Electrical Conductivity and Dielectric Permittivity of Directly Doped LiNbO 3 :Zn,Mg Crystals in the Temperature Range 4
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trical Conductivity and Dielectric Permittivity of Directly Doped LiNbO3:Zn,Mg Crystals in the Temperature Range 450–900 K M. N. Palatnikova, *, V. A. Sandlera, N. V. Sidorova, I. V. Biryukovaa, and O. V. Makarovaa aTananaev
Institute of Chemistry and Technology of Rare Elements and Mineral Raw Materials (Separate Division), Kola Scientific Center Federal Research Center, Russian Academy of Sciences, Apatity, Murmansk oblast, 184209 Russia *e-mail: [email protected] Received December 25, 2019; revised January 30, 2020; accepted February 10, 2020
Abstract—The electrical conductivity σ of a series of multidomain LiNbO3:Zn,Mg crystals prepared via direct doping in the composition ranges ~1 ± 0.02 mol % MgO and ~2.8–4.6 mol % ZnO has been shown to undergo a sharp increase at T* ≈ 800 K, accompanied by anomalies in temperature dependences of their dielectric permittivity, ε(T). The largest increase in their static bulk conductivity near T* has been observed in a narrow composition range (≈1 ± 0.02 mol % MgО and ≈3.9–4.3 mol % ZnO). Above T*, the LiNbO3:Zn,Mg crystals have an activation enthalpy for ionic conduction (Ha ≈ 1.76–2.15 eV) and enthalpy of transport (Hm ≈ 1.66–2.06 eV) unusually high for cation conductors, whereas below T* both quantities are characteristic of Li+ ion conduction in LiNbO3 crystals (На ≈ 1.15–1.35 eV and Нm ≈ 1.0–1.1 eV). The anomalous increase in Ha and Hm is caused by the formation of associated vacancies (divacancies) and correlated pair hopping of Li+ ions above T*. Keywords: crystals, lithium niobate, direct doping, ionic conductivity, dielectric permittivity, dielectric dispersion DOI: 10.1134/S0020168520080129
INTRODUCTION Lithium niobate (LiNbO3) crystals continue to attract great interest of experts in integrated and nonlinear optics and acoustoelectronics [1–4]. Research interest in LiNbO3:Zn and LiNbO3:Mg crystals is aroused by their high optical damage threshold and the possibility of utilizing them in the planar technology of optical converters [5–8]. At the same time, the absorption edge of LiNbO3:Zn,Mg crystals is shifted to shorter wavelengths and they have substantially increased nonlinear optical coefficients, which has generated considerable research interest in these crystals [9, 10]. In a study of temperature-dependent dielectric properties of multidomain LiNbO3:Zn crystals, the electrical conductivity of z-oriented samples was found to undergo an anomalous, sharp increase in the range T* ~ 760–810 K [11]. Similar temperature behavior of electrical conductivity was observed in the case of LiNbO3:Zn crystals after a multidomain to single-domain transition [12–16]. The jump in conductivity is accompanied by a considerable increase in the degree of unipolarity of the LiNbO3:Zn crystals [11, 14–16]. Note that, in the case of conversion to a sin-
gle-domain state, the residual domain structure undergoes breakdown [14–17]. The sharp rise in electrical conductivity is observed in the composition range ~5.4–6.8 mol % ZnO in the melt [18], near the main concentration threshold
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