STG07/UPNM/02/2

Electrical properties of stoichiometric BiFeO3 prepared by mechanosynthesis with either conventional or spark plasma sintering

Phase-pure powders of stoichiometric BiFeO3 have been prepared by mechanosynthesis. Ceramics sintered by either conventional heating in air or spark plasma sintering (SPS) followed by oxidative anneal in air are highly insulating at room temperature with resistivity, extrapolated from the Arrhenius plots, of ∼1016 Ωcm and activation energy 1.15(2) eV, comparable with those of a good-quality BiFeO3 single crystal. By contrast, the as-prepared SPS sample without the postsinter anneal shows lower resistivity, e.g., ∼1010 Ωcm at 25°C and activation energy 0.67(3) eV, indicating some reduction in the sample by the SPS process. The reason for the high conductivity reported for some ceramic samples in the literature remains unclear at present. © 2013 The American Ceramic Society.Peer Reviewe