Mat. Res. Bull., Vol. 21, pp. 609-619, 1986. Printed in the USA. 0025-5408/86 $3.00 + .00 Copyright (c) 1986 Pergamon Journals Ltd.

LITHIUM INSERTION REACTIONS OF SPINELS: EFFECT OF THE DISTRIBUTION AND REDUCIBILITY OF CATIONS IN SELECTED MANGANITE AND ZINC SPINELS

C.J. Chen, M. Greenblatt t Department of Chemistry

Rutgers, The State University of New Jersey New Brunswick, NJ 08903

and

J.V. Waszczak AT&T Bell Laboratories, Murray Hill, NJ 07974

(Received March 3, 1986; Communicated by A. Wold)

ABSTRACT

Lithium insertion reactions b~ n-BuLl and electrochemical techniques of various MMn204, M=Zn 2+, Ni z+, Cd 2+ compounds demonstrate that upon lithium insertion Mn 3+ is reduced to Mn 2+ and the cooperative Jahn-Teller effect present in the tetragonal host spinels is absent in their lithiated analoques as indicated by the tetragonal to cubic structural transformation that occurs. These studies and similar lithium insertion reactions of ZnB204, B=Co 3+, Cr 3+, [Fe3+Mn3+]0. 5 show that an important factor in the extent and kinetics of lithium insertion reactions in splnels is the ease of the reducibility of the transition metal cations.

MATERIALS INDEX: lithium insertion, spinels, manganese, zinc.

Introduction

Recent studies of topotactic lithium insertion/extraction, oxidatlon/reductlon reactions in a variety of spinel phases including Fe304 (I), Mn304 (2), Co304 (3), LiMn204 (2,4-7), LiTi204 (8-10), LiV204 (8,11) and various other compositions (12,13) have demonstrated fast Li + ion diffusion.

609

610 C . J . CHEN, eta] . Vo]. 21, No. 5

The ideal spinel structure, A[B2]O 4 can be described as a cubic-close-packed array of oxide ions in which only one half of the octahedral and one-eigth of the tetrahedral sites are occupied. There are eight molecules per unit cell and oxygens are located at the 32e position of space group Fd3m. The B cations occupy octahedral sites at 16d, the empty octahedral sites are at 16c. The 64 tetrahedral interstices are at the three nonequivalent positions 8a, 8b, and 48f; the A cations occupy site 8a. Each 8a tetrahedron shares common faces with four neighboring empty 16c octahedra which allows a possible diffusion pathway for the A cations, 8a+16e÷Sa+16c etc. through the structure. In contrast, the 8b tetrahedra share faces with the 16d octahedra occupied by the B cations, which renders them energetically unfavorable for cation occupation. The 48f tetrahedra share faces with both 16d and empty 16c octahedra. A cation distribution of A[B2]O 4 and B[A,B]O 4 is classified as a normal and inverse spinel, respectively. Mixed (or intermediate) splnels with a cation arrangement corresponding to AxBy[Al-xBl-y]O 4 are also possible (12).

It has been established by X-ray (I) and neutron diffraction (I0) studies that the [B2]O 4 sublattlce of the A[B2]O 4 spinel structure remains intact upon lithium insertlon/extraction and provides a three-dimenslonal (3D) framework and an interstitial space of edge-shared octahedra connected in three dimensions for Li + ion diffusion as shown in Fig. i. Previous studies

8b 48f

,

, I f

I /

I / / d L r o 0

FIG. 1 Half of a unit cell of the spinel structure (after Ref. 7) showing the positions of:O8a and • 16d cations; Qsome of the 32e 8b,• 48f oxygens; and *16c interstitial sites in one quarter (two octants) of the unit cell.

indicate that on lithium insertion into spinel the lithium ions in LixA[B2]O 4 occupy the octahedral 16c vacant sites up to x = 1 and the tetrahedral A (Sa) cations are cooperatively displaced to octahedral 16c sites producing partially ordered rock-salt-like phases, e.g. {LiFe}[Fe2]O 4 where the curly bracket stands for the 16c octahedral sites (2). Additional lithium ions inserted (l<x<2) must therefore occupy the empty 8a, and/or 48f sites (1,7,10). Because the Li + ions in these tetrahedral sites scatter X-rays weakly the fully lithiated phases Li2A[B2]O 4 also show a rock-salt like X-ray diffraction powder pattern.

