Ag+ β aluminas by means of the paramagnetic probe Ag2+

Solid State Ionics 9 & 10 (1983) 267-272 North.Holland Publishing Company

267

STUDY OF Ag + AND MIXED Na+/Ag + ~ ALUMINAS BY MEANS OF THE PARAMAGNETIC

PROBE Ag2 +

Didier Gourier and Daniel Vivien

Laboratoire de Chimie Appliqu4e de l'Etat Solide (LA 302), ENSCP ii rue P. et M. Curie, 75231 Paris Cedex 05, France 2+ Ag ions are created in mixed Na+/Ag + and pure Ag + 8 alumina by X-ray irradiation. Whatever the silver content of the crystals, Ag 2+ are always trapped by the interstitial oxygen ions responsible for the charge compensation of the excess of monovalent cations. The effect of Frenkel defects and the origin of the lack of silver hyperfine structure are discussed.

1.

INTRODUCTION

Sodium ~ and 8" aluminates are the superionic conductors which present at this time the greatest technological interest because of their application as solid electrolyte in sodiumsulphur battery. Although significant progress have been made for the realization of reliable sodium-sulphur cells, a number of factors controlling the durability of the cells are still not understood. It is now recognized that the degradation of 8 alumina during cell cycling is due to the propagation of sodium dendrites through the electrolyte tube (i). However the initiation stage of degradation remains a subject of investigations since a variety of factors have been recognized to influence the apparition of dendrites (i). Among them the Gontamination by foreign cations such K + and Ca z+ introduced in the electrolyte by ionic exchange during the cell cycling, have been found to play an important role in the electrolyte failure (2). These two ions are responsible for the so called mixed alcali effect (3, 4) in which the conductivity of the electrolyte passes through a minimum as the fraction of foreign cations is increased. However their localization in the structure at low levels of ionic exchange (a few %) remains unknown. Paramagnetic probes introduced in ~ alumina by ionic exchange may allow the study of the behaviour of foreign cations at low level of contamination. This method has for instance been used to follow the disappearance of the Frenkel defect during the transformation of non stoichiometric alumina into the stoichiometric one (5), the paramagnetic probe being Mn 2+ ions located in tetrahedral sites of spinel blocks. Another example is the study of local motion of conducting cations around F + centers located at 05 positions in the conduction planes (6). We report here the use of Ag 2+ (4d 9) ions as paramagnetic probes to study pure Ag + ~ alumina and mixed Na+-Ag + 8 alumina, which exhibits a mixed alkali effect very similar to those of Na+-K + ~ alumina. The investigation of these divalent cations in ~ alumina will give an insight into the reason which makes ~ alumina a very poor divalent cation conductor while ~"

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

alumina is a rather good one (4). An ESR study of Ag 2+ in Czochralski grown ~ alumina single crystals partially exchanged by Ag + has already been published (7). The main features of this work are the following : -i- The divalent silver ions are created in non stoichiometric mixed Na+-Ag + 8 alumina under irradiation by X-rays from a copper target tube. They are located near the interstitial oxygen ions responsible for the charge compensation of the excess conducting ions. -ii- Crystal field disorder affects the linewidth as found earlier in our study of Cu 2+ doped ~ alumina (8). -iii- No Ag 2+ ions could be created in pure Ag 2+ alumina. 2.

