The northwestern Indian Ocean hosts globally crucial marine biodiversity, which is relatively understudied. There has, however, been significant research and conservation effort focused on marine ...turtles across the region in recent decades. We undertook an exhaustive review of 251 relevant publications to extract and collate the current information concerning marine turtles. To ensure completeness, we received input on a preliminary draft from a large group of experts who have worked extensively in the region. Data presented here are from all 13 countries with significant coastlines in the region (Bahrain, Djibouti, Egypt, Eritrea, Iran, Kuwait, Oman, Qatar, Saudi Arabia, Somalia, Sudan, United Arab Emirates, and Yemen) on occurrence, nesting biology, morphology, foraging areas, population status, threats, and relevant national legislation and international conservation strategies. The experts independently provided details on what they felt were the (1) most significant threats to marine turtles, (2) most critical knowledge gaps for marine turtle ecology, (3) impediments to practical conservation, and (4) essential strengths and opportunities in the region.
It has recently been asserted that the donor charge in La+3-doped BaTiO3 is always compensated by Ti vacancies, and that electrons are never the primary compensating defect. It was also stated that ...the conductivity observed in reduced, donor-doped BaTiO33 results from the loss of a very small amount of oxygen not directly related to the donor content. However, the observed reproducible and reversable weight loss on reduction, or gain on oxidation, is exactly that expected for a change between ionic and electronic compensation. It corresponds to the loss or gain of the "excess" oxygen contained in the donor oxide, e.g. LaO1.5 vs. the BaO it replaces. The amount of this weight change is proportional to the donor concentration. This is in agreement with the observation that the equilibrium conductivity in the P(O2)-independent region of electronic compensation is proportional to the donor concentration. Thus the conductivity observed in reduced samples is directly coupled to the donor concentration, and the carrier concentration is equal to the net donor content. In fact, the equilibrium conductivity of donor doped BaTiO3 conforms to the behavior expected from classical defect chemistry, and exhibits regions of both ionic and electronic compensation of the donor charge, as expected. Phase studies by TEM have shown that donor-doped BaTiO3 sintered in air self-adjusts its composition, by splitting out a second phase if necessary, so that the appropriate amount of compensating Ti vacancies are formed. However, when sintered in a reducing atmosphere, the composition self-adjusts to accommodate charge compensation by electrons.
The effect of aliovalent dopants on the defect chemistry of metal oxides is reviewed. The discussion focuses on the stoichiometric solid solution obtained by combination of the stoichiometric dopant ...oxide and the stoichiometric host oxide without gain or loss of oxygen. The charge of the dopant centers is then compensated solely by ionic defects. As the oxygen activity is raised above that in equilibrium with the stoichiometric composition the lattice defects are gradually replaced by holes. Conversely, when the oxygen activity is reduced below that in equilibrium with the stoichiometric composition the lattice defects are gradually replaced by electrons. The near-stoichiometric region of a pure metal oxide is replaced by two regions with charge compensation by a product of reduction, e.g. oxygen vacancies or electrons, in the region of lower oxygen activity, and by a product of oxidation, e.g. cation vacancies or holes, in the region of higher oxygen activity.
A recent study of the defect chemistry of undoped polycrystalline and acceptor-doped single crystalline BaTiO3 attempted to obtain defect and thermodynamic parameters without any a priori ...assumptions, e.g. about the magnitude and temperature dependence of the carrier mobilities. The parameters found for the undoped sample differ significantly from those obtained from the acceptor-doped sample, and even more substantially from those reported from several earlier studies. It is suggested that these discrepancies result from the lack of sufficient data in certain critical regions of temperature and oxygen activity. As a result, the attempt to extract parameters from the equilibrium conductivity by multiparameter curve-fitting has considerable uncertainty. However, the electron and hole mobility products and ratios, mu(n)mu(p) and mu(n)/mu(p), were determined by linear extrapolation of the well-established log-log slopes of +1/4 and -1/4 for the equilibrium conductivities and oxygen diffusivities to their intersection. Both the products and the ratios were independent of temperature, indicating that both mobilities are also independent of temperature. This validates the thermodynamic parameters obtained in earlier studies in which it was assumed that the mobilities are temperature independent or nearly so, and that the carrier concentrations are proportional to the conductivities. 16 refs.
Factors that determine the concentrations of mobile charged species and their contributions to the electrical conductivities of perovskite ferroelectrics are reviewed. Possible charge carriers ...include electrons, holes, and oxygen vacancies. BaTiO
3and
Pb(
Zr
1 −
x
Ti
x
)
O
3 (PZT) are oxygen-excess
p-type insulators when equilibrated in oxidizing atmospheres with extensive trapping of holes at acceptor centers. Electrons are not trapped in BaTiO
3, and the reduced material is an
n-type semiconductor. In the case of PZT there is evidence for the presence of deep electron traps. Neither electrons nor holes have thermally-activated mobilities in BaTiO
3, but the hole mobility is thermally-activated in PZT. Oxygen vacancies can be the dominant charge carrier in both materials under some circumstances.
Equilibrium dc conductivity and thermopower measurements at 650-800 C on undoped and 1% acceptor-doped SrBi2Nb2O9, (SBN), indicated that the n-type conductivity was similar to that of a simple ...transition metal oxide that contained 1-2% donor excess. The donor content was attributed to the presence of Bi+3 on Sr+2 sites in the perovskite-like layers of the structure. These centres arose from cation place exchange between these ions in the alternating layers of the crystal. The exchange was apparently not completely self-compensating and there was local charge compensation in each layer. While the equilibrium conductivity of SrBi2Ta2O9, (SBT), was dominated by ionic conduction in the Bi layers, conduction by electrons in the perovskite-like layers prevailed in SBN. The difference in behaviour was attributed to the expected smaller band gap of the niobate. The electron mobility in SBN was extremely small, of the order of 10 exp(-5) cm2/v.s at 750 C, and was highly activated with an activation energy of about 1.6 eV. The resulting low mobility at ambient temperatures was proposed as the basis for an observed resistance to ferroelectric fatigue. Reports of metallic Bi on the surface of SBT and SBN by XPS analysis resulted from the highly reducing atmosphere of the XPS apparatus. 22 refs.
Recent results have been obtained on the defect chemistry of perovskite ferroelectrics and have been applied to polarization fatigue in thin film ferroelectric memories, in other words the loss of ...switchable polarization after repeated switching cycles. Recent evidence indicates that this effect is caused by the pinning of domain walls by charged point defects, and that both electronic and ionic species are involved.
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