A large number of transition- metal oxides with the general formula AB204 adopt the spinel-type structure. There are many different possibilities for varying the ionic sizes, the valence states and the

site-distribution of the constituent cations in the lattice and opportunities to study the effect of these parameters on the structural-physical properties of the lithium inserted phases. We have studied the effect of cation distribution on the extent of lithium insertion in a series of iron spinels

Vol. 21, No. 5 SPINELS 611

and found that the lithium content decrease in the order inverse > "mixed" > normal splnels (13). It appears that only in the inverse spinals, where a reducible cation (Fe3+[Fe 2+, Fe3+]O4 ) is present in the 8a tetrahedral site, can more than one Li + ion be inserted per formula unit (e.g. Li2Fe304 vs LiMn304). Furthermore, in these splnels Li + ions in excess of x > 1 can be removed by chemical or electrochemical delithlation (LiFe304 can not be delithiated) (13). In contrast, the normal lithium spinels with Li + ions in the 8a sites, Li[B2]o 4 with B = Ti, V, Mn undergo lithium insertion reversibly to form Li2[B2]O 4 phases (2-10).

In this paper we report lithium insertion reaction studies of various manganese splnels, MMn20 ~ with M = Zn 2+, Ni 2+, Cd 2+, and zinc spinels, ZnB204 with B = Cr 3+, Co 3+ and [Fe 3+, Mn3+]0.5 .

We have chosen the tetragonal manganese splnels to demonstrate how the cooperative Jahn-Teller effect, due to the presence of Mn 3+ (d 4) ions in excess of a critical concentration disappears upon lithium insertion and reduction of Mn 3 to Mn 2+, with a concomitant phase transformation to the cubic structure. Furthermore, we have previously shown that the presence of reducible cations in the tetrahedral sites facilitate the uptake of lithium ions in splnels. Lithium insertion reactions of the inverse manganese spinel Mn[Mn,Ni]O4, versus Zn[Mn2]O 4 and Cd[Mn2]O 4 studied here should provide additional evidence of that effect. Investigation of the lithium insertion of normal zinc splnels was intended to provide some understanding of the effect of the reducibility of the octahedral cations on the extent and relative diffusion rate of lithium insertion in spinnels.

Experimental

MMn204 with M = Zn 2+, Ni 2+ and Cd 2+ were prepared from equal molar quantities of high purity MO and Mn203. The fine powder mixtures were pelletized and calcined in a Pt crucible at 950°C for 48 hours in air.

ZnCr204 was prepared from mixtures of ZnO and Cr203 by the same method. ZnCo204 was prepared as follows: zinc and cobalt powder was mixed in the molar ratio of 1:2 and dissolved in concentrated HNO 3 slowly; the red-purple solution was heated to dryness on a hot plate and the solid powder obtained was fired at 450°C for i0 to 20 minutes in air and quenched. The same procedure was used to synthesize NiCo204, except that the metal nitrate hydrates were used as the starting materials, which were dissolved in water before the heat treatments. Chemical lithlatlon of the spinel samples was carried out with excess n-butyl-llthlum (n-BuLl, 1.5M in hexane) in a hellum-filled dry box at ambient temperature and/or 50 ° - 55°C for 7-10 days. The products were thoroughly washed with dry-hexane and dried under vaccum for several hours. The unreacted n-BuLl was determined by reaction with a known excess of O.IN HCI followed by back titration with O.IN NaOH. The lithium content of the llthlated products was also determined by plasma emission spectroscopy. The results of these two methods generally agreed within 5 to 10%. Lithium extraction reactions of the lithiated compounds were performed by treatment with 12 or DDQ (2,3 dichloro-4,5 dicyano-benzoqulnone) in acetonitrile (12/CH3CN , DDQ/CH3CN ) . The lithium concentrations of the delithlated products were determined by plasma emission spectroscopy. All the host splnels and their reaction products were identIEied by powder X-ray diffraction using Ni-filtered Cu radiation wlth.Si as internal standard. The llthlated and dellthlated samples were protected from air and moisture

