Crystal Structure and Electrical Properties of La_0.45Ce_0.1Li_0.27TiO₃ Synthesized by Sol-Gel Technique

Néstor Fernández, Purificación Escribano, Eloisa Cordoncillo, Héctor Beltrán, Mario F. García-Sánchez, Issis C. Romero-Ibarra, Nahum Masó

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DOI: 10.4236/njgc.2012.21009                

Abstract

The lithium ionic conductingLa_0.45Ce_0.1Li_0.27TiO₃ has been synthesized by sol-gel method. This solid is the result of substitutional doping with Ce(IV) in La_0.45Ce_0.1Li_0.27TiO₃ compound. The aim of the replacement of La(III) by Ce(IV) is increase the number of vacancies in the structure and favors the ionic mobility. Structural characterization shows that the obtained material have the expected tetragonal P4/mmm perovskite structure. Chemical analysis shows that composition was homogeneus in all the sample. The bulk conductivity measured at room temperature is about the same as previously reported for its related lanthanum lithium titanate. However, the lower activation energy for ionic conduction encourages further searching for better conductors in this system.

Keywords

Lithium Ion Conductors; Sol Gel; Impedance Spectroscopy; Perovskite

1. Introduction

Fast lithium ion conductors with the general formula RE_0.66–xLi_3xTiO₃ (RE: La, Pr, Nd, Sm) have been intensively studied since the last decade [1-9]. The materials were prepared mainly by ceramic methods and showed perovskite-like crystal structure. The presence of RE(III) in the perovskite A-site causes the existence of vacancies in the crystal structure and allows lithium ion to move through the solid leading to a DC conduction. The maximum bulk conductivity found at room temperature in this kind of solids was 1.1 × 10^–3 S·cm^–1 for lanthanum lithium titanate system (LLTO) La_0.66–xLi_3x5_1/3–2xTiO₃ with x = 0.11, which results is the better compromise between number of vacancies and lithium ions [2,3]. In this equation, 5 represents the number of vacancies. Substitution of other RE for A-site La always results in lower bulk conductivity. This fact has been related to the shortening of cell parameters found as a result of the smaller substitute cation ionic radius [9]. Many other related systems were prepared by substitutional doping for RE or Ti [10-12]. As a rule, none of these related solids improves the maximum conductivity value previously reported.

The accepted conduction mechanism consists on hopping lithium ions from its lattice site to adjacent vacancy across the so called “bottleneck” formed by four TiO₆ octahedra [8-10]. The reduction of cell parameters shrinks the bottleneck and raises the activation barrier for ionic movement. Conductivity values for these solids depend on the number of ionic charge carriers and the number of vacancies as well.

Crystal radii for Ce(IV) and La(III) in 12-coordination are reported by Sannon as 1.28 Ả and 1.50 Ả respecttively [13]. This value is in the borderline of the usually accepted 15% size difference limiting range for substitution without disrupting the structure [14]. It follows that Ce(IV) could substitute La(III) in LLTO A-site. Substitution of Ce(IV) for La(III) creates additional vacancies and must facilitate the ionic diffusion movement through the solid and improves its conduction properties, despite of the expected bottleneck shrinkage effect.

The sol-gel method preparation for precursor powders was chosen owing to its known lower reacting and sintering temperature and time. In addition, the materials synthesized by this method have uniform particles size and shape. These features are expected that improve the total material conductivity [15,16].

This paper discusses the preparation of Ce(IV) substitutional doped perovskite-like solid La_0.45Ce_0.1Li_0.27TiO₃ via sol-gel and its structural and electrical characterization. This solid could be considered as a member of the hypothetical series La_0.66–x–yCe_0.75yLi_3xTiO₃ with x = 0.09 and y = 0.133. This material have the same lithium content that the equivalent material without Ce (i.e. La_0.57 Li_0.275_0.153TiO₃), but the number of vacancies is about 20% higher (0.187).

2. Experimental Section

The synthesis was carried out by sol-gel technique. 5 ml ethanol (99.8% Scharlau), 16.74 g Ti(isoO-Pr)₄ (98% Strem Chemicals) and 6.07 g acac (99.5% Panreac) were placed in a beaker with continuous stirring. Thereafter, another solution with 8 ml ethanol, 11.11 g La(NO₃)₃. 6H₂O (99.9% Strem), 3.16 g (NH₄)₂[Ce(NO₃)₆] (99.9% Strem) and 1.07 g LiNO₃ (99% Strem) was added to the previous one. The solvent was evaporated during three days below IR lamp and the obtained gel was thoroughly dried in the oven at 100˚C. The dried gel was ground up in aghata mortar, placed into alumina crucible and heated at 2˚C/min up to 500˚C. This temperature was maintained for 2 h to complete the decomposition of organic precursors in the sample. The precursor powder obtained was ground up, pressed into 13 mm disk pellets and placed in Pt crucibles. The sample was covered with isocompositional powder and closed with Pt foil in order to prevent evaporation of lithium compounds [12,15,17- 19]. The sample was heated for reacting and sintering at 15˚C/min up to 1100˚C, at which temperature stood for 2 hours. Then it was slowly cooled to room temperature during about 10 hours.

Phase identification was done by X-ray diffraction (XRD) in a Siemens D-500 diffractometer using Cu K_a radiation. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) analysis were performed in an Oxford Instrument LEO 440i microscope. AC electrical measurements were performed from room temperature up to 300˚C in air using an HP 4192 Impedance analyser over the frequency range 100 Hz-13 MHz. Samples for electrical measurements were prepared from the pellets as they came out from synthesis, coated with gold paste and assembled between Pt electrodes. Dimensions of samples were measured with 0.01 mm precision caliper and they were weighted with 0.0001 g precision.

3. Results and Discussion

Figure 1 shows the XRD pattern obtained from solid sample. The obtained peaks were indexed according to PDF 87 - 936 with reports parameters of P4/mmm tetragonal perovskite-like structure. Similar results were obtained in compounds without cerium synthesized by sol gel [16]. The phase tetragonal is usually found in compounds obtained by this method due to the low tem-

Conflicts of Interest

The authors declare no conflicts of interest.

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