Physical properties of LiAlxFe1−xCl4 ionic conductors

Solid State Ionics 9 & 10 (1983) 103-106 North-Holland Publishing Company

PHYSICAL

103

PROPERTIES

OF L i A l x F e l . x C l 4

IONIC C O N D U C T O R S

Michel SPIESSER, Pierre PALVADEAU, Catherine GUILLOT, Jacky CERISIER Laboratoire de Chimie des Solides, L.A. 279, 2, rue de la Houssini~re - 44072 NANTES C@dex, France.

LiFeCl 4 and LiAICI 4 have been prepared by direct reaction of FeCI 3 and AICI 3 with LiCI. Crystallographic data show that both compounds are isostructural. For this reason it has been possible to obtain a solid solution LiAlxFel_ x CI4 with 0 < x 4 I. Physical properties and particularly ~lectrical conductivities are affected by the substitution of Fe 3+ by AI3+ . The conductivity is higher in disorded phases (for example LiAIo.25Feo.75CI4) than in the well-ordered phase (LiAIo.5oFeo.50CI4). LiFeCl 4 has been found to be antiferromagnetic. In solid solutions, the magnetic behaviour depends strongly upon the compositions, with the magnetic interaction decreasing with increasing x.

i.

INTRODUCTION

Ionic conductivity studies of alkali metal tetrachloroaluminates and tetrachloroferrates have been reported (1-4). Interesting features of these systems are their very low eutectie temperatures and high electrical conduetivities in the molten state. CI

It is well known that the creation of a structural disorder increases the conductivity. For this reason we have realised a solid solution LiAI x F e l _ x CI 4 and have studied the evolution of physical properties when iron is progressively substituted for aluminium. 2.

0

M

•

Li

PREPARATION AND X-RAY RESULTS

LiFeCI 4 and LiAICI 4 have been prepared by reaction of FeCI 3 and AICI 3 with LiCI according to the conditions described in a previous paper (4). The melting points of LiFeCI 4 (154°C) and LiAICI 4 (146°C) were checked by D.T.A. using indium as the standard (M-P = -156,6°). These results are in agreement with previous papers (4). Initial X-ray studies with a Debye-Scherrer camera showed a very bad crystallization of LiPeCl 4 and also that LiFeCI 4 is isostructural with LiAICI 4. Despite this bad crystallization, it has been possible to select single crystal of sufficient quality for study by the Bragg and Weissenberg methods. The measured crystal parameters and space group confirm the analogy of the two structures. The parameters are (in Angstroms and degrees) : for LiFeCI 4 [a = 7.02(i), b = 6.33(1), c : 12.72(4), 8 = 92°(30)] and for LiAICI 4 [ a = 7.007(3), b = 6.504(4), c = 12.995(i0), B : 93.32°(5)](5). The symmetry is monoclinic and the space group is P21/c. (Figure I) The phase diagram of the LiFeCI4-LiAICI 4 system has been studied by D.T.A. (Figure 2). It indicates the formation of a solid solution LiAlxFel_ xCl 4 with 0 < x< i. X-ray studies show

0 167-2738/83/0000-0000/$ 03.00 © 1983 North-Holland

I

@ y

Figure i : Crystal structure of L i F e C ~ (isotype of LiAICI4) projected along the b axis.

the regular evolution of parameters. The structure consists of [ M C ~ ] - tetrahedral anions in which the monovalent cation is octahedrally surrounded by six chlorine atoms belonging to four [MCI4]- tetrahedra.

3.

ELECTRICAL MEASUREMENTS

Since the sample are hygroscopic, electrical measurements have been performed in glass cells sealed under vacuum. Tungsten was used for blocking electrodes. Compounds were melted directly under vacuum in the cell to obtain good contact with the electrodes. Conductivity was measured by a.e. techniques using a Philips conductimeter at a 3000 Hz. The high temperature

M. Spiesser et al. / LiAlxFel_xC14 ionic conductors

104

TT

4°c

OT (~'!cm-1. K)

145°C ~ ' ~

'5

~'÷~ !

