Local and Translational Motion of the 6 Li and 7 Li Ions in the Li 3 Sc 2 (PO 4 ) 3 and Li 3 In 2 (PO 4 ) 3 Compounds
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LOCAL AND TRANSLATIONAL MOTION OF THE 6Li AND 7Li IONS IN THE Li 3Sc 2(P04)3 AND Li 3In2(P04)3 COMPOUNDS , SERGEI E. SIGARYOV , ANDREI A. VASHMAN IGOR S.PRONIN * Institute of Crystallography USSR Academy of Sciences, Leninsky pr.59, Moscow 117333, USSR All-Union Scientific Research Institute of Inorganic Materials, ul.Rogova 5, Moscow 123060, USSR
ABSTRACT Basing on impedance and NMR spectroscopy data it is shown that there are two types of lithium ion motion in the each polymorhic modification of the Li 3 Sc 2 (P0 4 ) 3 and Li 3 In 2 (P0 4 ) 3 solid electrolytes. First type is a translational motion of these ions which determines ion motion the a values, while the second one is Li within the lithium ion-ion pairs at the distance about the sum of two lithium ionic radii. Substitution of 7Li by 6Li leads to the mixed behaviour of a.
alkali effect-type
INTRODUCTION Investigation of features of ion transport in crystals allows one to obtain important data on the nature of superionic state. In this sense a joint studies of the same samples by impedance and NMR spectroscopy are very useful to determine the characteristics both translational and local ion motion. This paper present results of lithium ion transport process 3-
the three-dimensional framework [M2 P3 0 2 133 the isostructural [1,21 compounds Li 3 Sc 2 (P0 4 )3 and Li 3 In 2 (PO
of
investigation in
4
)3
(M = Sc,In).
EXPERIMENTAL All the samples under consideration were prepared
by conven-
tional solid state synthesis [3]. Substitution of 7Li by 6Li was made by using 6Li enriched (up to 88 at%) Li 2 CO3 as an initial reagent instead
of 7Li
2
C0 3 one.
Conductivity
values were obtai-
ned for ceramic pellets (8+12 mm in diameter, 2+4 mm thick) from the analysis of the immittance plots of the C/solid electrolyte/ /C electrochemical cells. The measurerements were carried out in range 5+5 105 Hz at temperatures 290+700 K. The 6,7Li NMR spectra were recorded in the temperature range from 200 K to 575 K and obtained by Fourier transformation of "solid echo" signal following the 900 - r - 900 pulse sequence. x y the frequency
The durations of the 90°- pulses were selected on the basis the condition of observation of the maximum amplitudes of the signal
Mat. Res. Soc. Symp. Proc. Vol. 210. 01991 Materials Research Society
586
(10.0 ps for 6Li and 4.5 ps for 7Li). The pulse delay T of 20 tts was used in the "solid-echo" sequence. The phase cycling of the first pulse was carried out to minimize artefacts that are appeared after the second pulse in the "solid-echo" sequence. The spin lattice
relaxation time,
TI
of 7Li
was measured
in
the temperature range 300+575 K using the "saturation-recovery" technique with a variable interval t between the end of the series of saturating pulses and start of "solid-echo" sequence. RESULTS AND DISCUSSION 1.
of
Li
3
Sc 2 (PO
4
)3
Fig.l shows the temperature dependences of ionic conductivity the 6 Li3Sc2(P04)3 and 7Li 3 Sc 2 (P0 4 ) 3 compounds [4].
4@0J 0.5 El E 0
-3.5
V) I--
-7.5
-11.5
1 1.30
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