IL NUOVO CIMENTO VOL. 19 D, N. 7 Luglio 1997
Study of defects in Nasicon compounds
by thermoluminescence experiments
M. LUCCO-BORLERA(1), S. RONCHETTI(1) and M. SALIS(2)
(1) Dipartimento di Scienza dei Materiali e Ingegneria Chimica del Politecnico - Torino, Italy (2) INFM-GNSM, Istituto di Fisica Superiore dell’Università - Cagliari, Italy
(ricevuto il 23 Aprile 1997; approvato il 7 Luglio 1997)
Summary. — Samples of Nasicon compounds with different silicon and phosphorus contents were investigated with spectrally resolved thermoluminescence. Evidence of the presence of charged point defects originated by exchanges of silicon and phosphorus ions lying in different cells was obtained. Other defects are ascribed to displacements of sodium ions in unoccupied sites.
PACS 78.60 – Other luminescence and radiative recombination.
PACS 61.72.Ji – Point defects (vacancies, interstitials, color centers etc.) and defect clusters.
PACS 78.60.Kn – Thermoluminescence.
Nasicon compounds Na11xp42xZr2SixP32xO12(p standing for a Na1 vacancy) have
been extensively studied in the last two decades because of their good Na1 ionic
conductivity which make them interesting solid electrolytes for applications in high-temperature Na/S batteries, as well as in sodium potentiometric sensors [1-3]. The anionic framework consists of SiO4and PO4tetrahedra linked by shared oxygen ions to
ZrO6octahedra (see fig. 1). These tetrahedra are structurally equivalent and their real
distribution in the lattice is statistical [4]. The anionic negative charge is compensated by sodium cations lying in a sublattice of the crystal. Actually, sodium cations partially occupy two kinds of sites, that is, Na(1) and Na(2) sites which are coordinated with six and eight nearest oxygen ions, respectively [2]. In the range 1.8 GxG2.4, the symmetry of the Nasicon structure is monoclinic (SG C2Oc) at room temperature and rhombohedral (SG R3–c) above 423 K [5]. In the range 0 GxE1.8 and 2.4 ExG3, rhombohedral symmetry was found already at room temperature. Jumping between two adiacent Na1 sites is made possible through channels formed by the edges of
neighbouring polyhedra. The opening of channels connecting Na(1) to nearest Na(2) sites becomes maximum for x 42, which corresponds to the highest conductive Nasicon phase [2].
Owing to their different charges, exchanges of Si14and P15 ions among tetrahedra
belonging to different neutral cells localize in the lattice point-charges of opposite
Fig. 1. – Unit cell of Nasicon crystal structure. For the sake of simplicity, the figure lacks some tetrahedra. The central Na(1) and the nearest Na(2) sites are represented by white and shaded circles, respectively. Other sodium sites are omitted.
signs. Analogous defects were already encountered in aluminosilicates, in which exchanges take place between Si14 and Al13 ions [6]. Keeping this state of affairs in
mind, and in order to get information useful in characterizing defects in Nasicon compounds, we performed thermoluminescence (TL) experiments on x 40, x42 and
x 43 phases. On exciting these materials at room temperature with ionizing radiations,
pairs of electrons and holes are injected into the conduction and valence bands and become trapped by the defects of opposite-sign charges. When crystals thus activated are heated, the electrons or holes thermally released from traps recombine, thus originating TL emission.
Samples with x 40 (a), x42 (b) and x43 (c) were prepared through a sol-gel technique, by means of the preliminary formation of an amorphous solid (xerogel) [7, 8]. The latter was obtained by the following procedure. Three solutions were utilized. Solution A: a suitable volume of tetraethylorthosilicate was added to an equal volume of ethanol and half a volume of distilled water. The pH of the solution was adjusted to 0.5–1 with HNO3 (1 M). Solution B: stoichiometric quantities of NaNO3 (BDH
Chemicals, purity D 99.5%) and of zirconium dinitrate oxide (Ventron, 45.68% of ZrO2)
STUDY OF DEFECTS IN NASICON COMPOUNDS BY THERMOLUMINESCENCE EXPERIMENTS 951
Fig. 2. – Contour plot of the spectrally resolved thermoluminescent emission of sample a.
solution A while stirring until a clear phase was obtained. Solution C: a suitable portion of (NH4)2 HPO4 (Merck, purity D 99.5%) was dissolved in distilled water and then
added to the (A + B) solution prepared as above. In this way, a dense gel was obtained, which was first dried at 105 7C (while stirring) and then cautiously heated in air up to 400 7C, thus removing excess water, alcohol and gaseous products formed by nitrate decomposition. On examination with X-ray diffraction, the products thus prepared were found to be amorphous. By submitting samples to simultaneous DTA and TG analyses, crystallization temperatures were assessed to lie in the 680–800 7C range. Therefore all samples were treated at 1200 7C for one hour so as to get complete crystallization.
The samples thus prepared were irradiated for one hour at room temperature by soft X-rays (Cu Ka) so as to achieve saturation of the TL yield. Their TL emissions were recorded in standardized conditions for temperatures rising from 300 K to 680 K and within a spectral range from 380 nm to 720 nm, utilizing an apparatus operating under vacuum [9]. Warming speed was 1 Ks21. Figures 2 and 3 show, by means of
contours, the TL emissions of samples a and b, respectively. The sample-a emission takes place in a temperature range from 350 K to more than 500 K and with wavelengths above 720 nm. The TL yield, that is, the TL intensity integrated over the entire temperature and wavelength ranges, is 200 arbitrary units. Sample b shows an emission with a maximum at 400 K and 600 nm, accompanied by an emission at the same temperature and wavelength exceeding 720 nm. The TL yield is 90 arbitrary units. The
Fig. 3. – Contour plot of the spectrally resolved thermoluminescent emission of sample b.
TL emission of sample c is very faint (TL yield 10 arbitrary units) and appears drowned in the noise. For this reason, and for the sake of brevity, it is not presented here.
The 400 K and 600 nm peak, which characterizes the TL emission of sample b, is connected with the presence of both kinds of SiO4 and PO4 tetrahedra. Thus, as
expected, it may be ascribed to P15-Si14 ion exchange defects. In fact, this peak is not
present when only one kind of tetrahedra exists. As for the red emission, a plausible explanation is based on the displacement of sodium ions in vacancies existing in the cation sublattice. In fact, in the compound with x 40, only one sodium site out of five is occupied. This allows an easy displacement of sodium ions towards vacancies lying in different lattice cells, thus originating charged defects active in the TL process. As x increases, owing to a reduced vacancy density, the displacements become less frequent. This explains why the TL emission above 720 nm is reduced in sample b and becomes negligible in sample c. Of course, other kinds of defects may intervene in originating the red emission, such as interstitial oxygen ions. Owing to their double negative charge, oxygen ions can trap holes deeper than the single negative charges which appear, when Na1 ions are displaced. This, indeed, allows for recombination levels
nearest to the conduction band. Interstitial oxygen ions may be introduced into the lattice during the crystallization process at high temperature. As for the electron traps, attention is to be focused on the fact that the positive charge of defects can be identified either with the charge of
[
P(
( 1 O2) O)
4]
11 or Na1 ions. Analogously, holeSTUDY OF DEFECTS IN NASICON COMPOUNDS BY THERMOLUMINESCENCE EXPERIMENTS 953
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