Thermal Stability and Application Limits of LiNb_(1-x)Ta_xO_3 solid solutions

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

The present subproject aims to determine the influence of lithium content on the high-temperature properties of lithium niobate-tantalate (LNT) solid solutions and to evaluate their application limits. In the first phase, it was established that the electrical conductivity of nearly stoichiometric LNT up to about 600-650 °C is mainly determined by Li-ion conduction, similar to Li-deficient LNT. Above this temperature, the electronic contribution is evident. At 900 °C, the electronic conduction mechanism dominates in stoichiometric LiNbO3, while ionic conductivity dominates in Li-deficient LiNbO3. The electronic contribution decreases with increasing Ta content. These findings are practically relevant as they suggest that the conduction type in LNT can be influenced by adjusting the Li stoichiometry and the Nb/Ta ratio. Furthermore, the long-term stability of LNTs was investigated by measuring the electrical conductivity and resonant frequency during extended periods of uninterrupted thermal treatment in air. For example, it is shown that the resonance frequency of the LiNb0.5Ta0.5O3 sample fluctuates by only ± 100 ppm during the 350 hours at 700 °C. In the second phase of the project, particular attention will be paid to the effects of doping on the high-temperature stability of LNT crystals. In this context, dopants such as Mg are expected to reduce the conductivity of LNTs and consequently increase the stability of the material. Another focus is on the stability of LNT thin films and thin film systems under extreme temperatures and conditions. In addition, it must be determined how the dopants influence the thin films in terms of high-temperature stability. Furthermore, the influence of Li stoichiometry on the high-temperature stability of electrical, acoustic, optical, and structural properties of single-crystalline LNT as a function of temperature, time, and oxygen partial pressure will be further investigated. A special focus is on the domains and domain walls in LNT. It must be determined up to what maximum temperature the domain structures remain stable with variable Li stoichiometry and a given Nb/Ta ratio. Another important aspect to be investigated is how the Li2O expansion influences domain stability. The project includes close collaboration with other subprojects involving sample preparation (TP1, TP9), transport (TP2, TP4, TP6, TP7), domain investigation (TP5, TP6, TP7), and theoretical modeling (TP8). The findings from this subproject will help deepen our understanding of the thermal stability and application limits of LNT solid solutions and could provide important impetus for the development of future applications in high-temperature sensor technology and related fields.

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