W. D. JOHNSTON, R. C. MILLERA N D R. MAZELSKY
198
Vol. 63
for their kindness in supplying the source and some gratefully acknowledges also the F. P. Whitaker of the fluorescent materials. One of us (C. F.) Fellowship of 1956-1957.
A STUDY OF SEVERAL SYSTEMS OF THE TYPE Liz[CO,Ni,l- v)l (1 BY W. D. ,JOHNSTON,R. C. MILLER A N D R. MAZELSKY Westinghouse Research Laboratories, Pittsburgh 35, Pa. Recekued J u l y 11 1968 ~
Compositions in the systems Liz [CoyNi(l-y)](l-z)O, Liz [Mn,Cocl-,)](l-,)O and Liz [NuZn(l-v)](l-z)O have been prepared by reaction of Li202and the appropriate transition metal monoxide solid solution. I n general the products have been shown to be single phase materials of rock salt symmetry with the lattice parameter decreasing with increasing lithium content. I n the case of Li, [Co,Ni(l-,)](l-,)O, the lattice parameter data have been interpreted in terms of n. preferential oxidation of cobalt to the trivalent state. Where z = 0.08 in this system, the lattice parameter has been measured as R function of temperature. From the data the energy required for the oxidation of divalent to trivalent nickel in the crystal was shown to be 3 kcal./mole greater than that for the corresponding cobalt oxidation.
The electrical and magnetic properties of a number of lithium substituted transition metal oxides have been reported recently. 1 - 3 I n particular, the electrical conductivities of the systems LizMn(1.-,)0, Li,Co(l-.,O, LizNi(l-z)O and LizC U ( ~ - ~have ) O been found t o increase with increasing atomic nuniber of the transition metal. This increase in electrical conductivity comes about through z1 corresponding decrease in the activation energy required for conduction. In order t o provide materials which might yield additional information about the conduction process, a study was made t o determine whether solid solutions of some of the above systems could be prepared. h i example of such a solid solution would be Li,[Co,Ni(l-,,](I-.,O
where 0
< x < 0.5 and 0 5 v 5
1
Similar formulas can be written if Ni+2 is expected to be preferentially ionized. On the other hand, if the ionization is assumed t o be random, the formula should be written Liz [ C O + ~ , +3(1N~
y)
Iz [Co +zyNi+2+),
](1-2~)0~
Solid solutions of lithium substituted transition metal oxides have not been studied before, so suitable preparational methods had to be developed. I n addition, it was not a t all clear that such a solid solution would form, particularly if one transitioii metal ion was preferentially oxidized. I n such a case a two phase mixture might develop, e . g . , Li3Co(l--2)0 and NiO. Other possibilities would involve the ordering of the transition metal ions, or, in the extreme case, the formation of new crystal structures. Such possibilities would, of course, preclude the electrical study planned for these materials. This paper deals with the preparation and the evaluation of the structures of a number of these systems. I n addition, the distribution of charges of the transition metal ions was investigated in one of these systems.
I n systems of this type the substitution of a given number of lithium ions into the crystal lattice causes the oxidation of an equal number of divalent transition metal ions t o the trivalent state, I n the case where two different divalent transition metal ions are present in the lattice, a ' Discussion question arises as t o whether one or both of the ions are oxidized. A consideration of the exThe method used for the preparation of comperimental evidence based on electrical conduc- pounds of this type involved, first, forming the tivity studies in perovskite systems containing required solid solution of the transition metal two different transition metaI ions such as [Las- oxides, e . g . , Co,Ni(l-,)O, and then treating this Sr(l-z)][Mn,Fe(!-,) suggests that oiily one material with lithium peroxide. The second step ion will be oxidized. of the procedure is given in the reaction If we accept this assumption and further assume that Co+z is more easily oxidized than Ni+2, we 2 LilOs (1 - r ) C o , N i ( l - , ) ~+ may write Li, [Co,Ni(l- 11)1(1-~)0 L~+,CO+~,CO+~~(,_~)-~N~+~(I-,)(L-~)O~ It is essentially the same procedure that has been where the lithium coiitent is smaller than the cobalt used previously in the preparation of lithium subcontent (:v < y ( l - m)). When the lithium con- stituted transition metal oxides and can be used tent is higher than the cobalt content (x > essentially independently of the mixtures of transiy(1 - x)), the chemical formula becomes tion metals involved. Li,Co+a,(i-z)Ni +3z- g(,-s)Ni +2(1-~.)Oc. The lattice parameters of the oxide systems Co,Nic,-,)O,. Mii,C0(~-,)0 and Ni,Zn(l-y)O are ( 1 ) W. D. Johnston and R. R. Heikes, J . Am. Chem. Soc., 7 8 , 3255 shown graphically in Fig. I , I n all cases the graphs (1980). (2) R. R. Heikes and W. D. Johnston. J. Cliem. Phys., 26, 582 show that the lattice parameter is linear with com(1954). position within experimental error. The CoO(3) W. D. Johnston and R. R. Heikes a n d E. Sestrich, J . P h y s . NiO and the Coo-Ma0 systems are single phase ,Ckem. Solids, 7 , No. 1, 1 (1958). rock salt structures in their entirety. The NiO(4) G. H.Jonker, Physica, 20, 1118 (1954).
]Y3
+
A STUDY OF SYSTEMS OF
Feb., 1959
ZnO phase is a single phase niaterial with rock salt symmetry 117hen the Zii content is between 0 to 28 atom %. Beyond this poiiit the hcxagoniil ZiiO phase appears. These lithium substituted systenis have been prepared in t,his work Li, [RInuCo(~-,,,](~-Z,O y = 0.5 Li,[clo,~i~i(1..,)](1-2)0 y = 0.025, 0.05, 0.075, 0.1, 0.2,, 0.5,. 0.94 Li,[~i,Zn(,-Y)](,--x)O y = 0.75, 0.925
The lattice parameters obtsliiied from the X-ray examination of many of these materials are given in Fig. 2, nit)h the data previously obtained for Li,Mn (l-s)O, Li,Co(,-,) 0 and Li,Ni(l- ,T)O included for comparison. In all cases the curves show a smoothly decreasing lat'tice parameter with iiicreasiiig lithium coi~tent~.I n many cases these curves were terniinated arbitrarily a t a = 0.15. This should not be interpreted as necessarily reflecting m y chemical instability or multiple phase behavior. I n none of the cases where a < 0.15 mas there any indication of impurity phases or ordering. The data for Li,[CoUnTi~l-,)](1--)O may be plotted in a more informative manner. In Fig. 3 are plott'ed lnttice paraniet,ers as a fuilction of 2/ a t const,nnt R: ( L e . , R: = 0.08). The figure shoms that the data ohtnined a t 25' can be represented npproximately as two straight lilies ilitert?ecting a t the colialt content (where in this case y = 0.087) which is e q u d to the lithium content (N = 0.08). Similar behaT7ior is also found for other values of x with the intersection of the straight lines always occurring roughly a t the equivalence point of Co aiid Li. This behavior immediately suggested that cobalt was preferentially oxidized t o the +3 state. I n order to test this idea, the lattice parameter was calculnted as a function of the cobalt composition, assuming preferential cobalt ioiiization and using Vegard's law. Vegard's law states that the lattice parameter is a function of the sum of the radii of the individual ions multiplied by their mole fractions. This calculation gives the results that where y(1 - x) 3.2: fl/2 = y [ ( 1
- r ) ( t ? c ~ +-~ R N i + ? ) ]
and where y( 1 - 2:) fl/a =
