ABSTRACT
Ba1-xSrxTiO3 solid solutions prepared by a solid-state reaction in air at 1200-1400 °C, followed by slow cooling to room temperature at the end of the reaction, were essentially oxygen-stoichiometric and p-type. Their conductivity increased reversibly when either p(O2) in the surrounding atmosphere was increased or a dc bias as small as 1 V was applied across the samples. The enhanced p-type conductivity is attributed to the creation of mobile holes on underbonded oxide ions. The same samples quenched from >â¼1400 °C were increasingly oxygen deficient and n-type. They showed reduced conductivity with either a dc bias or increased p(O2), attributed to the trapping of mobile electrons. These materials provide a rare example of a switch between n-type and p-type conductivity, in the same material, linked to oxygen stoichiometry variation. In both n- and p-type materials, the samples responded to external stimuli in a way similar to that of a leaky capacitor; polarization processes at sample surfaces led first to charge storage and second to a reversible change in bulk electrical properties.
ABSTRACT
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 â¼3 to 20 V mm(-1). 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.
ABSTRACT
Oxygen-deficient tetragonal tungsten bronzes ceramics with general formula Ba(2)NdTi(2+x)Nb(3-x)O(15-x/2) (0 ≤ x ≤ 1) have been prepared by low temperature solvothermal synthesis with final firing of ceramics at 1100-1300 °C in air. Rietveld refinement of X-ray powder diffraction (XRD) and neutron powder diffraction (ND) data at room temperature of Ba(2)NdTi(3)Nb(2)O(14.5) shows that Ba and Nd are ordered on the 15-coordinate and 12-coordinate sites, respectively, Ti and Nb are disordered nonrandomly over the two octahedral sites, and oxygen vacancies locate preferentially in the coordination sphere of Nd and Ti/Nb(2) atoms. Variable frequency impedance measurements show that samples are poor electronic conductors with activation energies â¼0.8-1.7 eV, conductivities â¼1 × 10(-5) S cm(-1) at â¼725 °C and with some evidence of oxide ion conduction at high x values. Composition dependence of the dielectric properties shows a transition from classic ferroelectric behavior with Ba(2)NdTi(2)Nb(3)O(15) to a relaxor-like behavior with Ba(2)NdTi(3)Nb(2)O(14.5). At intermediate compositions, both a first-order phase transition and relaxor-like behavior are observed.
ABSTRACT
BaTiO3 containing Ca substituted for Ti as an acceptor dopant, with oxygen vacancies for charge compensation and processed in air, is a p-type semiconductor. The hole conductivity is attributed to uptake of a small amount of oxygen which ionises by means of electron transfer from lattice oxide ions, generating O(-) ions as the source of p-type semiconductivity. Samples heated in high pressure O2, up to 80 atm, absorb up to twice the amount expected from the oxygen vacancy concentration. This is attributed to incorporation of superoxide, O2(-), ions in oxygen vacancies associated with the Ca(2+) dopant and is supported by Raman spectroscopy results.