A phase diagram for morphotropic ͑Pb 0.985 La 0.01 ͒͑Zr 1−x Ti x ͒O 3 is proposed based on a combination of X-ray and neutron powder diffraction experiments and complemented by transmission electron microscopy. Dependent on composition three regions are characterized. The stability range of tetragonal microdomains for high Ti contents, the stability range of rhombohedral microdomains for low Ti contents, and an intermediate stability range of nanodomains. All three regions exhibit a corresponding low temperature configuration. Temperature dependent diffraction revealed that lanthanum doping reduces the sensitivity of the structure to changes in temperature and composition. A continuous transition from pseudorhombohedral to tetragonal symmetry with an intermediate two-phase region at the morphotropic phase boundary is observed. A similar transition of low temperature superstructure phases from pseudorhombohedral to pseudotetragonal with an intermediate monoclinic phase governed by a continuous change in the oxygen octahedral tilt system from a − a − a − over a − a − c − to a 0 a 0 c − is identified.
The symmetry of PbZr x Ti 1-x O 3 in the region of the morphotropic phase boundary is still under debate. Noheda et al. [1] claimed the existence of a monoclinic phase. In contrast to this Jin et al. [2] showed for relaxor ceramics that, if the width of tetragonal microdomains is smaller than the diffraction coherence length, the measured crystal lattice constants are of monoclinic symmetry. With convergent-beam electron diffraction (CBED) very small volumes can be examined. So crystal symmetry can be investigated on single domains. To distinguish the most probable phases with space group symmetry P4mm, R3m and Cm, just one zone-axis is needed. At most two projected CBED-patterns of neighbouring domains are necessary. The method will be explained by the use of simulated and experimental CBED-patterns.
Keywords: ferroelectric oxids, in situ synchrotron xray powder diffraction, phase stability, nanodomains, Ferroelectric lead zirconate titanate, PbZr 1-x Ti x O 3 (PZT), is currently used in a wide range of applications converting electrical into mechanical energy. Highest strains and piezoelectric properties are found at the socalled morphotropic phase boundary (MPB) between tetragonal and rhombohedral symmetry. Previous results describe the structure at the MPB as monoclinic [1], whereas our recent studies using high-resolution synchrotron x-ray powder diffraction in combination with TEM and EPR [2,3] were able to correlate XRD observation with a nanodomain structure. The internal symmetry of the nanodomains is difficult to determine due to strong coherence effects in diffraction experiments, which is in correspondence with findings for relaxor ceramics using martensitic theory [4]. The stability field of these nanodomain structures is strongly dependent on sample composition, temperature and electric field. In situ synchrotron diffraction experiments in transmission mode at the beamline B2, Hasylab, Hamburg, reveal changes related to these three parameters. Furthermore, the microstructural reaction of the material is investigated under operating conditions. While the domain structure of samples with low nanodomain content is changed under electric field into that of the adjacent tetragonal or rhombohedral phase field, samples with a distinct balance between Zr-content and tetragonal c/a-ratio show stable nanodomains. These samples undergo reversible phase transitions between a mixture of tetragonal microdomains and nanodomains at the coercive field and rhombohedral microdomains at high voltage. In-situ electric field diffraction above the transition temperature between nanodomain structures and the tetragonal phase show tetragonal domain switching. Furthermore, temperature dependent measurements of the dielectric constant are able to clarify the character of the ferroelectric-to-paraelectric transition and gives new information on the behavior of nanodomain structures under field. This enables us to correlate properties of this material with the relaxor systems PMN-PT and PZN-PT.
domains in an isotropic arrangement. There are also other weaker patterns with axial symmetry about the Bragg rod. Application of a field brings about large changes. The diffuse scattering becomes much more diffuse, apparently involving a migration closer to the film Bragg peak, evidence of an increased spread in domain size and domain disorder. After removing the field the domain distribution does not relax back to the initial state, presumably reflecting the influence of domain wall pinning in stabilizing a field-induced domain geometry.
