Electronic and Ionic Transport Mechanisms of Stoichiometric Lithium Niobate at High-Temperatures
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Electronic and Ionic Transport Mechanisms of Stoichiometric Lithium Niobate at HighTemperatures Anke Weidenfelder1, Michal Schulz1, Peter Fielitz2, Jianmin Shi3, Günter Borchardt2, KlausDieter Becker3 and Holger Fritze1 1 Clausthal University of Technology, Institute of Energy Research and Physical Technologies, Am Stollen 19 b, 38640 Goslar, Germany 2 Clausthal University of Technology, Institute of Metallurgy, Robert-Koch-Str. 42, 38678 Clausthal-Zellerfeld, Germany 3 Braunschweig University of Technology, Institute of Physical and Theoretical Chemistry, Hans-Sommer-Str. 10, 38106 Braunschweig ABSTRACT The electrical and electromechanical properties of lithium niobate single crystals are investigated at high-temperatures. The total electrical conductivity is determined as a function of temperature by impedance spectroscopy for Z-cut crystals with different lithium content. For stoichiometric lithium niobate (sLN) the activation energy is found to be (1.49 ± 0.03) eV in the temperature range from 500 to 900 °C. Further, the piezoelectric properties (resonance frequency, Q-factor) of X-cut lithium niobate crystals are determined at high temperatures for samples with compositions ranging from congruent to stoichiometric and, subsequently, compared to the conductivity data in order to identify loss contributions. In this context, the high-temperature stability is examined for X- and Z-cut samples with compositions ranging from congruent to stoichiometric. Series of samples with and without additional alumina protection layers are annealed in air at 900 °C for approximately 50 h. Subsequently, depth profiles are measured by SNMS. In all cases, no lithium loss is observed and, therefore, a high-temperature stability of sLN for at least 50 h at 900 °C can be assumed in ambient air. Further, the influence of protective layers with different thicknesses and compositions is investigated for X- and Z-cut samples. A lithium loss in the first 300 nm is observed for the Zcut samples, while the X-cut samples show a behavior dependent on the type of protecting layer. INTRODUCTION Lithium niobate (LiNbO3, LN) single crystals posses an unusual richness of physical properties. This ferro-, pyro-, and piezoelectric material shows large electrooptic, accoustooptic, and photoelastic coefficients as well as strong photorefractive and photovoltaic effects which make it an important material for many different technical applications. LN is commonly used in the congruent composition (cLN) with a lithium to niobium ratio of Li/(Li+Nb)~0.48. The data for LN that can be found in the literature is predominantly determined for congruent crystals at room temperature. This is due to the temperature limitation of cLN to 300 °C. At temperatures above 300 °C, the congruent material starts to degrade [1] and is, therefore, inappropriate for long-term applications. In contrast, the stoichiometric crystals exhibit improved high-temperature stability up to at least 900 °C [2] which makes them potentially applicable for high-temperature devices.
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