EXPERIMENTAL

Single crystals of sodium ~ alumina with composition 8.5 A1203, Na20 and with suitable size for ESR experiments (about 2 x 2 x 0.5 mm 3) were synthesized by slow cooling from the melt following the usual procedure (9). Mixed Na+/Ag + and pure Ag + 8 alumina single crystals were prepared by ionic exchange (IO) for various times in AgNO 3 at 6OOK. The crystals were washed in distilled water to remove the residual nitrate and then heated at 9OOK for 24 hours prior to irradiation. One experiment was performed on a stoichiometric Ag + ~ alumina single crystal, with composition close to ii A1203, Ag20 prepared according to the Colomban's procedure (ii). Ag 2+ ions were produced by X-ray irradiation of these crystals at room temperature, with 45 kV, 6 mA X-rays from tungsten and copper target tubes. After irradiation, the crystals were immediately cooled at 14OK in the ESR spectrometer to avoid recombination of Ag 2+ ions. X-band ESR spectra were recorded at temperatures ranging from 14OK to 3OOK with a JEOL ME 3X spectrometer. Magnetic field and microwave frequency were calibrated respectively by using a BRUCKER BNM 12 NMR proton probe and a wavemeter. 3.

RESULTS

Ag 2+ ions were produced in all the samples under irradiation with X-rays from the tungsten tube.

268

D. Gourier, D. Vivien / S t u d y o f Ag + and mixed Na+ /Ag + ~aluminas

However, when using a Cu target tube, no paramagnetic Ag 2+ centers were observed in pure Ag + alumina while Ag 2+ could be created in mixed compounds with silver content up to about 95 % Ag +. Even in small quantities in the conduction planes, Na + ions appear to be necessary for the creation of Ag 2+ ions with low-energy X-rays. The intensity of the ESR spectra did not vary markedly with silver composition of the crystals, but was strongly enhanced by a factor of 5 to iO when heating the crystals at 90OK prior to X-ray irradiation. When the magnetic field B o is colinear to the crystallographic c axis, the ESR spectra of Ag 2+ in all the compounds consist of a single line, 2.3 to 2.5 mT width, without any resolved structure. The main features of the Ag 2+ spectra appear when the magnetic field rotates in the aa* plane. In that case the spectra consist of two sets of three lines (Figure i) whose field position exhibit a z/3 periodicity (Figure 2). This behaviour is very similar to those of Cu 2+ ions in Na + 8 alumina (8). In mixed Na+/Ag + ~ alumina single crystals in the composition 5 %- 95 %, two Ag 2+ centers hereafter called I and II are observed. An increasing Ag + content of the crystal resulted in an increase of the intensity of the Ag 2+ (1) centers with respect to those of Ag 2+ (II) center. In Ag + ~ alumina, one observes also two Ag 2+ centers, hereafter called I' and II', slightly different from those of the mixed crystals. These two centers were obtained only in pure Ag + ~ alumina, in which their formation needs the W short wavelength X-rays. The principal values of the g tensors and the corresponding linewidths for the four Ag 2+ centers are gathered Table I. The orientation of the principal axes of the g tensors with respect to the crystallographic axes were determined by Late X-ray d i f f r a c t i o n patterns. The values found for mixed Na+/Ag + crystals in this study are very similar to those found for mixed crystals obtained from Czochralski boules (7). From the principal g values, which exhibit a large (2.24 2.27) And two small (2.O1 - 2.07) components, one can infer that the unpaired electron lies in a predominantly 4 dx2-y 2 orbital (12). The m o s t striking feature of these spectra is the absence of hyperfine (h.f.) structure which should arise from the two I = i/2 silver isotopes iO7Ag (52 %, gN = -0.226) and 109Ag (48 %, gN = 0.260). From the data of Table I, it follows that the h.f. parameters must be smaller than half the linewidth, i.e. A i (i = x, y, z) < i mT. Measurements at Q band have shown that the linewidths are mainly due to crystal field disorder (7), thus confirming that the h.f. parameters are close to zero. This behaviour appears very surprising if we consider ESR results on Ag 2+ complexes (12) or Ag 2+ in KCI (13) and in alkaline oxides (14), in which h.f. parameters ranging from 2 to 4 mT have been observed. Furthermore, for Ag 2+ in zeolites (15), with the same dx2-y 2 ground state as for ~ alumina, the largest h.f. component is A z = 3.7 mT. The absence of h.f. coupling for Ag 2+ in ~ alumina,

i

IOrnT

%

i

m

A0

I

I A0 li

[ A0 I'

