Project Details
Projekt
Crystal Growth and Basic Properties of Lithium Niobate - Lithium Tantalate Solid Solution Crystals
Applicant
Dr. Steffen Ganschow
Subject Area
Synthesis and Properties of Functional Materials
Term
since 2020
Project identifier
Deutsche Forschungsgemeinschaft (DFG) - Project number 426703838
The present proposal describes a subproject for the continuation of the Research Unit 5044 'Periodic low-dimensional defect structures in polar oxides', which is dedicated to the correlation of defect structure, electron and ion transport as well as electromechanical properties using the model system lithium niobate-lithium tantalate (LiNb1-xTaxO3, LNT). This subproject is focussed on investigations of the growth of LNT single crystals from the melt. In the first funding period, the phase diagram of the LiNbO3-LiTaO3 (LN-LT) system was studied by means of thermoanalytical methods and a thermodynamically consistent phase diagram was created. The investigations were confined to mixtures of the congruently melting compositions of the end members LN and LT. In the second funding period, the focus is on the controlled manipulation of the defect landscape of the solid solutions. Two approaches are being pursued to this end: (1) Variation of the lithium content of the crystals by growing (a) from melts with excess lithium and (b) from melt solutions using K2O as a solvent. In both cases (a) and (b), the underlying phase diagrams are not yet known and must be determined as part of a fundamental investigation (thermal analysis, crystallisation experiments, X-ray diffractometric analysis). (2) Doping the solid solution with Mg. It is known from numerous studies on LN that Mg ions are incorporated into the Li site and thus reduce the number of Nb ions on Li sites ("antisites"). The relationship between solid solution composition and the incorporation of Mg into them is investigated in crystal growth experiments. The composition of the crystals and the distribution of the components are determined using optical emission spectrometry (ICP OES) and X-ray fluorescence analysis (XRF). Emphasis is also placed on the controlled generation of periodic structures in the solid solutions. These are generated by periodic fluctuations in the growth rate by shifting the crystal rotation axis relative to the thermal axis ("off-centered Czochralski"). As a result, the effective distribution coefficient of components (Nb, Ta) and dopants (e.g. Y) is subject to periodic fluctuations, and a striation pattern is formed in the crystal, to which ferroelectric domains, for example, can align. Alternatively, the limits within which the diffusion boundary layer in front of the growth front can be affected by an electric field applied between the crystal and the melt are being investigated. It is expected that, in addition to diffusion, electromigration also contributes to the transport of species in the boundary layer and thus an applied alternating field also leads to the formation of striations. Due to the expected small distance between the striations of a few micrometres at most, scanning electron microscopy is used as an additional method of analysis.
DFG Programme
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