If the crystallites in a polycrystalline sample are oriented, the intensity of a reflection will vary as a function of the orientation of the sample in the X-ray beam. This means that even if two reflections overlap in a conventional powder diffraction pattern (i.e. have the same d-value), their intensities are likely to vary differently as a function of sample orientation, so their individual intensities can be deduced if data are collected at different sample orientations. It has been shown that this principle can indeed be exploited to obtain more single-crystal-like data from a polycrystalline material [1]. Although the initial study was performed in reflection mode, it was soon realized that a transmission geometry offered several advantages and the experiment was adapted accordingly. In particular, the problem of sample homogeneity was eliminated (the sample is bathed in the X-ray beam), the severe correction of the data for the sample tilt was no longer necessary, and less synchrotron beamtime was required. To start with, a 2-dimensional image plate detector was used, but the resolution of the diffraction patterns, both in d min (2θ range) and in peak width proved to be a limitation [2]. To overcome this, the experiment was changed once again to accommodate the 1-dimensional Mythen I Si-microstrip detector that was available on the Materials Science Beamline at SLS [3,4]. Now, a new version of this detector, Mythen II, has become available, and further optimization of the experiment can be undertaken. The detector now has a 2θ range of 120˚ (vs. 60˚), has a much larger dynamic range, and is not plagued by random dead and hot channels. Consequently, a different, more efficient, data collection strategy can be employed. Improvements in the data analysis software Maud [5] have also made it possible to reduce the data collection time. Constant 5˚ steps in the sample rotation (φ) and tilt (χ) angles are no longer required, so the number of sample orientations to be measured can be reduced from 1368 to 302 without sacrificing information content. The first measurements with the new detector and the new data collection strategy have now been performed. Textured samples of phlogopite mica and the aluminophosphate AlPO 4 -17 (ERI framework type), with known crystal structures, and of a niobium silicate with an unknown structure have been measured. Preliminary analysis of these data show that sensible orientation distribution functions can be derived, and the full intensity extractions are in progress. The crystal system of the niobium silicate was ambiguous because unit cells in several different crystal systems (hexagonal, orthorhombic, monoclinic) were possible. The texture analysis has now shown that only the orthorhombic unit cell is consistent with the measured pole figures.[1] Baerlocher, Ch.; McCusker, L.B.; Prokic, S.; Wessels, T. Z. Ferroelectric lead zirconate titanate solid solutions, PbZr 1-x Ti x O 3 (PZT), are frequently used in industrial applications exploiting the reaction of both the lattice and t...
Despite extensive studies the microstructure of the morphotropic phase boundary (MPB) in the ferroelectric material lead titanate zirconate Pb(Zr1-xTix)O3 is still under discussion. Whereas some groups (Noheda et al. [1]) fitted diffraction data by monoclinic symmetry, other groups describe the MPB as composed of a complicated system of micro-and nanodomains [2]. The composition range from x = 0.40 to x = 0.55 is investigated by our group by a combination of X-ray diffraction and transmission electron microscopy (TEM) as a function of temperature and electric field [3]. Tetragonal Pb(Zr0.45Ti0.55)O3 and rhombohedral Pb(Zr0.60Ti0.40) O3 feature small microdomain widths coupled with a bimodal distribution. Around the morphotropic phase boundary an increase in microdomain widths associated with the formation of nanodomains is observed. In situ electric field synchrotron powder diffraction across the entire compositional range of the MPB shows changes in phase fractions and phase transitions dependent on the nanodomain content [4]. These observations are complemented by in situ studies of the domain structure under electric field in TEM [5]. Convergent beam electron diffraction (CBED) has been applied to elucidate the symmetry of the nanodomains. The results will be discussed together with calculations of the energy of formation of domains and on results obtained by electron spin resonance (EPR) [3]. The compounds MOX (M = Ti, V, Cr; X = Cl, Br) crystallize in the FeOCl structure type with space group Pmmn and lattice parameters a = 3.778, b = 3.355 and c = 8.027 Å for TiOCl at room temperature. All compounds go through phase transitions in dependence on temperature, which involve magnetic ordering as well as lattice degrees of freedom. TiOCl and TiOBr have attracted particular attention as one-dimensional (1D), S = 1/2 spin-chain compounds that are in a dimerized, spin-Peierls state at low temperatures (Seidel et al., 2003). Both compounds possess incommensurately modulated crystal structures at intermediate temperatures (van Smaalen et al., 2005). Here we give an overview of the temperature-dependent phase diagrams of TiOCl and TiOBr, and we discuss possible mechanisms for the normal-to-incommensurate and lock-in phase transitions (Schönleber et al., 2008). The pressure dependencies of the crystal structures and physical properties of these compounds are also discussed (Kuntscher et al., 2007). These results are analyzed in view of the phase transitions observed in VOCl and CrOCl.