612 C . J . CHEN, et al. Vol. 21, No. 5

throughout. The electrochemical llthlatlon was performed using a small test cell which was galvanostatically discharged at a constant current under helium. Thin pellets, 3/8 inches in diameter, made of finely powered mixture of the splnels and 20 weight percent graphite were used as cathodes, Li metal loll was used as anode and IM LICIO 4 in propylene carbonate (PC) as electrolyte. Open-clrcult voltages were obtained as a function of composition after the cell achieved equilibrium for several days. The qualitative electrical resistivity on pressed powder pellets was measured using a 2-probe method (12), The magnetic susceptibility (4-300°K) was recorded using the Faraday method described previously (14).

Results and Discussion

AMn204~ A = Zn~ Cd and Ni

Chemical llthiatlon of ZnMn204, CdMn204, and NiMn204, yield LiZnMn204, LiCdMn204 and Lil.sNiMn204 at room temperature as well as at 50-55°C. The red-brown color of the host compounds gradually turned black on llthiatlon. Crystallographic parameters and final composition of llthlated and delithlated phases are summerlzed in Table I. Lattice parameters were determined by least squares fit to the observed powder X-ray diffraction data, which was indexed on the basis of space group Fd3m (Oh7) for the two nickel manganites; the Zn and Cd hosts and their llthlated phases fit the tetragonal space group F41/ddm (D4h 19, an alternate setting for space group 141/amd , used to show the splitting of the cublc-spinel reflections). The normal tetragonal splnels ZnMn204 and CdMn204 both can accommodate ~ one inserted lithium ion per formula unit on reaction with n-BuLl. The unit cell volume of the LiZnMn204 and LiCdMn204 phases increase by ~ 6% and ~ 7% compared to their respective hosts. Upon lithium insertion, the axial ratios of both host compounds are reduced from c/a = 1.14 for ZnMn204 to c/a = 1.06 for LiZnMn204 and c/a = 1.19 for CdMn204

TABLE 1

Cr~stallosraphic Data and Lithium Content of Selected Man~anltes

host c/a(1) x(2) c/a(3) x(4) cla

Mn[Mn2]O4(5) 1.157 I.I±0.I 1.054 - - Zn[Mn2]O 4 1.144 1.0±0.2 1.058 0.8±0.2 1.060 Cd[Mn2]O 4 1.192 1.0±0.I 1.042 0.8±0.2 1.056 Mn[NiMn]O 4 1.0 1.8±0.2 mixed - -

phases Li[Mn4+Mn3+]O4 (5) 1.0 1.0±0.2 1.161 - -

(I) c/a of the host. (2) x = lithium concentration in the llthlated compounds. (3) c/a of the lithiated phases. (4) x = lithium concentration in the dellthlated compounds. (5) reference 2.

Vol. 21, No. 5 SPINELS 613

to c/a = 1.04 for LiCdMn204 (Table I). The increase of unit cell volume and the reduction of axial ratios are similar to those previously reported for the chemical lithiation of Mn304 (included in Table 1 for comparison) and confirm the observation that (i) the charge-compensatlng electrons from the inserted lithium ions reduce Mn 3+ to Mn 2+ and occupy the d states of the host lattice and (2) that the magnitude of the cooperative Jahn-Teller distortion is to a first approximation, a function of the concentration of the Mn 3+ ions in octahedral sites.

