Self-diffusion coefficient of lithium in molten xLi 2 O–(1  x)B 2 O 3 system using high-temperature PFG NMR Takahiro Ohkubo a,⇑ , Mallory Gobet b,c , Vincent Sarou-Kanian b,c , Catherine Bessada b,c , Muneharu Nozawa a , Yasuhiko Iwadate a a Graduate School of Engineering, Chiba University, 1-33 Yayoi-cho Inage-ku, Chiba 263-8522, Japan b CNRS, UPR3079, CEMHTI, 1D avenue de la Recherche Scientifique, 45071 Orléans cedex 2, France c Faculté des sciences, université d’Orléans, avenue du Parc-Floral, BP 6749, 45067 Orléans cedex 2, France article info Article history: Received 18 November 2011 In final form 30 January 2012 Available online 8 February 2012 abstract Lithium diffusion was measured in xLi 2 O–(1  x)B 2 O 3 melts as a function of temperature and composition by high-temperature pulsed field gradient NMR. The 7 Li self-diffusion coefficients were found to increase with increasing x when compared at the same temperature. It was of much interest that E a in the molten xLi 2 O – (1  x)B 2 O 3 system decreased monotonously with increasing Li 2 O content up to x = 0.4 and then increased at x = 0.45 and 0.5. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction The understanding of structural aspects of ionic conducting glasses and the correlation with their dynamic properties have progressed in the field of solid state ionics [1]. Their investigations have made it possible to develop new materials for specific appli- cations such as electrochemical devices. Boron oxide glasses desig- nated as the xLi 2 O–(1  x)B 2 O 3 system are well-known as stable solid-state materials containing conducting Li + over a wide range of x values [2,3]. Since the glass network structure in these materi- als is crucial for ionic conduction, much of the research has in- volved structural modeling of the boron network using Raman [4,5] and solid-state NMR [6–9] spectroscopies. These studies re- vealed that the diverse bonding structures of boron to oxygen are characterized by a three-dimensional network that strongly de- pends on the glass composition and the temperature. On the other hand, structural information regarding their melts at high temperature is also required in order to discuss the proper- ties of not only the melts but also the resulting glasses. Reported studies on alkali borate glasses and melts using high-temperature NMR [8] and Raman spectroscopy [5] showed that the fraction of BO  4 units decreases with increasing temperature. This observation was also confirmed by solid-state NMR on glassy samples prepared with different quenching rates [9]. However, to the best of our knowledge, Li + ions dynamics in the melts have not been investi- gated yet. In this Letter, we present the first measurements of lithium dif- fusion in xLi 2 O–(1  x)B 2 O 3 melts at different compositions and temperatures. All diffusion measurements were made using a recently developed high-temperature pulsed field gradient (PFG) NMR equipment [10,11]. 2. Experimental The starting materials were xLi 2 O–(1  x)B 2 O 3 glasses with x being 0.2–0.5, which were synthesized from reagent grade Li 2 CO 3 and H 3 BO 3 . These reagents were carefully mixed and transferred to a Pt crucible, and heated to 973 K for decarbonation. The tem- perature was then raised to 1073 K for 30 min to melt the mixture. The melt was poured onto a copper plate in air. All of the obtained glasses were homogeneously transparent. The weight loss after fu- sion was determined in order to control their composition. No lith- ium loss was detected, indicating a composition close to nominal composition. The obtained glasses were ground to powders for NMR measurements. The NMR measurements were performed in situ in the liquid phase from 1130 to 1260 K. The high-temperature PFG NMR spec- tra were recorded using a Bruker Avance WB 400 MHz spectrome- ter, operating at 9.4 T. The NMR probe is a 10 mm liquid probe equipped with z-axis gradient coil and adapted to high tempera- ture in CEMHTI (Orleans, France). Experimental details of high- temperature PFG NMR are available in previous papers [10,11], which have succeeded in good agreement with conventional tracer methods using radioactive isotope for diffusion measurements. In this Letter, we used Double-Stimulated-Echo pulse sequence to suppress convection artifacts [12]. At least eight gradients of line- arly increasing strength ranging from 0.5 to 50 gauss/cm were ap- plied. Gradient application time ranging from 1 to 5 ms and experimental diffusion time ranging from 80 to 100 ms were used. The 90° pulse width was 25 ls. When a recycling delay greater 0009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2012.01.076 ⇑ Corresponding author. E-mail address: ohkubo.takahiro@faculty.chiba-u.jp (T. Ohkubo). Chemical Physics Letters 530 (2012) 61–63 Contents lists available at SciVerse ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett