W. D. Johnston Westinghouse Research Laboratories Pittsburgh 35, Pennsylvania
Mixed Valence Oxides
I t has been recognized for a number of years that transition metal oxides generally are insulators when they are very pure, but when the composition of these same transition metal oxides deviates from stoichiometry resulting in an excess or deficiency of the metal ion or oxygen ion, the materials show increased electrical conductivity. The first systematic correlation between the electrical conductivity of transition metal oxides and their departure from stoichiometry was made by Wagner and co-workers (1) in the early 1930's. This work has more recently been extended by Hauffe (2). Wagner's work may be illustrated by his study of p-type conducting compounds such as NiO. In a p-type conductor the electrical conductivity comes about through a flow of positive charges known as electron holes in contrast to an n-type conductor where the current results from a flow of electrons. When these compounds are close to stoichiometric in composition the materials are insulating. However, these materials may be caused to deviate from stoichiometry by increasing the oxygen pressure above the compound, resulting in an oxygen excess material NiOl+, or, more properly, a nickel deficient material NicI-,,O. I n such a case the electrical conductivity increases. Deviation from stoichiometry in NiO results in an occasional divalent nickel ion being missing from a site normally occupied by Ni+2 in the crystal lattice. These are called nickel vacancies. As a consequence, two additional positive charges must be present in the lattice in order to preserve electrical neutrality. These additional positive charges may be referred to as Ni+8ions in chemical terminology or as "electron holes" in semiconductor language. Wagner applied the law of mass action to this type of a system. At a sufficiently elevated temperature (80&100O0C), where oxygen can be reversibly taken up by the bulk oxide, the equilibrium which applies may be represented as follows: NiO crystal
+ one molecule of 0 1 s NiO crystal containing 2 N i 0 + 4Nita
Here NiO represents a Ni vacancy in the NiO lattice. Associated with each Ni vacancy are 2Ni+' ions in order to keep the crystal electrically neutral. An equilibrium constant for this reaction may be written as K=
(Ni0)4Xi+a)( PO,
but since 2 NiO
=
Ni+J
The electrical conductivity, v, is taken as proportional to the number of Ni+' ions in the lattice. In this way the electrical conductivity should increase as the l/s power of the oxygen pressure. (4Kpq)l/-
(Ni+q
m s
Actually Wagner found a I/, power dependence but the relationship was qualitatively established. The lack of quantitative agreement is probably due to the assumption that the energy required for electrical conduction is independent of the concentratioil of electron holes, in this case Ni+'. As we will see later this assumption is not valid for similar materials. Similar equilibrium relationships have been applied to n-type semiconductors such as ZuO where there is an excess of zinc in the crystal lattice leading to conduction by electrons. I n this case the electrical conductivity has been shownto decrease with increased oxygen pressure. I n experiments of this type it mas established beyond question that a relationship exists between electrical conductivity and non-stoichiometry. However, there were very definite practica! limitations to the application of such semiconductors. These limitations were chemical in nature. In the first place, it is generally very difficult to obtain large deviations from stoichiometry. Accordingly, only limited variations in electrical properties are possible. As the temperature is increased, the amount of deviation from stoichiometry is generally observed to increase. This has been frequently interpreted in terms of the entropy associated with vacancy formation (3, 4). The wiistite or FeO,+, phase field (5) of the iron-oxygen system is a good illustration of this point. The phase field increases with increasing temperature (i.e., x increases with temperature). However, the deviations that are achieved a t high temperatures may well be metastable or unstable a t lower temperatures. I n addition the degree of non-stoichiometry a t any temperature is a function of the oxygen partial pressure and thus difficult to control. Therefore, the limited range of composition, the lack of thermal stability, and the problems involved in preparing reproducible samples sharply limit the utility of this type of material. Controlled Valence Principle
Division of Inorganic Chemistry at the 134th Meeting of the American Chemical Society, Chicago, September, 1958.
In the years 1948-50 these serious deficiencies were overcome by Verwey and co-workers (6) a t the Philips Volume 36, Number 12, December 1959
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Laboratories a t Eindhoven. Verwey introduced the concept of controlled valence semiconductors. His techmque has the effect of controlling the concentration of the conducting species but doing away with the necessity of a large vacancy concentration. I n this way semiconducting behavior and chemical stability became compatible. The controlled valence principle may be illustrated by the system Li,Nicl-,,O. This material may he most easily described as a NiO lattice in which somc of the Ni+%ions have been removed and replaced hy an equal number of Li + ions. Since lithium has a fixed valence of + l and oxygen is expected to remair. 4ouhlr minus. some of the remaining nickel ions are fo.ccrl to take on a +3 charge in order to preserve electrical n~utrality. Actually, one Ni+a is formed for every lithium ion substituted as shown in the expanded chemical formula Li,+Ni,+3Ni+?cl_2,,0=. I n this vay a lattice is built up consisting of lithium randomly distributed over some of the cation sites and a random array of Ni+2 and Ni+3 ions on the remaining sites. The + 3 charge on the nickel is not localized and is free to move from one nickel to another and thus gives rise to electrical conductivity. Verwey has substituted 10% Li in the NiO crystal lattice in this manner and has increased the electrical conductivity from 10-lo W1cm-' for pure nickel oxide to approximately lW1 cm-'. In addition, the material is chemically stable. Verwey suggested that Li,Ni,-,O may be of use as a thermistor material. One of the reasons for choosing lithium for this substitution is the fact that it is nearly equal in size to the divalent nickel ion. The ionic radius for Ni+%is 0.74A while the Li+ radius is 0.70. Sodium, for example, having a radius of 1.00 A, has been found to be too large to suhbtitute in the NiO lattice (7). When a lithium ion enters the crystal lattice it is necessary for either the Li+ ion to he oxidized to +2 or the nickel ion to he oxidized to + 3 in order for the crystal to remain electrically neutral. An additional possibility, which not be described here, is the formatioil of - 1 oxygen ions (6). It is indicated in the formula that it is the nickel ion which is oxidized. This may be justified on the basis of the ionization potentials of the metals. The second ionization potential for lithium is 75 ev while for nickel the third ionization potential is only 36 ev. Thus, the oxidation of the nickel requires much less energy. Metals which adopt mixed valence states are usually found among the transition metals where the energy required to change the valence is small. On the other hand, non-transition metals generally do not form mixed valence compounds. For example, if magnesium were used instead of nickel in this case, a mixed valence state would not result since the third ionization potential for magnesium is 80 ev. Additional cationic impurities in these materials may have various effects. For example, the introduction of Fe+e+3 in Li,Ni(l-,,O has a poisoning effect and decreases the electrical conductivity due to the formation of Li+-Fe+3 pairs with the result that fewer Ni+3 ions are formed. I n fact, several workers have been able to run what are essentially solid state titrations in this manner (4, 6 ) . An impurity that does not contribute in a positive way to this conduction process generally contributes in a negative way since any for606
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lournol of Chemical Education
eign ion which substitutes for the transition metal ion in the crystal lattice exerts a blocking effect. For example, if Mg+2 were to substitute for some of the Ni+2 in Li,Nil-,O, both the resistivity and the activation energy for conductivity (6) would increase. In addition to compounds having rock salt structures such as NiO, perovskite compounds of the type AB03 may he used in the formation of mixed valence systems. Here the A ion is a large fixed valence ion and B a small variable valence transition ion. There are several such systems of interest. A system which is currently of very practical interest is BaTiOs in which a very small amount of the divalent barium has been replaced by trivalent ions such as lanthanum. In order to maintain electrical neutrality in such a compound, a small amount of Ti+' ions must be reduced to Ti+3 forming the system La,+3Ba+20-,,Ti,+aTi+4~1111033. I n this way the material becomes semiconducting due to the presence of titanium ions of two different valence states on equivalent crystallographic sites. In this case the + 3 charge may move from one titanium to another due to the motion of an electron, and the material will exhibit n-type conductivity. At present a number of laboratories are considering this material as a control device (8, 9). Double Exchange
Several years before mixed valence BaTiOa was developed, Jonker and Van Santen (10,11) produced a mixed valence manganese containing perovskite compound. A general formula for this mixed valence perovskite is La,+' Sr+T,_,,. [Mn,+91n+4(,_,,]OC
The end members of the solid solution where x = 0 and x = 1 are not mixed valence materials. If x = 1, the formula is LaMnOBand the valence of Mn is +3. If x = 0, the formula is SrMnOa and the valence of Mn is +4. The end members of the solid solution are insulators. At compositions between x = 0 and x = 1, both tri- and tetravalent manganese are present. In the intermediate composition range this material shows excellent electrical conductivity and, in fact, in the range x = 0.6 to x = 0.8 the conductivity is metallic in type. In the same composition range the material is ferromagnetic with the moment approaching that required for complete alignment of all manganese moments. A completely ferromagnetic arrangement has been confirmed recently by neutron diffraction (12). In addition, this result has been duplicated in recent experiments on non-stoichiometr;~LaMnOI (15). In this material, linear chains of Mn+30=Mn+4 exist. Electrical conduction presumably occurs by electrons moving from a + 3 to a +4 manganese ion. Zener (14) explained this electron transfer on the basis of an interaction called double exchange. Since the configuration MntJ 0- Mnf4
is equal energetically to the alternate configuration Mn+'O- Mnts
Zener suggested that the proper way to describe such a. system is by combining these two states in a resonant. system. In this way an electron can be visualized as
moving from one manganese to another without any expenditure of energy. This electron motion gives rise to the electrical conductivity noted in these materials. The theory also favors the parallel alignment of the spins of the 3d electrons of the manganese ions which results in ferromagnetism. Thus, the theory explained both the non-activated metallic electrical conductivity in these materials as well as their ferromagnetism. It seemed reasonable that double exchange might occur in the lithium substituted transition metal oxides as well as in the perovskites. Unfortunately, however, no magnetic data had been reported for compounds of the type LizNicl-,,O. Thus, it became of interest to us t o prepare a number of these materials in order to determine both their magnetic and electrical properties. One of the first materials studied in this program was lithium-substituted manganese oxide (15). The work was later extended to Li,Col-,O, LizN1-,0, Li,Cn-,0 (16) and other related compounds. Lithium-Substituted Transition Metal Oxides
Preparatim. There are several methods which may he used to prepare these materials. Verwey fired an intimate mixture of Li20 and NiO in air a t 1200°C. This mixture takes up oxygen from the air as indicated in the equation and results in the required composition. z/2Li20
+ (1 - z)NiO + 2 / 4 0 ,
-
Li,Ni