I A0 II'

Figure 1 9.2 GHz ESR spectrum at room temperature of X-ray irradiated ~ alumina single crystals. The magnetic field B o is perpendicular to the c axis and makes an angle of 15 ° with the a axis a) Na + ~ alumina exchanged in molten AgNO 3 for 2 minutes b) pure Ag + ~ alumina. particularly when B o is colinear to the a cristallographic axis (z axis of the g tensor) appears then clearly anomalous. Let us consider the expression for the h.f. parameters of a d 9 ion in axial symmetry, with a dx2-y 2 ground state (16) : .

Az

=

P

Ax =Ay

4

[-K-~

2

3

-~(ge-gx)-(ge-gz)]

226 = P [ - K + ~ -~(ge-gx ) ]

[i]

where ~2 is a covalency parameter which depends on the degree of electron delocalisation on the ligands, and where the dipolar coefficient P is defined as : P = ge gN ~ ~N 4d

[2]

Three terms are involved in the expression of

269

D. Gourier, D. Vivien / Study of Ag + and mixed Na+ /Ag + 13aluminas

Table I Spin hamiltonian

6 alumina

parameters

Center

gc

(Y)

ga

I

(Ag+-Na + )

and ESR linewidths (z)

2.2734

ga*

(in mT) of Ag 2+ in ~ alumina (x)

2.0239

2.0685 2.2355

I'

aBa,

2.7

1.7

2.011

2.0

1.6

2.0260 2.0682

1.8

2.4

t 2.3

2.246

II'

1.7

2.0062

i

I

AB a

2.5

II

Ag +

ABc

i

i

a)

a

~" 330 f

32O 310

300 290

the component A i of the h.f. tensor : -i- %~he -KP term, or isotropic hyperfine coupling (16) in which K is the Fermi contact constant which describes the inners' shell polarization by the d shell;~ -ii- The direct dipolar terms -# e2p and +~ ~2p; -iii- The pseudodipolar term which involves the deviation ag i = ge - gi of the gi value from the free electron spin ge value. In the case of rhombic symmetry, the expression [i] will be slightly more complicated because of some admixture of other 4d orbitals to the 4dx2-y 2 ground state. However whatever the site symmetry may be, the sum of the three h.f. components in [I] leads to the simple expression Ax+Ay+A z = P[-3K-(Agx+agy+Agz) ]

i

i

i

i

i

b)

o

340

330 320 310 300

0°

i

i

i

i

i

10 °

20 °

30 °

40 °

50"

Figure 2 Variation of the field position of the ESR lines when the magnetic field E o rotates in the aa* plane, squares : Ag 2+ (I) and (I'). circles : Ag 2+ (II) and (II'). a) mixed Na+/Ag + ~ alumina b) pure Ag + ~ alumina.

[3]

in which only the isotropic and pseudodipolar terms have non negligible contribution. If we take A i ~ O and IAil = aBi/2 as the limiting values for the h.f. parameters, one can estimate that K values are in the range -O.I0
4.

STRUCTURE OF THE Ag

DEFECT CENTER

4.1.

Structure of conduction planes of Ag + and + Na ~ alumina

Figure 3 shows the theoretical positions of Na + and Ag + in the conduction planes of pure sodium and silver ~ alumina. The site occupation is significantly different in these two compounds. First of all the occupation rate of aBR position is larger in silver ~ alumina (0.84) than in sodium ~ alumina (~ O.01) (18). An Ag + ion in the site has two first neighbours oxygen ions 04 at 2 . 4 1 A and three 05 at 2.78 A (18). Its coordination is then nearly 2 (linear). It is known that this coordination is particularly favored for the +i ions of the group IB metals (with d IO configurations) (19). We then suggest that there is a relation between the electronic configuration of Ag + ions and its preference for the aBR sites. In Ag + ~ alumina X-ray diffraction studies have shown that instead of

