Electrical and electromechanical properties of stoichiometric lithium niobate at high-temperatures
Highlights
► Electrical and electromechanical determination of lithium niobate ► The electrical conductivity is dominated by electrons at pO2 < 10− 5 bar. ► The ionic conductivity shows a linear dependence of σi = 6.3 ⋅ 10− 4 + 0.022[VLi]. ► Lithium ion diffusion via lithium vacancies at 0.2 bar is verified. ► Oxygen vacancies formation of [V••o]/[LiNbO3] < 5.2 ⋅ 10− 5 under reducing atmosphere.
Introduction
Lithium niobate (LiNbO3, LN) is one of the most important electro-optical and nonlinear optical single crystals. Its large pyroelectric, piezoelectric, electro-optic and photoelastic coefficients make it a key material for many different technical applications. LN has a broad homogeneity range with compositions from congruent (cLN, 48.35–48.6 mol.% Li2O [1], [2]) to stoichiometric (sLN, ~ 50 mol.% Li2O [3]). The congruent composition leads to an intrinsic defect structure that is accepted to be dominated by lithium vacancies [4], [5], [6], [7] and corresponds to the formula [LiLi]1 − 5x[NbLi]x[VLi]4xNbNbO3 (lithium vacancy model). This defect structure controls key physical properties. At temperatures above 300 °C the congruent material degrades [8] and is, therefore, inappropriate for high-temperature applications. In contrast, our preliminary tests of the stoichiometric crystals indicated that the material is stable up to at least 900 °C [9]. It is suspected that these crystals contain less lattice defects. However, the defect chemistry is not yet understood. While cLN crystals are grown from a melt of congruent composition by the conventional Czochralski method, several attempts have been made to prepare crystals with stoichiometric composition. For thin samples the vapor transport equilibrium (VTE) method can be used to get stoichiometric crystals. For thicker crystals, however, extremely long diffusion times have to be applied [1]. Large stoichiometric single crystals can be grown from a Li-rich melt by the recently developed double-crucible Czochralski method with an automatic powder supply system [10] or by the top-seeded solution growth (TSSG) method from potassium containing flux [11].
The objective of this work is to investigate electrical and electromechanical properties of several LN samples with different compositions grown by the methods described above to get a better understanding of the defect chemistry and transport mechanisms in sLN.
Section snippets
Materials and methods
Several single crystalline wafers grown by the methods described in Section 1 are purchased. Samples are prepared by cutting wafers into plates with X-cut and Z-cut orientation. Subsequently, the samples are polished and keyhole-shaped platinum electrodes are deposited by pulsed laser ablation for electromechanical characterization and circular platinum electrodes are deposited by screen printing for conductivity measurements. The samples vary in composition and crystal orientation, see Table 1
Experiment
The experiments are performed in a furnace where the oxygen partial pressure (pO2) can be controlled by passing an Ar/H2 gas mixture into the furnace. For precise temperature and pO2 measurement a thermocouple and an yttrium-doped zirconia sensor, respectively, are used. The pO2 is adjusted by an oxygen ion pump. Thereby, measurements up to 900 °C and at pO2's from 10− 14 bar to 0.2 bar are realized. Two types of experiments are performed. First, the electrical properties are analyzed by AC complex
Low oxygen partial pressure/electronic conductivity
The conductivity of sample 3 (stoichiometric) is determined at 900 °C as a function of pO2 in the range of 10− 12 bar to 0.2 bar (see Fig. 1). The trend of the electrical conductivity is similar to that presented by Smyth [7] and Jorgensen [15] found for cLN. Both of them presented a pO2 dependent conductivity σ ~ pO2m with m = − 1/4. Our investigation exhibits a slope of m = − 0.22 ± 0.01 and, therefore, a value of m = − 1/4 is conceivable as well as a value of m = − 1/5. In both cases we can assume that the
Conclusions
The electrical and electromechanical behaviors of lithium niobate as function of crystal composition, temperature and oxygen partial pressure have been investigated. Using the Arrhenius diagram of the temperature dependent total conductivity the activation energy is calculated. These values show a decrease from cLN to sLN. For the oxygen partial pressure dependence it is found at 900 °C and pO2 < 10− 5 bar that the electrical conductivity of sLN is dominated by electrons. At high pO2 the diffusion
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