Solid State Ionics 42 ( 1990) 223-226

North-Holland

Electrical conductivity due to ammonium ion transport in ( NH4) 3 [ MF6 ] (M:Al, Ga, In) and ( NH4) 2K [ Al& ] crystals

Yoshihiro Furukawa, Ayako Sasaki and Daiyu Nakamura Department of Chemistry, Faculty ofScience, Nagoya University, Nagoya 464-01, Japan

Received 20 March 1990; accepted for publication 3 1 May 1990

The electrical conductivity c of (NH,), [AIF and (NH4)?K[AlF6] was measured from room temperature to ca. 400 K by an

ac complex impedance analysis. The conductivity of each complex obeyed the Arrhenius relation, uT=unexp( -&/RT). The CJ

value at 320 K and the I?, value are 3.0~ 1O-4 and 29 and 1.0~ 10e6 S m-’ and 69 W mol-‘, respectively, in the order given

above. ‘H and 19F NMR second moment and spin-lattice relaxation time measurements on (NH4)s[AlF6] confirmed that the

electrical conduction is attributable to the self-diffusion of NH: ions. The results obtained are compared with those of

( NH,)4 [GaF,] and (NH,), [ InF,] recently reported. The high CJ values for these complexes are related to their crystal structures and anionic reorientational motions.

1. Introduction

Recently, we studied ionic motions in ( NH4)3 [ GaF,] and ( NH4)3 [ InF,] crystals by means of the temperature dependence of ‘H and 19F NMR second moment Mz and spin-lattice relaxation time T, [ 11. In the study, it was revealed that the NH: and [ MF6] 3- ions undergo reorientational motions below and near room temperature, respectively, whereas above room temperature translational self- diffusion of the NH.$ ions is activated. The occur- rence of the cationic diffusion was confirmed by electrical conductivity 0 as high as an order of low4 S m-’ at ca. 350 K. The avalue obtained for the gal- lium complex is larger than that of the indium com- plex at a given temperature, suggesting that the cat- ionic conduction in (NH,), [ MF,]-type complexes depends on the ionic radius of M3+ ion or the space available for the NH: ions in crystals.

In this study, we measured the temperature de- pendence of 0 in (NfL)3[AlFel and ( NH4)2K[ AlF6] crystals in order to investigate an- ion size effect and also mixed ion effect on 0 in the (NH,),[MF,]-type complexes. For (NH4)3[AlF6], the temperature dependences of the spin-lattice re- laxation times, T, and T,,, in the laboratory and ro- tating frames, respectively, and the second moment

0167-2738/90/$03.50 0 1990 - Elsevier Science Publishers B.V. ( North-Holland )

M2 of ‘H and 19F NMR absorptions were measured to get some information on ionic motions. Except for the indium complex the complexes given above have the cubic Fm3m structure at room temperature [ 2,3 1. The indium complex is known to possess the Fm3m structure above 353 K [ 21.

2. Experimental

( NH4) 3 [ AlF6] was crystallized by mixing hydro- fluoric acid solutions separately including NH: and [ AlF6] 3- ions [ 41. The crystals obtained were iden- tified by X-ray powder diffraction. The sample of ( NH4)*K[ AlF6] was the same as that used in our previous experiments of ‘H and 19F NMR [ 5 1.

The g measurements were carried out on pressed pellets by the ac complex impedance analysis from 0.1 to 100 kHz [ 11. NMR absorption curves were recorded on a JEOL JNM-MW-40s spectrometer operated at 40 MHz. T, and T,, of ‘H and 19F NMR in ( NH4 ) 3 [ A1F6 ] were determined on a Bruker SXP 4/ 100 spectrometer by the usual pulse sequence [ 41. The temperatures were measured with a copper-con- stantan thermocouple to 2 1 K.

224 Y. Furukawa /Ammonium ion transport in (NH,),[MF], and (NH,),[AIFJ crystals

3. Results and discussion

3.1. NMR second moment and spin-lattice relaxation time in cubic (NH,),[AlFd

Fig. 1 shows the temperature variation of M2 ( ‘H) and M2 ( 19F) of ‘H and i9F NMR absorptions, re- spectively, above room temperature. At 295 K, the values of M2( ‘H) and M2( 19F) were 3.7 and 2.3 G2, respectively. By referring to the theoretical second moment calculations for (NH,), [ InF6] [ 11, the small M2 values at 295 K indicate that both NH: and [ AlF6] 3- ions perform rapid overall reorienta- tions. With increasing temperature, M2( ‘H) sharply decreased to 0.6 G2 at ca. 360 K and further to 0.05 G2 at 440 K. On the other hand, M2( 19F) decreased gradually to 0.9 G2 in the same temperature range as above. A plateau of M2( ‘H) and M2( 19F) was sug- gested to exist in a narrow temperature range around 370 K. These results are very similar to those ob- tained for the gallium and indium analogs and in- dicate that above room temperature the transla- tional self-diffusion of the NH: ions is excited in two steps whereas the anions or the fluoride ions do not perform self-diffusion [ 11.