D. Gourier, D. Vivien / Study of Ag + and mixed Na+/Ag + 13aluminas

270

two well individualized BR and mO sites, as in Na + ~ alumina, there is a quasi-continuous electronic density between these two positions, with two weak maxima at positions called BRDI and BRD2, with occupation rates of 0.79 and 0.84 respectively (18). In BRD2 position, Ag + ion has two neighbours oxygen 05 at 2.61 ~ and 4 02 at 2.71 while in BRDI position it has 4 02 at 2.74 ~ and two O5 at 2.90 ~ (18). Sodium and silver ~ alumina have however in common a partial occupation of each mO site by interstitial oxygen ions which account for the charge compensation of the excess M + conducting ions (20). Furthermore, an important proportion of Frenkel defects exists in the spinel blocks. This Frenkel defect results from a displacement of one A13+ ion from its octahedral All position to a tetrahedral site delimited by three oxygen ions of the All coordination polyhedra and the interstitial oxygen in mO position (Figure 4). 4.2. Localization

of Ag 2+

Analysis of the g tensors allows a precise determination of the sites occupied by Ag2+ ions and their environment. First of all, one can notice that all Ag 2+ ions (I, If, I' and II') have nearly similar g values and have their gz tensor axis along the a crystallographic axis. Furthermore, their gz values are all consistent with a

/

k\

iI

~\

i/

c)/A~

mO , 11 "\

,~

\', \

II

"

I II

BR i/

,

\\ I('aBR l

\\ ~

11

/I

\

",,>,'

i

X \

(

)0 2Ag 2÷

I

',

% I

I ) Ag + ~..__i/

%

~\ x\ \\

~11 X

\\

X

I

X BRDIII ~

'<

.... \

/

5 I;

-'1/{/" "~, i1 ',,,'~ ',I V\\ // \\

/11--"%%

\

/ Xx

iI

\x\

~o = a 14dx2 - y2>

+ b

14dz2>

[4]

with a = 1 and b ~ i. A further lowering of symmetry (C 2 or C s symmetry) would lead to an admixture of 4dxy orbital into T o. The g shifts gi = gi - ge for a d 9 ion with an electronic ground state ~o given by [4] may be expressed as (21) :

~ \

\\

predominantly dx2-y 2 ground state. It follows that the four kinds of Ag 2+ centers correspond to minor modification in the environment of the same site. Since for a d 9 ion, the ground state orbital is the most destabilized by the crystal field, the dx2-y 2 lobes must point toward negative 02- ions along the a* and c crystallographic axes, which correspond to the x and y axes of the g tensor. As already pointed out by Barklie et al (7), this possibility arises only if the two ligands along a* are the oxygen in 05 position and one interstitial oxygen in mO position. The role of this oxygen is confirmed by the fact that we have failed to create Ag 2+ ions in stoichiometric Ag + ~ alumina (with approximate composition II A1203, Ag20 ) in which interstitial oxygens no longer exist. This Ag 2+ site, depicted figure 4, can be described as a distorted square planar arrangement of oxygen with C2v symmetry, in which two of the ligands, the oxygen ions O ~ are slightly displaced from the y axis of the g tensor. Two possible sites with exactly the same symmetry are available on each side of the interstitial oxygen along a* direction. These two sites, located near the normal BR and aBR positions, have their y axes pointing respectively toward 02 and 04 oxygen ions (Figure 4). In C?~ 7 symmetry, there is a mixing of the 4dx2-y and 4dz2 orbitals. Thus, the ground state wave function is :

,/ \ x,/ v

Figure 3 alumina. The Structure of the mirror planes of ions are drawn to size. a) Theoretical positions of Na + ions. b) Positions occupied by Ag + ions (see ref.18), c) Localization of Ag 2+ centers. Among the two possible sites on each side of the 02- defect, only the nearest from the normal position of Ag + ions have been represented on the figure.