LiAICI 4

I

LiFeCI4

LiFeCI 4 I LiAlo,2sFeo,TsCI4

10J Figure 2 : The LiFeCI4-LiAICI 4 phase diagram.

-iA10,75Feo,25CI4 part of the curves have been checked with an alternating current source at 80 Hz. Figure 3 shows the log ~ T versus I/T. Below I05°C, follows an Arrhenius-type variation, with an activation energy of 0.78 eV. The conductivity increases rapidly above I05°C. In the liquid state, LiFeCI 4 and other compounds show a low activation energy (~ 0.15 eV) and high conductivity typical of molten salt electrolytes, as observed in LiAICI 4.

I

LiAIo,5Feo,sCI4 ~

LiAICI4 103/T ( K.1 )

I

I

I

2,2

2,3

2,4

2,5

2,6

2,7

Figure 4 : Conductivities in the high-temperature range measured by an a.c. technique

(8o Hz). Log(~.T.) ~ "lcm'l 2

4.

=0.16 eV

1 0 -1

-2

~

C M.P.

I. . . . . . . 2.5

"~.~= 3

0.78eV ~(lmlo3K 3.4

Figure 3 : Semi-logarithmic plot of the conductivity of LiFeCI 4.

Figure 4 shows the conductivities in the high temperature range for four compositions relative to LiAICI 4 . We observed that the conductivity is higher for LiFeo.25AIo.75CI 4 and LiFeo.75 AIo.2s CI 4. This is probably due to the disorder created by the replacement of one or three of the four [FeCla]tetrahedra surrounding the Li + cation by [AICI4]- . In contrast, the substitution of two tetrahedra in LiFeo.50 AIo.50CI 4 gives a ordered phase, and in this case to the conductivity is reduced.

NUCLEAR MAGNETIC RESONANCE

NMR spectra of lithium in L i F e C ~ and L i A I C ~ show a very pronounced narrowing of the linewidth at about I05°C and IO0°C, respectively. At lower temperature, a classical motional narrowing is observed, particularly in the case of LiAICI 4. LiFeCI 4 represents a more complex situation due to the paramagnetic ions which increase strongly the linewidth. Above this transition the linewidth (< 0.2 G) indicates that lithium is in a "liquid-like state". It is noteworthy that this situation occurs before the melting point (~ 150°C).

Line width (G) .Io(LiAICl4)

Line width(G) ( Li FeCI4) 201

~ X

• Li FeCI4 LiAtCI4

x

-5

0

~

10-

~ - - 'I - - 50

Figure

i

100

-:--x:---

I

T ( °C ) O.

150

5 : Lithium NHR spectra of LiFeCI 4 and LiAICI 4 (two different scales have been used to represent the linewidth)

M. Spiesser et al. / LiAlxFel_xC14 ionic conductors Spin lattice relaxation times (T 1 ) were measured for LiFeCI 4 using the classical - < - ~/2 sequence. The correlation time ( • c ) can be described by an Arrhenius law. The maximum of T~ I was found at 77°C. Between this temperature and I05°C it as been possible to estimate an activation energy. The value 0.9 eV is close to those obtained from electrical measurements. NMR measurements of the solid solution are under way to study the contribution of the paramagnetic ions.

5.

6. ~OSSBAOER

300 K

Susceptibilities were measured with a Faraday balance between 4.2 K and 500 K. All samples wore contained in evacuated sealed synthetic silica tubes. At higher temperatures (above 50 K) the susceptibilities are well described by a Curie-Weiss law. The effective magnetic moment for all compositions is very close to 5.92 B.M., which corresponds to the spin-only contribution of high-spin Fe 3+.

X

15(

~

LIAIO'2FeOJ3CI4

~A|o~3Fe07

SPECTROSCOPY

Initial measurements have been performed at 300 K and 7T K for several compounds. Table i summarizes the M~ssbauer data for five compositions.

