Journal of Crystal Growth 306 (2007) 406–412 Partitioning of ionic species and crystallization electromotive force during the melt growth of LiNbO 3 and Li 2 B 4 O 7 Shinji Koh à , Satoshi Uda, Xinming Huang Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, Miyagi 980-8577, Japan Received 27 January 2007; received in revised form 26 April 2007; accepted 3 May 2007 Communicated by R. Feigelson Available online 22 May 2007 Abstract The dependence of the crystallization electromotive force (EMF) on the Li 2 O composition of the melt in the growth of LiNbO 3 (LN) and Li 2 B 4 O 7 (LB4) crystals was measured using a micro-pulling-down (m-PD) method. The crystallization EMF developed during the growth of LN crystals continuously decreased with increase in Li 2 O composition. The eutectic compositions in the Li 2 O–Nb 2 O 5 system were found to be peculiar compositions, at which the development of the crystallization EMF was prevented owing to no degree of freedom in the Gibbs phase rule. It was found that no segregation of the ionic species occurred in the LB4 growth from the melt with the stoichiometric composition, which suggested that the equilibrium partitioning coefficients for all the chemical species in the stoichiometric LB4 melt were unity. r 2007 Elsevier B.V. All rights reserved. PACS: 64.70.Dv; 73.50.Lw; 81.10.Fq; 81.30.Fb; 84.37.+q; 87.50.Rr Keywords: A1. Electric fields; A1. Interfaces; A2. Micro-pulling-down growth; B1. Borates; B1. Lithium compounds; B1. Niobates 1. Introduction Partitioning of the chemical species in the melt to the solid phase is generally discussed in terms of the equilibrium relationship. However, if ionic species are present in the melt, partitioning is not determined only by the phase diagrams because the phase diagrams do not include information on partitioning of the ionic species. In the liquid phase of oxide materials, such as LiNbO 3 (LN) and Li 2 B 4 O 7 (LB4), several ionic species exist, resulting in ionic conductivity of the melt [1–4]. The ionic species are partitioned with their own equilibrium partitioning coeffi- cients, k 0 , and are accumulated or depleted at the growth interface according to the k 0 , the diffusivity D and the growth rate, which results in the formation of net ionic charge distribution around the growth interface. In our previous study [5], we investigated an electrical phenomenon, i.e., development of crystallization electromo- tive force (EMF) at the growth interface of congruent LN (c-LN), which was first experimentally suggested by D’yakov et al. in 1985 [6]. A micro-pulling-down (m-PD) method was exploited to measure the intrinsic electric potential distribu- tion. By systematic experiments, the Seebeck coefficients for the liquid and solid phases of c-LN were determined to be 0.2370.03 and 0.7170.04 mV/K, respectively, and the growth-rate dependence of the crystallization EMF was evaluated. The mechanism of the crystallization EMF during the growth of c-LN from the melt was explained using a model, wherein redistribution of the ionic species in the melt formed a net ionic charge distribution at the growth interface resulting in the development of an EMF. The nonlinear dependence of the crystallization EMF on the growth rate was analyzed on the basis of a one-dimensional differential equation for the m-PD growth that included electric-field- driven transport in the melt [7,8]. The use of the m-PD method allowed us to carry out one-dimensional analysis ARTICLE IN PRESS www.elsevier.com/locate/jcrysgro 0022-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2007.05.008 à Corresponding author. Tel.: +81 22 215 2102; fax: +81 22 215 2101. E-mail address: koh@imr.tohoku.ac.jp (S. Koh).