2 1 Ea + ~ b 7 2 Agx = - Ayz

2

2~[~_~b~2 2

Agy = - ax---z 8 i Agz = - Axy

[s]

a 2 ~2

where Ayz, Axz and axy are the energy separation between the 4dx2-y 2 orbital and the 4dyz, 4dxz and 4dxy orbitals respectively. The 4dxz orbital lies entirely in the aa* plane. Since £gy is determined by Axz, one should expect it to be very sensitive to changes in the cationic and anionic environment of Ag 2+ in the mirror plane. However the four kind of Ag 2+ ions have the same gy values (Table I). It follows that they probably do not differ significantly in the neighbouring cations in the mirror planes. As a consequence the g factors cannot allow the distinction between the two possible sites on each side of the interstitial oxygen. The disti n c l o n between the four Ag 2+ involves their gx and gz values, which are mainly determined by the energy separation between the ground state and respectively the 4dyz and 4dxy orbitals, whose lobes point toward the spinel blocks. The cations of the spinel blocks which lie in the second shell are AI3 and A1 i in tetrahedral sites (Figure 4). Because of their localization

D. Gourier, D. Vivien / Study of Ag + and mixed Na+ /Ag + (3aluminas

1(C')

O

02. 02-in mO site

xia') f - ' ' ~x

02-°ut of the

t a'c" plone AI s*

4dx2_y2 z

Y

I

271

interaction K. In the absence of strong covalent u-bonding, K stems from the polarization of inner s electrons by the unpaired d electron, and is generaly positive. In ~ alumina, the small K value can be explained assuming that an important negative contribution to K adds to the predominant positive one. A small negative component may arise from a partial occupancy of the outer ~ $ orbital whose mixing With dx2-y 2 and dz 2 is allowed by the low symmetry of the occupied site (22). However the covalency of the Ag-O bonds may be the major negative contribution to the Fermi contact interaction as often observed in cupric coordination complexes (22). Such reduced K values have been found for Cu 2+ ions in ~ alumina (8, 23). 5.

CONCLUSION

Ah siles empty

4 dxy......... 4- dz 2 A[i sites occupied

4-dxz .........t Ali sites empty 4dyz

~'Ali sites occupied

Figure 4 Partial view of the unit cell of ~ alumina showing the localization of Ag2+ center and its relation with the interstitial oxygen ion in mO position and the interstitial AI3 + ions The l orientation of the lobes of both 4dx2-y 2 and 4dxy orbitals are indicated. This figure also presents a schematic diagram showing the splitting of the 4d orbitals of Ag 2+ in ~ alumina and the influence of the Frenkel defects. in the ca* plane, defects in these tetrahedral sites will act as additional crystal field components influencing essentially the 4dxy orbital. If the two A1 i sites were empty (no Frenkel defect), the destabilization of the 4dxy orbital would be larger than if they were occupied, i.e. if two Frenkel defects were associated with one interstitial O~- ion (Figure 4). ~xy being minimal for Ag 2+ (I) and maximal for Ag 2+ (II), these two types of ions could correspond to Ag 2+ associated to two empty A1 i sites and two occupied A1 i sites respectively. To a lesser extent, the small differences between the gx values of Ag2+(1)and(II) could have the same origin since the 4dyz orbital also points toward spinel blocks. The differences between Ag2+(I~ andIII ~ appear more difficult to interpret since they bear identical gz and g values (Table I). However the model remains ~alid if we suppose that there are still a small difference between the gz values of Ag 2+ (I) and (II') which could not be determined experimentally because of the important linewidth of the spectra. In the preceeding section it has been emphasized that the lack of h.f. structure was due to an anomalously small value of the Fermi contact