Fig. 2 shows the temperature dependence of T, and T,, of ‘H and 19F nuclei in cubic (NH,), [AIF ( T> 22 1 K) [ 41. With increasing temperature, T, of both nuclei increased, reached maxima near 350 K, and decreased on further heating. The log T,, values of both nuclei decreased linearly with decreasing 1 / T above ca. 250 K, and T,, (‘H) was shorter by a factor of ca. 2 than T,, ( 19F) at a given temperature. T, below 350 K is dominantly governed by the NH: and/or [A1F613- ionic reorientation, and T, above 350 K as well as T,, above 250 K is clearly assignable to translational diffusion of the NH: ions on the basis of the foregoing M2 results. The slope of the log T,, versus 1 /T curves above 250 K of the both nuclei yielded an activation energy E, of ca. 26 kJ mol-’ for the cationic self-diffusion.

3.2. Electrical conductivity

The 0 measurements were carried out above room temperature. The results obtained in the present study as well as those of (NH,), [GaF,] and (NH,), [ InF,] in our previous study [ 1 ] are shown

Fig. 1. Temperature dependence of the second moments of ‘H

and 19FNMR absorptions for (NH4),[AIF6]: (0) ‘Hand (0) 19F.

400 300 T/K

I” ’ I I 1

/

0.:

0 no== . (NHJJAIFGI

.

2 3 4

103K/ 1

Fig. 2. Temperature dependence of T, and T,, of ‘H and 19F NMR

in the high-temperature cubic phase of ( NH4)3 [ AIF,]: (0 ) and

(A ) ‘H T, at 60 and 20 MHz; (0) and (A ) 19F I”, at 56.44 and

18.81 MHz; (m) and (0) ‘Hand 19F T,, at the rfstrength H, of

10 and 10.6 G, respectively.

in fig. 3, where log aT is plotted as a function of l/ T. Above ca. 400 K, the ~7 values observed for these complexes became less temperature-dependent, maybe, because of partial decompositions [ 6,7]. Hence, the data above 400 K are not given in fig. 3.

The avalue of (NH,)3[AlF6], being 1.4~ low4 S

Y. Furukawa /Ammonium ion transport in (NH,),[IWF]~ and (NHJ,[AIFd crystals 225

2.5 3.0 103K/ T

Fig. 3. Electrical conductivity u of (NH4)3[MF6] (M=Al, Ga,

In) and (NH4)2K[AIF,]. Values of log u Tare plotted against

1 /T. Discontinuities of the log aTversus 1 /T curve for the M = In

complex are due to the phase transitions already reported [ 1,2].

m-’ at 300 K, increased monotonically with increas-

ing temperature. From the log aT versus 1 /T curve, the activation energy E, and ao(ao=oT as T-co) were determined to be 29 kJ mol-’ and 5.0~ lo3 S m- ’ K, respectively. The E, value determined from the 0 measurements agrees fairly well with that for the NH,+ ionic diffusion determined from the ‘H and i9F T,, measurements. Therefore, the electrical con-

duction in this complex is attributable to the trans- lational diffusion of the NH: ions.

(NH,),[GaF,] yields 0 of 4x lo-’ S m-’ at 300 K. A change in temperature coefficient is observed near 350 K in the log aT versus 1 /T curve of this complex. From the slope of the curve, apparent ac- tivation energies are deduced as 37 and 41 kJ mol-’ below and above 350 K, respectively [ 11. For these complexes, it can be shown from the comparison be- tween the observed and calculated second moments

of ‘H and 19F NMR that the diffusion of the NH: ions occupying the tetrahedral (8~) sites in the Fm3m lattice is activated at relatively lower tem-

peratures than the NH: ions at the octahedral (4b) sites [ 11. Under the assumption that the (TT versus 1 /T curve observed is a superposition of two ex- ponential terms, E, of the NH: (8~) and NH: (4b) ionic diffusion can be estimated as 32 and 44 kJ mol-‘, respectively.