The first conclusion is further supported by the increase of electrical conductlvlties on lithium insertion. The room-temperature electrical conductivities of the lithlated samples are approximately two orders of magnitude larger than those of the host spinels. The presence of Mn 3+ and Mn 2+ in equivalent octahedral sites of LiZn[Mn3~In2+t]O4 probably facilitate a charge hopping mechanism of conduction. Compared with axial ratios found in related manganese spinels with different Mn 3+ ion concentrations in the B sites I e.g. c/a = 1.16 in Mn2+[Mn3+]PO4, c/a = 1.05 in Mn2+[Cr3+Mn3+]O4 and Zn[Ga3+Mn3+]04, c/a = 1.04 in Zn[Cr3~Mn3+]O4, and c/a = 1.0 in Zn[Fe3~n3+]O 4 Li[Mn3+Mn4+]04 and Li[Fe0.53+Mn0.53+Mn4+]04, the observed c/a values for the lithiated Zn and Cd manganites (1.06 and 1.04) are in accordance with approximately one octahedral Mn 3+ ion per formula unit (15,16).

The lithium composition determined by electrochemical lithiation of ZnMn204 is in good agreement with that obtained by chemical means (LiZnMn204). The shape of the open-clrcult voltage (V) vs lithium composition (x) shown in Fig. 2 is simillar to the V vs x behavior of Li[Mn3+Mn4+04] and is remarkably

5.5

5.0

2.5 >

2.0

1.5 I 0.2

I I I I 0.4 0.6 0.8 1.0

X in L i x Z n M n 2 0 4

FIG. 2 Open-clrcuit voltage vs. x for LI/IM LiCIO 4 in PC/LIxZnMn204

different from that of Mn304 (2) or ZnFe204 (12) which are also normal splnels. The original tetragonal phase persists to x = 0.06; the plateau in the compositional range 0.06 < x < 0.4 represents a two phase region and a new single tetragonal phase with reduced c/a appears at 0.4 < x < 1.0. In Lil+xMn204 the plateau in the range 0.i < x < 0.8 was attributed to the first-order character of the Jahn-Teller distortion, and the simultaneous presence of the cubic and tetragonal phases. The LixZnMn204 phase undergoes the reverse, tetragonal-to-cubic structural transformation in the range 0.06 < x < 0.4. The Zn 2+ ions, because of their strong

preference for the tetrahedral sites (17) probably remain in the 8a sites (Just as the original Li + ions occupy the 8a sites in Lil+xMn204) , rather than be displaced to the octahedral 16c site in the 0.06 < x < 0.4 composition. However, with increasing Li + content~ the Li+-Zn 2+ replusive interactions are finally strong enough to push the Zn 2+ ions into the 16c sites beyond x > 0.4, as indicated by the drop in V.

614 C . J . CHEN, et al. Vol. 21, No. 5

Dellthlation of LiZnMn204 and LiCdMn204 even with the strong oxidizing reagent DDQ/CH3CN , extracted negligible amounts of lithium, indicating that the pseudo-cublc phases are stabilized by the insertion of one lithium.

In Table 2 the relative X-ray intensities of the LiZnMn204 and LiCdMn204 when compared to those of LiMn304 suggest that the Mn204 sublattlce remains intact upon lithium insertion, and that in the fully lithlated phases, the cations are primarily in the 16c octahedral site, i.e.{Li,Zn}16c[Mn2]16dO 4 and {Li,Cd}16c[Mn2]16dO4 . The critical lithium concentration (x c) for the cooperative displacement of the tetrahedral 8a site cations, may be different for each sample.

The temperature variation of magnetic susceptibility of the llthiated Zn and Cd mangenltes is shown in Fig. 3. The data cannot be fit to a Curle-Welss

TABLE 2

X-ray Intensity Data* of Selected Man~anltes

hkl LiMn[Mn2]O4t LIZn[Mn2]O 4 LiCdMn204

111 27 13 11 202 ~ _ _ _ 220 J 113 - - - 311 - - - 222 I00 i00 I00 004 38 31 34 400 92 98 55 313 <I - - 331 <I - - 511~ . . . . 333~ 404 60 64 59 440 17 15 7

*estimated from the height of each reflection peak. testimated from the powder X-ray diffraction pattern shown in ref. 2

law in any range of the temperature measured. The antlferromagnetlc ordering observed in ZnMn20 ~ below 523 K and attributed to interaction between nearest neighbor B-slte Mn 3+ ions (18) is absent in the llthiated phase, and a disordered distribution of strong magnetic interactions are indicated. The LiCdMn204 shows magnetic behavior similar to that of LiZnMn204.

