Enhanced Conductivity and Nonlinear Voltage–Current Characteristics of Nonstoichiometric BaTiO₃ Ceramics

Abstract

The electrical conductivity of both BaO-deficient and TiO 2 -deficient BaTiO 3 ceramics shows nonohmic, low field characteristics at temperatures > ~ 200 o C in contrast to stoichiometric BaTiO 3 for which the electrical conductivity is independent of applied voltage. The non-linearity is observed in both bulk and grain boundary resistances of ceramics that are both porous (~82%) and non-porous (~98%) and is not associated with interfacial phenomena such as Schottky barriers and memristors nor with charge injection from the electrodes. Results, shown as a function of time over the temperature range 200 to 750 °C with field gradients in the range ~ 0.5 to 20 Vmm -1 , 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 non-stoichiometric BaTiO 3 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 2-3 orders of magnitude. IntroductionThe electrical properties of resistive materials in bulk form generally show linear voltage-current, V-I, behaviour at small applied voltages although at high voltages, insulating materials exhibit dielectric breakdown. 1 For materials in which conduction takes place by a hopping mechanism, of either ions or electrons, the conductivity can be modelled using random walk theory 2 and the effect of a small dc voltage is simply to apply a slight bias to the random motion of the conducting species, leading to a net drift in a particular direction. A small dc bias is insufficient to force ions or electrons to move; consequently, the V/I response obeys Ohm's Law.In contrast to bulk properties which obey Ohm's Law, non-linear characteristics are commonly associated with interfacial effects, such as electrode-sample contacts which give rise to Schottky barriers. 1 There is also current interest in memristor effects in which, for the specific case of TiO 2 / TiO 2- interfaces, coupled electronic and ionic charge transfer occurs across the interface giving rise to non-linear phenomena. 3 We have recently reported three examples of hopping electronic conduction in bulk ceramics of acceptor-doped BaTiO 3 (BT) that exhibit non-linear characteristics at low field and at temperatures in the range 140 to 650 ºC. [4][5][6] The dopants were Zn 2+ , Mg 2+ and Ca 2+ which substituted for Ti 4+ in the octahedral sites of the perovskite structure, with concentrations in the range 0.003 to 3 mole%.Both bulk and grain boundary conductivities increased by 1 to 2 orders of magnitude, at a rate that was dependent on temperature, on applying a dc bias in the range ~ 0.5 to 20 Vmm -1 at the same time as the ac impedance measurements were made. On removal of th...

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

“…It does not require electron injection at the cathode, although evidence for this has been obtained from electrocolouration effects in other materials [23][24][25][26][27][28][29][30][31]. It does not require a cation-based mechanism for hole creation at The results reported here on Ca-doped BiFeO3 are different in nature but not in mechanism from that seen with various acceptor-doped titanate perovskites [17][18][19][20][21][22]. In the present case, the ON/OFF switching occurs at much lower temperatures and more rapidly than in the titanates.…”

“…A new mechanism is required, and proposed, to explain this effect since none of the existing models can account for the observations reported here. This is shown schematically in Figure 11; it is based on earlier studies of voltage-dependent conductivity in perovskite-structured, acceptor-doped titanates of Ba, Sr and Ca [17][18][19][20][21][22].…”

“…The defect structure of Ca-doped BiFeO3 consists of a disordered distribution of dopant Ca 2+ ions on Bi 3+ sites and oxygen vacancies which tend to associate with the Ca 2+ dopants to preserve local electroneutrality, Figure 11(a). In the cases of acceptor-doped titanates [17][18][19][20][21][22], the dopant substitutes partially for Ti 4+ on the B sites whereas here, Ca substitutes for Bi on the A sites.…”

Non‐ohmic phenomena in Mn‐doped BaTiO3

“…Nonlinear low‐field behaviour has been observed recently 1–3 in p ‐type, acceptor‐doped BaTiO 3 ceramics in which the resistance decreased by 1–2 orders of magnitude on application of a small bias voltage, typically in the range 1–10 V, across pellets of thickness 1–2 mm; the resistance decrease was fully reversible on removal of the dc bias and was not associated with interfacial effects such as Schottky barriers. The rate of change of resistance with time was very temperature‐dependent.…”

“…The enhanced conductivity was attributed to the presence of underbonded oxygen atoms surrounding acceptor dopants in the BaTiO 3 crystal lattice; facile ionization of these underbonded oxygens generated holes, or O − ions, and the increase in hole concentration was responsible for the enhanced conductivity. The driving force for ionization was provided by the dc bias which caused the activation of electron trap states at the sample surface 4.…”