Donald M. Smyth

Titanium diffusion into LiNbO3 as a function of stoichiometry

The diffusivity of Ti into LiNbO3 has been measured as a function of crystal orientation and Li/Nb ratio in the temperature range 980–1112 °C. The Li/Nb ratios of single crystal samples were adjusted by vapor phase equilibration with fixed Li2O activities. No anisotropy of diffusivity with crystal orientation could be detected. The diffusion constants for Ti in LiNbO3 decrease with increasing Li2O content. At 1050 °C, the values decrease from 2.60×10−12 cm2/s at the Li2O‐deficient phase boundary (48.1 mol % Li2O) to 1.06×10−12 cm2/s at the congruently melting composition at (48.6 mol  % Li2O) to 0.4×10−12 cm2/s at the Li2O‐rich phase boundary (50.0 mol % Li2O). It is suggested that enhanced lateral diffusion observed for stripe geometry Ti sources results from increased diffusivity in a surface layer formed by Li2O outdiffusion.

Charge motion in ferroelectric thin films

Switching of the direction of polarization in ferroelectric thin films can be used as a mechanism for space-efficient memory storage in microcircuits. The fields required for adequate switching rates are of the order of 5 × 105 volts/cm. At these high fields, there are several potential mechanisms for the motion of charged species in the film with a resulting degradation of properties. The defect chemistry and charge transport mechanisms in perovskite-based systems are reviewed, and their pertinence for the development of stable, reliable memory elements is discussed.

Control of microchemical ordering in relaxor ferroelectrics and related compounds

Abstract A review is given of some of the ways to control microchemical domains (due to ordering on the B-sites) in relaxor ferroelectrics and related compounds. Depending on the system, ordering can he controlled by heat treatment, chemical composition and stoichiometry. Specific examples are given of both thermally- and chemically-induced ordering in relaxor ferroelectric and perovskite-related materials.

Defects and transport in LiNbO3

Abstract The defect chemistry of LiNbO3 has been studied by measurement of the equilibrium electrical conductivity as a function of temperature, oxygen activity, Li/Nb ratio, and impurity additions. Density changes indicate that Nb-excess defects result from both Li2O-deficiency and reduction. Oxygen vacancies are not a major defect in LiNbO3. Insensitivity to changes in Li/Nb ratio, and to additions of aliovalent impurities in amounts up to several percent, implies a very large degree of intrinsic ionic disorder. It is proposed that there is little energetic preference for the cation stacking sequences characteristic of the LiNbO3 and ilmenite structures and that this results in extensive cationic disorder.

Compositional control of ferroelectric fatigue in perovskite ferroelectric ceramics and thin films

The mechanism and control of ferroelectric polarization fatigue (loss of polarization with cycling) in donor‐ and acceptor‐doped BaTiO3 ceramics and Pb(Zr1−xTix)O3 (PZT) thin films has been investigated. Experimental results clearly demonstrate that fatigue behavior is related to the defects within the materials. Donor‐doped BaTiO3 ceramics showed significantly improved fatigue characteristics when compared with acceptor‐doped materials. A similar but reduced effect has been observed in donor‐doped PZT thin films. The electric‐field‐assisted migration of charged species within ferroelectric materials may be responsible for the degradation/fatigue behavior. Results support the expectation that oxygen vacancies play an important role in fatigue that occurs as a result of polarization switching.

Shallow Pb3+ hole traps in lead zirconate titanate ferroelectrics

A combination of electron paramagnetic resonance and band structure calculations is used to show that the Pb2+ ions in lead zirconate titanate materials can act as shallow hole traps to give Pb3+ centers. The Pb3+ centers are created by optical illumination and are found to be metastable at room temperature. Their decay kinetics follow a stretched exponential function which suggests dispersive transport. The traps have the same character as the upper valence band which is formed from the hybridization of Pb s and O p states.

The defect chemistry of donor-doped BaTiO3: A rebuttal

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 BaTiO3 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.

Local variations in defect polarization and covalent bonding in ferroelectric Cu2+-doped PZT and KNN functional ceramics at the morphotropic phase boundary

Cu(2+)-doped Pb[Zr(0.54)Ti(0.46)]O(3) (PZT) and Cu(2+)-doped [K(0.5)Na(0.5)]NbO(3) (KNN) ferroelectrics with a dopant concentration of 0.25 mol% were investigated by means of multi-frequency and multi-pulse electron paramagnetic resonance (EPR) spectroscopy. Through the use of high magnetic fields and pulsed microwave fields an enhanced resolution was achieved yielding valuable information about the structural distortion at the dopant site. The results obtained suggest that Cu(2+) substitutes for both systems as an acceptor centre for the perovskite B-site. For reasons of local charge compensation, different kinds of defect associates invoking one and two oxygen vacancies are formed. These two kinds of extended defects differ in their electric and elastic properties. The results obtained are analyzed in order to characterize differences of the local structure in the Cu(2+)-defect center for morphotropic phase boundary compositions between the two systems. In particular, it is found that Cu(2+)-doping in KNN creates 50% more oxygen vacancies than the same amount of copper in PZT. Furthermore, local differences in covalent and ionic bonding are monitored.

Ionic transport in ferroelectrics

Abstract Ionic conductivity in ferroelectric oxides having the perovskite structure is reviewed. Depending on the compound, doping levels, and equilibration conditions, ionic transport numbers approaching unity have been observed under equilibium conditions. The ionic conductivity decreases with decreasing temperature with an activation energy of about 1 eV over the entire measured temperature range of 1000-60°C. This is attributed to the activation energy of the oxygen vacancy mobility. The p-type conductivity decreases somewhat less rapidly for both equilibrium and quenched conditions. Evidence is presented that suggests that the initial leakage current contains a substantial ionic component under device use conditions. Subsequent time-dependent behavior is critically dependent on whether or not the electrodes are blocking to oxygen.

Comments on the Defect Chemistry of Undoped and Acceptor-Doped BaTiO3

In a recent study of the defect chemistry of undoped polycrystalline and acceptor-doped single crystalline BaTiO3 an attempt was made 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, μnμp and μn/μp, 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 found to be independent of temperature, indicating that both mobilities are also independent of temperature. This important conclusion 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.