Anna‐Maria Welsch

University of Sydney

Author Statistics

Papers

Citation

H-Index

i-10 index

Research Field

Pressure-induced structural transformations in advanced ferroelectrics with relaxor behaviour

High Pressure Research (2013)

Boriana Mihailova Naëmi Waeselmann Bernd Maier Anna‐Maria Welsch R. J. Angel

AbstractA series of model perovskite-type relaxor ferroelectrics (pure and A-/B-site doped PbSc1/2Ta1/2O3 and PbSc1/2Nb1/2O3 as well as of 0.9PbZn1/3Nb2/3O3−0.1PbTiO3) were studied by high pressure diffraction and inelastic light scattering in order to elucidate the mesoscopic-scale ferroic atomic arrangements responsible for the superb macroscopic properties of these materials. The combined analysis of the pressure-enhanced phonon anomalies observed by Raman spectroscopy and the pressure-induced long-range order detected by synchrotron X-ray and neutron diffraction revealed that at ambient conditions antiferrodistortive order coexists with the ferroelectric order on the mesoscopic scale. This suggests that the locally polarized spatial nanoregions known as polar nanoregions are ferrielectric in nature and their abundance and mean size depend on both the antiferrodistortive and ferroelectric coupling, which in turn can be tuned by appropriate chemical variations.Keywords: relaxor ferroelectricsX-ray diffractionRaman spectroscopyperovskitesphase transitions AcknowledgementsFinancial support by the Deutsche Forschungsgemeinschaft (SPP 1236, projects MI 1127/2-1 and MI1127/2-2) is gratefully acknowledged. The DFG-priority project SPP1236 was funded by the Deutsche Forschungsgemeinschaft from 2006 to 2013. The authors thank Marin Gospodinov, Bulgarian Academy of Sciences, for synthesizing the single-crystal samples, Carsten Paulmann and William G. Marshall for the excellent support at F1/DESY and Pearl/ISIS, respectively, Jing Zhao, Virginia Tech, for help in high-precision single-crystal XRD data collection, Claude Ederer, Trinity College Dublin, for DFT calculations, and Hauke Marquardt and Sergio Speziale, GFZ Potsdam, for the collaborative Brillouin spectroscopic study. The authors are also grateful to Björn Winkler, Goethe-Universität Frankfurt, for the opportunity to use the gas loader available in his lab.Notes†This contribution is part of the final report summarizing the results of the DFG-priority project SPP1236, which was funded by the Deutsche Forschungsgemeinschaft from 2006–2013.

Mesoscopic physics

10.1080/08957959.2013.806499

Cite

Citations (8)

Lithium conductivity in glasses of the Li₂O–Al₂O₃–SiO₂system

Physical Chemistry Chemical Physics (2014)

Sebastian Roß Anna‐Maria Welsch Harald Behrens

To improve the understanding of Li-dynamics in oxide glasses, i.e. the effect of [AlO4](-) tetrahedra and non-bridging oxygens on the potential landscape, electrical conductivity of seven fully polymerized and partly depolymerized lithium aluminosilicate glasses was investigated using impedance spectroscopy (IS). Lithium is the only mobile particle in these materials. Data derived from IS, i.e. activation energies, pre-exponential factors and diffusivities for lithium, are interpreted in light of Raman spectroscopic analyses of local structures in order to identify building units, which are crucial for lithium dynamics and migration. In polymerized glasses (compositional join LiAlSiO4-LiAlSi4O10) the direct current (DC) electrical conductivity continuously increases with increasing lithium content while lithium diffusivity is not affected by the Al/Si ratio in the glasses. Hence, the increase in electrical conductivity can be solely assigned to lithium concentration in the glasses. An excess of Li with respect to Al, i.e. the introduction of non-bridging oxygen into the network, causes a decrease in lithium mobility in the glasses. Activation energies in polymerized glasses (66 to 70 kJ mol(-1)) are significantly lower than those in depolymerized networks (76 to 78 kJ mol(-1)) while pre-exponential factors are nearly constant across all compositions. Comparison of the data with results for lithium silicates from the literature indicates a minimum in lithium diffusivity for glasses containing both aluminium tetrahedra and non-bridging oxygens. The findings allow a prediction of DC conductivity for a large variety of lithium aluminosilicate glass compositions.

10.1039/c4cp03609c

Cite

Citations (54)

The structural state of lead-based relaxor ferroelectrics under pressure

IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control (2011)

