An analysis of the temperature dependence of the spontaneous polarization of LiNbO 3 crystals
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ICAL PROPERTIES OF CRYSTALS
An Analysis of the Temperature Dependence of the Spontaneous Polarization of LiNbO3 Crystals R. I. Shostak, S. V. Yevdokimov, and A. V. Yatsenko Taurida National University, Simferopol, Ukraine e-mail: [email protected] Received October 13, 2008
Abstract—The temperature dependence of spontaneous polarization and the primary pyroelectric coefficient of stoichiometric lithium niobate crystals are calculated and the specific features of structural ordering of lithium niobate in the ferroelectric phase are analyzed using a modified electrostatic model. The reasons that experimentally observed anomalies of the primary pyroelectric coefficient occur are discussed. PACS numbers: 77.70.+a, 77.84.Dy DOI: 10.1134/S1063774509030195
INTRODUCTION Ferroelectric lithium niobate (LiNbO3) crystals are a very popular object of study due to their wide practical application. Despite the abundance of information obtained to date, many effects present in LiNbO3 crystals are still insufficiently studied. Such phenomena include the instability and anomalies of optical, piezoelectric, and pyroelectric properties [1–4] which are observed in the practically important temperature range from 300 to 400 K; the mechanisms of their formation are not completely clear. Moreover, the fundamental questions of the nature of structural phase transition and the specific features of structural ordering of cation sublattices of LiNbO3 in the ferroelectric phase are still debated [5, 6]. Additional information on the noted questions can be obtained by simulating the temperature dependence of the spontaneous polarization P0(T) of a crystal and comparing the obtained results with the known experimental data on the pyroelectric properties of LiNbO3. The value of P0(T) in ferroelectrics is estimated by different methods: for example, on the basis of analyzing the temperature dependences of the spontaneous strain of the crystal unit cell [7], the difference in the refractive indices [8], or the coefficient of second-harmonic generation (SHG) [9]. However, the most informative calculations of P0(T) can be performed on the basis of ab initio simulation of the structure [10] or using a fairly complete electrostatic model of the corresponding crystal [11]. The purpose of this study is to analyze the specific features of structural ordering of LiNbO3 in the ferroelectric phase and the causes of the anomalies of the primary pyroelectric coefficient. The analysis is performed on the basis of the electrostatic model from the
results of calculating the temperature dependence of P0 and the potential profile in stoichiometric crystals. CALCULATION The standard expression describing spontaneous polarization in the displacive ferroelectrics has the form 1 P 0 = --V
N
∑ ( q d + p ), i
i
i
(1)
i
where V is the unit-cell volume of the crystal, N is the number of ions per unit cell, qi is the effective charge of the ith ion, di is the displacement of the ith ion with respect to the nonpolar position, and pi is the induced electric dipole momen
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