Acoustic loss and electromechanical properties of LiNb_(1-x)Ta_xO_3

• Applicant: Professor Dr.-Ing. Holger Fritze
• Subject Area: Synthesis and Properties of Functional Materials ; Experimental Condensed Matter Physics
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 426703838

Crucial high-temperature properties of LNT solid solutions such as bulk and domain wall conductivity as well as acoustic losses were determined as a function of temperature and oxygen partial pressure (pO2) and correlated with the atomistic transport processes. Above 400°C, the acoustic losses are governed by the relaxation of piezoelectrically excited charge carriers and thus the electrical conductivity, so that their reduction is a fundamental requirement for high-quality piezoelectric resonators. Below this temperature, the losses decrease and reach values that correspond to that of phonon scattering. The electronic conductivity tends to be suppressed by high Ta contents, which becomes apparent above 600°C and allows a reduction in losses. High mechanical resonance frequencies also lead to a reduction in losses, so that small structures or even thin films are desirable. The pO2 dependence of the conductivity can be explained by a defect mechanism that is not linked to the unwanted evaporation of Li2O. Fundamental findings, such as the unexpectedly strong change in the activation energy of the electrical conductivity at the transition between the ferroelectric and paraelectric phase, are now also available. Furthermore, domain wall currents at temperatures up to 400°C are determined and interpreted. In the second project phase, a further understanding of the electroacoustic properties will be obtained, particularly with regard to thin films and the incorporation of dopants. Mechanical stresses represent a further possibility for tailoring the defect structures in LNTs. As mechanical stresses can be varied much more in films than in bulk crystals, the question arises to what extent conductivity, phase transformation, poling behavior and the stability of the interfaces are influenced and how this correlates with the bulk properties. Furthermore, the charge transport in domain walls of bulk crystals and films will be determined with the objective of tailoring it. Long-term perspectives for conductive low-dimensional structures in devices can be assumed. The work is carried out from an experimental perspective in close cooperation with TP2, 3 and 4 with regard to charge transport and with TP5 and 6 in the area of domain walls. For example, conductivity data are provided and transport coefficients determined there are used for modeling. Furthermore, the exchange with TP1 and 9 on the preparation of crystals and films as well as the modeling in TP8 contribute significantly to the knowledge gained. Special investigation methods such as resonant ultrasound spectroscopy, laser Doppler vibrometry and micro-impedance spectroscopy are used up to a temperature of at least 1000°C.

• DFG Programme: Research Units
• Subproject of: FOR 5044: Periodic low-dimensional defect structures in polar oxides