MAGNETIC MEASUREMENTS

Figure 6 shows the X versus T curves in the low-temperature range (T < 30 K). X depends strongly upon the composition, with the magnetic interactions decreasing with increasing x . LiFeCI 4 is antiferromagnetic with a transition temperature T ~ = i0 K. In the solid solution LiFel_ x AIxCI4, for x 4 0 . 3 0 , the transition temperature depends on x.

105

77 K

~mm/s

Amm/s 6totals

%ds

LiFeC~

O. 20

0.51

0.31

0.54

LiFeo.5AIo.5CI 4

0.20

0,52

0.31

0.56

LiFeo.3AIo.7CI 4

O. 20

O. 52

0.31

0.58

LiFeo.2AIo.8CI 4

0.20

0.52

0.32

0.58

LiFeo. IAIo.gCI 4

0.42

0

0.53

0

isomer shift with respect to Fe 57 A quadrupole splitting Table I : M~ssbauer spectroscopy. Isomer shift and quadrupole splitting values are characteristic of iron III in a tetrahedral environment. In the last compound, LiFeo. I AIo. 9 CI4 , the situation is different. In this case the isomer shift is higher (0.42 mm/s), and there is no quadrupole splitting. This may be due to a dilution effect, because the paramagnetic ions are so far apart from each other that dipole or exchange coupling becomes negligible. This concentration dependence of spin-spin relaxation has been observed in dilute systems (like Fe in AI203 (6)).

CI4 It is also possible that in this compound the symmetry of the [FeCI4]- tetrahedra is higher. This problem is not totally solved (7).

LiAIo~FeEsCl4 x 7.

i lo

o Figure

6

i 20

i TIK)I~ 3o

: X versus T curves in the low temperature r a n g e (T < 30 K ) .

For x > 0.30, all compounds become paramagnetic. Magnetic studies are under way to study this transition in using MSssbauer and neutron diffraction spectroscopy.

CONCLUSION

If L i F e C ~ appears to be good ionic conductor, it is possible to increase it conductivity by substituting iron by aluminium, The most favorable composition seems to be LiFeo.25 AI0.75 C ~ . An interesting feature of these compounds is the considerable increase in the lithium mobility below the melting point. Other physical properties (eg., magnetic) are also affected by the substitution.

M. Spiesser et al. / LiAl xFe l_xC14 ionic conductors

106

REFERENCES -3 1

-2 [

-1 i

0 i

1 i

~ Fer'l~ 4 ni

."':

2 i

3 i

V(mm/s)

D

(i)

Weppner, W. and Huggins, R.A., J. Electrochem. Soc., 124 (1975) 35.

(2)

Weppner, W. and Huggins, R.A., Phys. Letters 58A (1976) 245.

(3)

Weppner, W. and Huggins, R.A., Solid State lonics i (1980) 3.

(a)

Palvadeau, P., Venien, J.P., Spiesser, and Rouxel, J., Solid State lonics 6

293K

"

=.

(1978)

/

LiFICI

4

• .

:S

LiAIo,9Feo,ICI

.

4

/.

........

:

293K

.

4

•

.[: .:

77 K

:...I -3

I -2

I -1

(6)

Wertheim, G.K., Remeika, J.P., Proc. XII colloque Ampere. Univ. of Leuven (1964), North Holland, Amsterdam 1965.

(7)

Palvadeau, published.

. :~7~,:'.--:::.,,:,*--.::~-.'-~.~

...

Li AIo,9Feo,ICI

Mairesse, G., Barbier, P. and Vignaoourt, J.P., Cryst. Struct. Comm. 6 (1977) 15.

'..,

,,.~.-; ~,:,:v.r.:.~.:.:,'::',.:~,,:t::

[ 0

231-236.

(5) 77K

: :,,

A I

I 2

o 3

VLmmlsl

Figure 7 : M~ssbauer spectra of LiFeCI 4 and LiFeo. IAIo.9CI4 •

M.

P. and Fatseas, G., to be