Boriana Mihailova R. J. Angel Bernd Maier Anna‐Maria Welsch Jing Zhao

The exceptional properties of lead-based perovskite-type (ABO(3)) relaxor ferroelectrics are due to their structural inhomogeneities. At ambient conditions, the average structure is pseudocubic but rich in ferroic nanoregions too small to be directly studied by conventional diffraction analysis. However, combining in situ temperature and pressure diffraction and Raman scattering allows us to resolve the structural complexity of relaxors. Because of the different length and time scales of sensitivity, diffraction probes the long-range order, i.e., the structure averaged over time and space, whereas Raman spectroscopy can detect local structural deviations from the average structure via the anomalous Raman activity of the phonon modes that, when the symmetry of the average structure is considered, should not generate Raman peaks. Hence, the combined analysis of the long-range order induced at low temperatures or high pressures and of the phonon anomalies enhanced on temperature decrease or pressure increase can reveal the energetically preferred structural nanoclusters existing at ambient conditions. In this regard, high-pressure experiments are vital for understanding the nanoscale structure of relaxors. Using X-ray diffraction, neutron diffraction, and Raman scattering on stoichiometric and doped PbSc(0.5)Ta(0.5)O(3) and PbSc(0.5)Na(0.5)O(3), we demonstrate the existence of a pressure-induced cubic-to-rhombohedral continuous phase transition. The high-pressure structure has suppressed polar shifts of B-site cations, enhanced correlation of Pb-O ferroic species, and long-range ordered antiphase BO(6) octahedral tilts. The critical pressure is preceded by an intermediate pressure at which the coupling between off-centered Pb and B-cations is suppressed and octahedral tilting detectable by neutron diffraction is developed.

10.1109/tuffc.2011.2030

Cite

Citations (19)

Lithium Mobility in Borate and Phosphate Glass Networks

Zeitschrift für Physikalische Chemie (2017)

Anna‐Maria Welsch Harald Behrens Dawid Murawski I. Horn

Abstract In order to improve our understanding of the Li-mobility in oxide glass networks with Li as the principle mobile particle, electrical conductivity and self-diffusivity of lithium was studied in two phosphate (0.2 Li 2 O·0.8 P 2 O 5 , 0.3 Li 2 O·0.2 MgO·0.5 P 2 O 5 ) and one borate (0.25 Li 2 O·0.75 B 2 O 3 ) glass compositions. Conductivity measurements provided information about ion dynamics while isotope-exchange experiments involving isotopically enriched Li diffusion glass couples provided information about long-range diffusivity of Li-isotopes through borate and phosphate networks. Due to the limitations of individual glass stabilities, the temperature range for selected experiments was very small, e.g. as in the case of Li–phosphate composition between 373 and 520 K. The activation energy for Li-migration derived from conductivity measurements was similar for Li–Mg–phosphate and Li–borate, 90.4 and 85.2 kJ/mol, while for pure Li–phosphate the value was 74.7 kJ/mol. In the case of self-diffusion, the activation energies were comparable with Li–Mg–phosphate having the highest value of 76.9 kJ/mol while Li–phosphate and Li–borate had almost the same value of 72.9 and 72.2 kJ/mol, respectively. In these glass compositions with similar Li-cation concentration, the differences in the mobility predominantly depend on structural arrangement of building units and the spatial distribution of negative potentials, as reflected in the value of H R / f , i.e. the Haven ratio divided by the correlation factor, as a mean to better understand the diffusion mechanism in glass structures, where vacancy vs. interstitial diffusion cannot be clearly defined. For Li–phosphate almost unconstrained Li-migration was indicated with the H R / f value of 0.98, while Li–Mg–phosphate had the most structural constraint on mobilized Li-cations, with the H R / f value of 0.30. Findings are compared with silicate (Li 2 O·3 SiO 2 ) and aluminosilicate (Li 2 O·Al 2 O 3· 4 SiO 2 ) glasses from our previous studies in order to elaborate the effect of network topology.

Lithium borate

Phosphate glass

Isotopes of lithium

10.1515/zpch-2016-0927

Cite

Citations (10)

Lithium Diffusion in Oxide Glasses – Bulk Materials vs. Powders

Diffusion fundamentals. (2017)

Harald Behrens Anna‐Maria Welsch I. Horn Michael Binnewies Martin Lerch

10.62721/diffusion-fundamentals.28.922

Cite

Citations (0)

Self-Diffusion of Lithium in LiAlSi₂O₆ Glasses Studied Using Mass Spectrometry

The Journal of Physical Chemistry A (2011)

Anna‐Maria Welsch Harald Behrens I. Horn Sebastian Roß Paul Heitjans

In order to improve our understanding of the transport mechanisms of lithium in glasses, we have performed diffusion and ionic conductivity studies on spodumene composition (LiAlSi(2)O(6)) glasses. In diffusion couple experiments pairs of chemically identical glasses with different lithium isotopy (natural Li vs pure (7)Li) were processed at temperatures between 482 and 732 K. Profiles of lithium isotopes were measured after the diffusion runs innovatively applying femtosecond UV laser ablation combined with inductively coupled plasma mass spectrometry (LA ICP-MS). Self-diffusion coefficients of lithium in the glasses were determined by fitting the isotope profiles. During some of the diffusion experiments the electrical conductivity of the samples was intermittently measured by impedance spectrometry. Combining ionic conductivity and self-diffusivity yields a temperature-independent correlation factor of ~0.50, indicating that motions of Li ions are strongly correlated in this type of glasses. Lithium self-diffusivity in LiAlSi(2)O(6) glass was found to be very similar to that in lithium silicate glasses although Raman spectroscopy demonstrates structural differences between these glasses; that is, the aluminosilicate is completely polymerized while the lithium silicate glasses contain large fractions of nonbridging oxygen.