Vol. 21, No. 5 SPINELS 615

25C

20¢

~15C

IOC

50

G

200 250 300

T (K)

FIG. 3 Temperature variation of the magnetic susceptibility of

(a) LixZnMn204 and (b) LixCdMn204 .

125

IOO

75

5o o

25

Nickel manganite is an inverse spinel, Mn3+[Ni2+Mn3+]O4 due to the strong octahedral site preference energy of the Ni 2+ ion (17). Upon lithium insertion, Lil.8NiMn204 is produced versus LiZnMnO 4 and LiCdMnO 4 normal splnels. This result is similar to the lithium insertion products of normal vs inverse ferrites (12) and is further evidence that reducible cations in the 8a site facilitate the insertion of lithium ions. However, the lithiated compound

Lil.8NiMn204 was found by powder X-ray diffraction to consist of two phases: a cubic-spinel, and a rock-salt type. Interestingly, similar results were obtained for another inverse nickel spinel, NiCo204 (19) which affords Li2NiCo204 by lithlatlon with n-BuLl,

but this product was also found to be a two phase system.

ZnB204, B=Co~Cr and [Fe~Mn]o_5

ZnCo204, is a normal cubic spinel with a = 8.106±0.005A. Stoichlometric lithiation with n-BuLl at room temperature produced Li2ZnCo204 with a = 8.334±0.005A, which indicates an 8.7% increase of unit cell volume. The estimated X-ray intensity data of this host and its lithiated analoque is given in Table 3 along with previously reported data (1,12) for related cubic

TABLE 3

X-ray Intensity Data of Selected Spinels

h k 1 ZnCo204 Li2ZnCo204 ZnFe204 Li0.5ZnFe204 Fe304 Li2Fe304

1 1 1 14 - 7 3 8 18 2 2 2 37 - 40 36 30 - 3 1 1 i00 - i00 i00 i00 i0 2 2 2 - 54 i0 27 8 63 4 0 0 20 i00 18 64 20 I00 4 2 2 17 - 14 21 i0 - 333 5 1 1 35 - 27 48 30 -

4 4 0 33 46 34 64 40 52 I220/1440 I. 12 0.0 I. 176 0.562 0.75 0.0 1400/1422 1.18 ~ I. 286 3.048 2.00

616 C . J . CHEN, e t al. Vol. 21, No. 5

ferrlte spinels. It is surprising that the variation of the peak intensities of Li2ZnCo204 is similar to that of the inverse ferrlte Li2Fe304 rather than to that of the normal ferrlte Lio.5ZnFe204. This suggests a similar distribution of cations in Li2ZnCo204 and Li2Fe304 (i.e. Li8a or 48f {~i!Zn}16c[Co2116dO4) • The qualitative electrical resistivity of Li2ZnCo204 (R~I0 fl) was found to be comparable with that of ZnCo204~ which indicates that a negligible concentration

C 3+ of o ions are present in the llthlated product hence the absence of charge hopping. Dellthlatlon by of the fully lithlated spinel Li2ZnCo204 by DDQ/CHsCN yields LiZnCo204. The small, but significant change in lattice parameter (a = 8.326 ±0.005A for LiZnCo204 vs a = 8.334 ±0.005A for Li2ZnCo204) indicates the possible occurence of reversible topotactlc lithium insertion in LixZnCo204 in the composition range 1 < x < 2. The stolchlometry of this dellthlated compound is further confirmation of our previous conclusion (127 that the rock-salt type structure of the lithiated splnels is stabilized by insertion of up to one lithium ion per formula; the excess lithium ions x > 1 which are located in the interstitial tetrahedral sites (8a or 48f) are mobile and possible to extract. Li2ZnCo204 is the first example of a normal spinel compound in which two Li ions could be inserted per formula unit. This is probably due to the ease of reducibility of the Co 3+ ions (Table 4). The shape of the open-clrcult voltage vs composition x for Li/ZnCo204 shown in Fig. 4 approximately resembles that of the Li/Fe304 cell (1,127. Therefore the pathway of lithium insertion proposed for Fe304 by Thakeray et al (I) can also be applied here:

