Mechanisms of Variation of the Unipolarity during Thermal Processing of Heavily Doped LiNbO 3 :ZnO Crystals
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Mechanisms of Variation of the Unipolarity during Thermal Processing of Heavily Doped LiNbO3:ZnO Crystals M. N. Palatnikova,*, V. A. Sandlera, N. V. Sidorova, and O. V. Makarovaa aTananaev Institute of Chemistry and Technology of Rare Elements and Mineral Raw Materials, Federal Research Center Kola Scientific Center, Russian Academy of Sciences, Apatity, 184209 Russia *e-mail: [email protected]
Received February 13, 2018; revised January 21, 2020; accepted February 4, 2020
Abstract—The phenomenological mechanisms for increasing the unipolarity under thermal processing in the short-circuiting conditions for heavily doped LiNbO3:ZnO crystals are analyzed by comparing with the temperature behavior of nominally pure LiNbO3:ZnO crystals with the congruent composition. It is shown that an increase in the unipolarity and, hence, the disappearance of the domain structure in heavily doped LiNbO3:ZnO crystals is initiated by thermal decomposition of charged polar clusters stabilizing domain walls. The decomposition of polar clusters is accompanied with an abrupt jumpwise injection of extra charge carriers (Li+ cations). As a result, the conductivity of LiNbO3:ZnO crystals at a temperature above 800 K is an order of magnitude higher than that of nominally pure LiNbO3 crystals with the congruent composition. This leads to the degradation of the domain structure in LiNbO3:ZnO crystals in contrast to LiNbO3 crystals with the congruent composition. DOI: 10.1134/S1063784220080149
INTRODUCTION Lithium niobate (LiNbO3) crystals have been the center of attraction of attention for specialists in integrated and nonlinear optics, acoustoelectronics, quantum electronics, and solid-state physics for many year [1, 2]. The interest in testing of heavily doped LiNbO3:ZnO crystals (about 4.0–9.0 mol % ZnO in the melt) is primarily due to its high resistance to optical damage [3]. It is known that LiNbO3 crystals contain so-called “stubborn” domains that do not vanish upon polarization reversal and have the form of wedge-shaped halfbubble domains with charged skew walls [4]. The monodomainization of grown lithium niobate crystals does not lead to ideal unipolarity; as a result, antiparallel stubborn domains can remain in bulk of a singledomain crystal. The monodomainization process involves cooling of the crystal in a dc electric field from temperatures slightly higher than Curie point TC ~ 1400–1450 K and close to the melting temperatures. In this case, the ionic conductivity of crystal in Li+ exceeds 10–4 (Ω cm)–1 and the LiNbO3 crystal actually experiences solid-phase electrolysis in the electric field. In these conditions, it is impossible to obtain electrodes that are complexly reversible in Li+; this gives rise to concentration gradients of the main components in the bulk of the crystal and, accordingly, to the formation of structure defects that can
stabilize the stubborn domain structure. In addition, polar clusters formed by impurity and intrinsic defects appear in heavily doped LiNbO3 crystals such as LiNbO3:ZnO
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