MS17 Structure and function studies by powder diffraction 24 th European Crystallographic Meeting, ECM24, Marrakech, 2007 Page s189 Acta Cryst. (2007). A63, s189 (Grenoble) (λ=0.3748 Å) at 25 o and 250 o C, respectively, suggests that the coefficient of thermal expansion is highest for samples with low iron substitution. Split of triangular Cu2 positions increases with temperature and the halfoccupied Cu2 positions from different coordination triangles approach one another, down to 2.70-2.75 Å in tennantite at 250 o C. There is insignificant residual electron density between the split Cu2 half-sites in tetrahedrite at 25 o C and its increase with temperature is moderate. The inter-site density is substantially higher in tennantite and it increases considerably with temperature, especially in the low-Fe sample.
Keywords: ferroelectrics, in situ powder diffraction, domain switchingThe origin of the extremely high piezoelectric response of morphotropic ferroelectric lead zirconate titanate (PZT), PbZr (1-x) Ti (x) O 3 , is still under discussion. Noheda et al. [1] proposed a monoclinic phase at the morphotropic phase boundary (MPB) as an explanation, via which a rotation of the polarization direction is possible. However, interpretation of X-ray data so far has not taken into account the strongly varying domain structure across the MPB. We have undertaken roomtemperature and temperature-dependent synchrotron X-ray powder diffraction experiments with and without an applied electric field. A complete series of polycrystalline pellet samples across the MPB was analysed in transmission mode geometry in high-resolution and intermediate resolution image plate setup at B2, Hasylab Hamburg, Germany and also subjected to TEM imaging. The investigations show that the domain structure miniaturizes across the MPB down to nanodomains of 5-10nm width and as a consequence the diffraction patterns depict a strong increase in peak asymmetry as the tetragonal c/a-ratio decreases with increasing Zr content. These nanodomains appear in the same stability range as the proposed monoclinic phase. The existence of a monoclinic phase, however, is questioned by Jin et al. [2] for relaxor ceramics, assuming that it is just an effect of domain miniaturization and coherence in diffraction. The changes in domain structure have a dramatic influence on the poling behaviour of the material. While compositions at the edges of the MPB only show changes in domain orientation along the applied field, morphotropic samples exhibit strong changes in both lattice constants and intensities, accompanied by a strong increase in macroscopic strain. PZT 54/46 shows an increase in the intensity between the tetragonal 101 / 110 duplet under electric field and seems textured rhombohedral in-situ under 5kV/mm. The effects observed will be discussed in terms of reorientation of nanodomains through changes in domain configuration, stacking disorder, polarization rotation and possible phase transitions under electric field. The authors appreciate the financial support of the German Research Foundation (DFG) through the Sonderforschungsbereich 595 ''?Electric fatigue in functional materials'' and the virtual institute (VH-VI-102) of the Helmholtz Society. Phys. Rev. Lett. 91, 197601 (2003). Keywords: titanium dioxide, crystallization of thin films, X-ray reflection and diffractionIn recent years, titanium dioxide films have been widely used in various fields because of their excellent chemical stability, mechanical hardness and optical transmittance with high refractive index. The photocatalytic activity of TiO 2 can result in the decomposition of organic compounds on the TiO 2 surface or the reduction of the contact angle between water and the TiO 2 surface after ultraviolet irradiation, i.e., in selfcleaning and antifogging effects, respectively. In the present work...
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