I + (Zn2+)8a [ Co23+]04 ............... > {Lixc} 16c (Zn2+)8a [ C°23+ ] 16d04

xcLi

II ............... > {Li+Zn2+}16c[Co3+Co2+]16dO 4

(l-xc)Li

III ............... > (Li+)8a,48f{Li+Zn2+}16c[Co22+]16dO 4

1.0Li

where each stage represents the corresponding single phase region shown in Fig. 4. The equilibration time required to obtain a constant open-clrcult voltage after each discharge period was much longer in the low composition range (x < I), which suggest a slow movement of phase boundary in each particle. This may be due to the low octahedral site preference energy of Zn 2+ and its reluctance to move from the 8a to the 16c site.

The temperature variation of the magnetic susceptibility of ZnCo204 and Li2ZnCo204 4.2-300K is shown in Fig. 5. The data for ZnCo204 (Fig. 5a) fit to a Curle-Welss law in the temperature range 20-300K ~eff ~ 1.38~8, 8 ~I5K.

The effective magnetic moment (~eff) due to Co 3+ per mole is much too low for high spin (d 6) electronic configuration (~eff = 4.9~B). Blasse predicted, that ZnCo204 llke MgRe204 its Isoelectronlc analoque should show weak temperature-lndependent paramagnetlsm (20), due to scrambling via the orbital momentum of the low spln IAlg(t2g)6 ground state with the ITlg(t2g)5 (eg) excited state. Our results indicate anomalous magnetic behavior, corresponding to neither the low spin nor the high spin electronic conflrguratlon of Co 3+ ions. The magnetic susceptibility data of Li2ZnCo204 (Fig. 5b) can not be fit to the Currle-Weiss law with meaningful magnetic parameters in any range of the measured

V o l . 21, No. 5 SPINELS 617

o

5 0

DO

50

O0

50

\

~o ,~o ,;o ~8o ~o T (K)

~ 5 0 0

2 0 0 O 3 O

,ooo%_ u qO I =

mo o ~

3 0 0

I t m i i

,, i i

I I I i I I o!~ o4 o!6 o 8 i o ,L2 14 ~6

X in Li x ZnCo204

I I 8 2 !0

FIG. 4 Open-circuit voltage vs. x for Li/IM LiCXIO4.PC/LixZnCo204.

FIG. 5 Temperature variation of the magnetic susceptibility of (a) ZnCo204 and (b) LixZnCo204.

temperatures. At very low temperature (T < 25K) the saturation and time dependence of the susceptibility is characteristic of a spin glass state (21).

Zn[Cr2]O 4 is a pale-green cubic spinel with normal cation distribution. Treatment of this compound with n-BuLl produced no color change and insignificant amount of Li + insertion. This can be attributed to the relative stability of the Cr 3+ to reduction (Table 4).

Reaction of another zinc spinel, Zn[Fe3~n3+]O 4 with n-BuLl at room-temperature yielded Lil.2ZnFeMnO 4. It is noteworthy that the amount of lithium incorporated has significantly increased when half of the Fe 3+ ions in the octahedral 16d sites of Zn[Fe23+]O 4 have been substituted by Mn 3+, i.e. Lio.5ZnFe204 was the fully lithiated composition observed (12). The lattice parameters of Zn[FeMn]O 4 are a = 8.287A, c = 8.712~ (c/a = 1.04), in good agreement with previously reported data (18). The powder X-ray pattern of the lithiated compound was fit to space group Fd3m with a = 8.559A. The unit-cell volume has increased by 4.8%. The tetragonal-to-cubic transformation of Zn[FeMn]O 4 phase upon lithium insertion indicates that most of the Mn 3+ ions have been reduced. Moreover, the 57Fe Mossbauer data of Lil.2ZnFeMnO 4 confirms, that most of Fe 3+ ions are not effected by the lithium reaction. Thus the significantly greater aese of the reducibility of Mn 3+ compared to Fe 3+ as indicated by the reduction potentials of these species in aqueous solution in Table 4 is also operative in the solid state. Although the actual value of reduction potentials in the solid must be quite different, the relative order shown in Table 4 apparently is the same as the lithiation results of Zn[B2]O 4 spinels confirm (Table 4).