Ag 2+ in Na+/Ag + and Ag + ~ alumina are located in the same lattice site, which corresponds neither to a Na + nor to a Ag + normal position. In this site, Ag 2+ is trapped by the Coulombic potential of an interstitial oxygen ion, giving rise to point defect associations such as Ag2+-O~ - and Ag2+-O~ - (AI~+-~AII) 2. These associations can also be considered as neutral UAgO3 entities. The drastic drop of conductivity (4) observed in mixed Na+/M 2~ ~ alumina could then proceed partly from such a trapping mechanism, the (M2+-O~-)~ neutral species which do not participate to the ionic conduction replacing mobile Na + pairs (24). REFERENCES (i) For a review see Dell, R.M. and Moseley, P. T., J. Power Sources, 7 (1981/82) 45-63. (2) Lazennec, Y., Lasne, C., Margotin, P. and Felly, J., J. Electrochem Soc. 122 (1975) 734-737. Yasui, I. and Doremus, R.H., J. Electrochem. Soc. 125 (1978) 1007-1010. (3) Ingram, M.D. and Moynihan, C.T., Solid State Ionics 6 (1982) 303-310. (4) Farrington, G.C. and Dunn, B., Solid State Ionics 7 (1982) 267-281. (5) Colomban, Ph. and Vivien, D., Phys. Stat. Sol. (a) 76 (1983) in press. (6) Gourier, D., Vivien, D. and Livage, J., Phys. Stat. Sol. (a) 56 (1979) 247-257. (7) Barklie, R.C., O'Donnell, K. and Henderson, B., J. Phys. C. ii (1978) 3881-3887. (8) Gourier, D., Vivien, D., Th4ry, J., Livage, J. and Collongues, R., Phys. Stat. Sol. (a) 45 (1978) 599-606. (9) Antoine, J., Vivien, D., Livage, J., Th~ry, J. and Collongues, R., Mat. Res. Bull. iO (1975) 865-871. (10) Yu Yao and Kummer, J.T., J. Inorg. Nuclear. Chem. 29 (1967) 2453-2460.

272

D, Gourier, D. Vivien / Study o f Ag + and mixed Na+/Ag + ~ aluminas

(11) Colomban, Ph., Boilot, J.P., Kahn, A. and Lucazeau, G., Nouveau J. Chimie 2 (1978) 21.

(18) Boilot, J.P., Colomban, Ph., Collin, G. and Comes, R., J. Phys. Chem. Solids 41 (1980) 47-54.

(12) Wertz, J.E. and Bolton, J.R., Electron Spin Resonance (Mc Graw-Hill, New York, 1972).

(19) Huheey, J.H., Inorganic Chemistry. Principle of structure and reactivity. (Harper and Row, New York, 1978).

(13) Delbecq, C.J., Hayes, W., O'Brien, M.C. and Yuster, P.H.,.Proc. R. Soc. A271 (1963) 243-267. (14) Boatner, L.A., Reynolds, R.W., Chen, Y. and Abraham, M.M., Phys. Rev. B 16 (1977) 86IO6.

(20) Roth, W.L., Reidinger, F. and La Placa, S., Superionic conductor edited by Mahan, G.D. and Roth, W.L. (Plenum Press, London, 1977). (21) Buluggiu, E., Dascola, G., Giori, D.C. and Vera, A., J. Chem. Phys. 54 (1971) 2191.

(15) Barrer, R.M., Zeolites and Clay Minerals (Academic Press, Londres, 1978).

(22) Rockenbauer, A., J. Magn. Reson. 35 (1979) 429-438.

(16) Mc Garvey, B.R., J. Phys. Chem. 71 (1967) 51-67.

(23) Gourier, D., Th6se de doctorat d'4tat (sept. 1980).

(17) Brown, T.G. and Hoffman, B.M., Molec. Phys. 39 (1980) 1073-1109.

(24) Wang, J.C., Gaffari, M. and Sang-Choi, J. Chem. Phys. 63 (1975) 772.