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 Z-cut samples, while the X-cut samples show a behavior dependent on the type of protecting layer.
Stoichiometric Lithium Niobate / Transport Mechanism / Electromechanical Properties / Oxygen-18 Tracer DiffusionZ-and X-cut lithium niobate samples with varying lithium content from 48.3 to 50.0 mol % are investigated at high temperatures. Electrical and electromechanical properties are obtained by impedance spectroscopy. Mixed ionic and electronic conductivity in the temperature range from 500-900 • C is found to be thermally activated. For 700 • C and 1000 • C the diffusion coefficient of lithium ions is calculated. The resonance frequency and the inverse Qf product are determined as a function of temperature up to 900 • C for shear and thickness mode vibrations. In addition, the tracer diffusion of 18 O is investigated by SIMS/SNMS. The 18 O depth profile is analyzed and the lithium concentration dependent oxygen diffusion coefficient at 950 • C for 49.9 mol % Li 2 O is determined to be D O ≈ 3 × 10 −17 m 2 /s.
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