The ceactivity of the zinc spinels studied here (in terms of the amount of lithium ions inserted) parallels the reducibility of the B-slte cations. Since lithiatlon is an oxidation/reductlon reaction in which both electrons and ions are transfered from guest species to the host matrix, the facility of the host structure (e.g. spinels) to receive (or donate) extra electrons are essential in

618 C . J . CHEN, et 8/. Vol. 21, No. 5

determining the reactivity in this type of reaction.

Technically, these results also suggest the possibility of tailoring the redox potentials of a working electrode in secondary lithium batteries by the right choice of B-slte cations in the spinel framework.

TABLE 4 Reactivity and Reduction Potential in Spinels

Compound x % increase E°(V) ~ in LixZnB204 in unlt-cell

volume (%)

Zn[Cr2]O 4 0.0 0.0 -0.41 Zn[Fe2]O 4 0.5 2.0 +0.77 Zn[Mn2]O 4 1.0 6.0 +1.51 Zn[FeMn]O 4 1.2 4.8 +1.51 Zn[Co2]O 4 2.0 8.7 +1.802

*Reduction potential of B ~+ + e- ÷ B z* in aqueous solution

Acknowledgement

The authors thank F.J. DiSalvo for his help with the interpretation of the magnetic results. We thank R.H. Herber for his assistance with the Mossbauer measurements. This work was supported by the Office of Naval Research.

References

I. M.M. Thackeray, W.I.F. David and J.B. Goodenough, Mat. Res. Bull., 17, 785 (1982).

2. M.M. Thackeray, W.I.F. David, P.G. Bruce, and J.B. Goodenough, Mat. Res. Bull., 18, 461 (1983).

3. M.M. Thackeray and S.D. Baker, in preparation.

4. J.C. Hunter, J. Solid State Chem., 39, 142 (1981).

5. A. Mosbah, A. Verbaere and M. Tournoux, Mat. Res. Bull., 18, 1375 (1983).

6. M.M. Thackeray, P.J. Johnson, L.A. de Picclotto, P.G. Bruce, and J.B. Goodenough, Mat. Res. Bull., 19, 179 (1984).

8. D.W. Murphy, M. Greenblatt, S.M. Zahurak, R.J. Cava, J.V. Waszczak, G.W. Hull, and R.S. Hutton, Rev. Chlm. Min., 19, 441 (1982).

9. D.W. Murphy, R.J. Cava, S.M. Zahurak, and A. Santoro, Solid State lonlcs, and IO, 413 (1983).

i0. R.J. Cava, D.W. Murphy and S. Zahurak, J. Solid State Chem., 53, 64 (1984).

U. L.A. de Picclotto and M.M. Thackeray, Mat. Res. Bull., 20, 187 (1985).

Vol. 21, No. 5 SPINELS 619

12. C.J. Chen, M. Greenblatt and J.V. Waszczak, Solid State lonics, in press.

13. C.J. Chen, M. Greenblatt and J.V. Waszczak, J. Solid State Chem., submitted.

14. F.J. DiSalvo, J.V. Waszczak, Phys. Rev., B23, 457 (1981).

15. J.B. Goodenough, '~agnetism and the Chemical Bond" Table XV, Interscience and John Wiley NY 1963.

16. G. Blasse, Philips Res. Repts., 20, 528 (1965).

17. V.S. Urusov, Phys. Chem. Minerals, ~, 1 (193).

18. Ref. 15, p. 202.

19. C.J. Chen and M. Greenblatt, unpublished results.

20. G. Blasse, Phillips Res. Repts., 18, 383 (1963).

21. P.J. Ford, Contemp. Phys., 23, 141 (1982).