Self-diffusion

10.1021/jp209319b

Cite

Citations (22)

Fe-oxidation state in alkali-trisilicate glasses - A Raman spectroscopic study

Journal of Non-Crystalline Solids (2017)

Anna‐Maria Welsch Jaayke L. Knipping Harald Behrens

FERRIC IRON

10.1016/j.jnoncrysol.2017.04.033

Cite

Citations (21)

Li Diffusion and the Effect of Local Structure on Li Mobility in Li₂O–SiO₂ Glasses

The Journal of Physical Chemistry B (2013)

Ute Bauer Anna‐Maria Welsch Harald Behrens Johanna Rahn Harald Schmidt

Aimed to improve the understanding of lithium migration mechanisms in ion conductors, this study focuses on Li dynamics in binary Li silicate glasses. Isotope exchange experiments and conductivity measurements were carried out to determine self-diffusion coefficients and activation energies for Li migration in Li2Si3O7 and Li2Si6O13 glasses. Samples of identical composition but different isotope content were combined for diffusion experiments in couples or triples. Diffusion profiles developed between 511 and 664 K were analyzed by femtosecond laser ablation combined with multiple collector inductively coupled plasma mass spectrometry (fs LA-MC-ICP-MS) and secondary ion mass spectrometry (SIMS). Analyses of diffusion profiles and comparison of diffusion data reveal that the isotope effect of lithium diffusion in silicate glasses is rather small, consistent with classical diffusion behavior. Ionic conductivity of glasses was measured between 312 and 675 K. The experimentally obtained self-diffusion coefficient, D(IE), and ionic diffusion coefficient, D(σ), derived from specific DC conductivity provided information about correlation effects during Li diffusion. The D(IE)/D(σ) is higher for the trisilicate (0.27 ± 0.05) than that for the hexasilicate (0.17 ± 0.02), implying that increasing silica content reduces the efficiency of Li jumps in terms of long-range movement. This trend can be rationalized by structural concepts based on nuclear magnetic resonance (NMR) and Raman spectroscopy as well as molecular dynamic simulations, that is, lithium is percolating in low-dimensional, alkali-rich regions separated by a silica-rich matrix.

Self-diffusion

10.1021/jp408805e

Cite

Citations (26)

High‐Energy Mechanical Treatment Boosts Ion Transport in Nanocrystalline Li₂B₄O₇

Journal of the American Ceramic Society (2016)

Dominik Wohlmuth Viktor Epp Bernhard Stanje Anna‐Maria Welsch Harald Behrens

In many cases fast solid ion conductors are characterized by a large number fraction of defects and vacant positions that enable the ions to move over long distances in a facile way. The introduction of structural disorder via high‐energy mechanical impact represents a very promising possibility to improve and to tune the transport properties of otherwise poorly conducting solids. Lithium tetraborate, Li 2 B 4 O 7 , in its single crystalline form or with an average crystallite size in the μm range, is known as a very poor Li ion conductor and can serve as a model compound to study the influence of structural disorder on ion dynamics. In the present study, we used high‐energy ball milling to prepare nanocrystalline defect‐rich Li 2 B 4 O 7 characterized by a mean crystallite diameter of ca. 20 nm. With increasing milling time the sample became partly amorphous. Polycrystalline Li 2 B 4 O 7 with crystallite sizes in the order of 100 nm served as starting material. The nanostructured samples obtained show dc conductivities σ dc in the order of 2.5 × 10 −7 S/cm at 490 K which represents an increase by more than four orders of magnitude compared to the source material. While conductivity spectroscopy was applied to study the effect of different milling times on ionic conductivity in detail; Li ion self‐diffusion in nanostructured Li 2 B 4 O 7 as well as in the starting material was investigated by variable‐temperature solid‐state 7 Li nuclear magnetic resonance ( NMR ) relaxometry. While the first is sensitive to long‐range ion transport, lithium NMR is able to access also short‐ranged ion motions.

Nanocrystalline material

10.1111/jace.14165

Cite

Citations (31)

Pressure-Induced Phase Transition inPbSc0.5Ta0.5O3as a Model Pb-Based Perovksite-Type Relaxor Ferroelectric

Physical Review Letters (2008)

Boriana Mihailova R. J. Angel Anna‐Maria Welsch Jing Zhao Jens Engel

We report pressure-induced structural changes in PbSc(0.5)Ta(0.5)O3 studied by single-crystal x-ray diffraction and Raman scattering. The appearance of a soft mode, a change in the volume compressibility, broadening of the diffraction peaks, and suppression of the x-ray diffuse scattering show that a phase transition occurs near pc approximately 1.9 GPa. The critical pressure is associated with a decoupling of the displacements of the B site and Pb cations in the existing polar nanoregions, leading to the suppression of B-cation off-center shifts and enhancement of the ferroic distortion in the Pb-O system.

Decoupling (probability)

Distortion (music)

10.1103/physrevlett.101.017602

Cite

Citations (45)