(NH,),[ InF,] exhibits two phase transitions in the temperature range studied [ 1,2]. When this complex is heated from room temperature, the (T value increases discontinuously at each transition temperature and the 'E, value of 0 in each phase de- creases drastically as 100, 64, and 38 kJ mol-’ for the room-, intermediate-, and high-temperature phase, respectively.

The (T value observed for (NH4)3[AlF6], (NH,), [GaF,], and the cubic phase of ( NH4)3 [ InFs ] decreases in that order or as their molar volume increases. The cubic to non-cubic phase transition temperature reported for these com- plexes increases as the molar volume increases [ 2 1,

Table 1

Activation energy E. and pre-exponential factor us for electrical conductivity in the cubic phase of (NH,),[MF,] (M=AI, Ga, In) and

(N&)&lAtF61.

Compound & (kJ mol-‘)

Temp. range

(K)

Us=’

(S m-’ K)

(NfL)3[AlF61 29 3 1 O-400 5.0x 10s

(NH,),[GaF,l ‘) 32 300-350 2.1x109

44 350-400 2.4x IO5

(NH&ItnF,l b, 38 353-400 2.4x IO4 (NI-L)zKlAlF61 69 310-400 7.0x 10’

a> uT=q,exp( -EJRT).

b, Ref. [I].

226

10-3

7

5 \ :

D

10-Q

Y. Furukawa /Ammonium ion transport in (NH,),[MFJ6 and (NH,)dA[AIF,J crystals

. : (NH.,)sIAIF,I

n : (NH,)a[GaFel

A :(NH&[lflF,l

o :(NH&K[AIF~l 0

I I

200 300 400

Tmin/K Fig. 4. Correlation between electrical conductivity u4w at 400 K

and Ti minimum temperature T,,,,, due to the anionic reorienta-

tion in (NH,),[MF,] (M=AI, Ga, In) and (NH,),K[AlF,].

indicating that the stability of the cubic phase in- creases with decrease of the molar volume. Then, ~7 in the present complexes seems to be related to sta- bility of crystal packing of the Fm3m lattice.

The K+ ions in the (NH4)2K[AlF6] crystal are known to occupy the octahedral (4b) sites in the Fm3m unit cell [ 31. The o value of this complex is the lowest among the complexes studied, indicating that the K+ ions introduced hinder the cationic self- diffusion. Also, A3 [ AlF,]-type crystals having Li+ or K+ ion as A ions show rather low 0 compared with the present ammonium complexes [ 8,9] _ Therefore, the high cationic conductivity observed in the pres- ent study is characteristic of the A = NH: complexes with the cubic Fm3m structure and also of occu- pancy of both the tetrahedral (8~) and octahedral

(4b) sites by NH: ions. It is noted that reorientational motion of the

[ MF613- octahedral anions takes place easily in (NH,),[MF,] crystals [ 1,4]. The T, minimum (20 MHz) due to the anionic reorientation is observed

at ca. 200, 230, and 330 K for (NH,),[MF,l (M = Al, Ga, In), respectively [ 1,4]. The TI mini- mum temperature Tmin for the same motion in (NH4)*K[AlF6] isca. 355 K [5], indicatingthat the anionic reorientation is rather hindered in compar- ison with those in the foregoing (NH,), [ MF6] CW-

tals but is much less hindered than those in the A3[AlF6] (A=Na, K) crystals [lo]. Fig. 4 shows the plot of a400, CJ extrapolated to 400 K, against Tmin

for the ammonium complexes. A strong correlation between 0400 and Tmin is recognized among

( NH4)3 [ MF6] crystals, and the replacement by K+ ions of NH: ions blocks the cationic self-diffusion as mentioned previously. This correlation can be understood by considering that the NH: ions can get more chance to jump to vacant nearest-neighbor sites through enlarged openings surrounded by fluor- ide ions during the transient of the [MF613- ionic reorientations. In the cubic structure of ( NH4) 3 [ MF6 ]-type crystals, the orientations of both the NH: ions should be disordered and the cation is possible to make many hydrogen bonds with neighboring fluoride ions. This unique situation for NH: ions, not expected for alkali-metal ions, sta- bilizes enthalpically and entropically the cubic lat- tice of ( NH4) 3 [ MF,]-type crystals [ 41, makes the reorientations of the octahedral anions easier [ 5 I, and enhances the cationic translational diffusion by

a paddlewheel action operative between the constit-

uent ions [ll].

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