Xov., 1953
THECHEMISTRY OF PHOSPHORS
773
THE CHEMISTRY OF PHOSPHORS BY ROLAND WARD Department of Chemistry, Universitg of Connecticut, Storrs, Conn. Received March IS, 1963
The chemist's business is not only to perform chemical synthesis but also t o determine the structure and the nature of the chemical bonds in the products which he synthesizes. When dealing with phosphors, he is concerned, in many instances, with the effects of an impurity in very low concentration, and it is the structural features of this active center upon which his attention finally comes to rest. The problem is not unlike that of determining the structure of an ion or molecule in dilute solution inasmuch as optical methods are about the only recourse, but the difficulties of the task are compounded by the influence of the matrix on the optical behavior of the so-called active center, an influence which may be so extensive that the optical effects become the properties of communities of centers or even of the entire crystal. Absorption bands which the active center of luminescence may possess are frequently masked by the absorption of the host crystal, and in a great many instances the broad emission bands are not resolved even a t the lowest temperatures. The revelation of the structural features of activated crystals by optical methods is in general of such a diffuse nature that it permits a fairly wide range of speculation. The field is somewhat restricted, however, by other optical properties such as decay rates, temperature dependence of luminescence, stimulation and quenching phenomena, and photoconductivity; but too often this additional information only serves t o demonstrate the complexity of the problem. It is this striving toward an understanding of the configuration of active centers in impurity-activated phosphors that has lured many chemists into the realm of solid state physics, and his interests become one with the physicists. Phosphor research has from the beginning provided another happy field of collaboration between the two disciplines. While most physicists have only a casual interest in how a phosphor has been prepared, they are now thirsting for a detailed knowledge of the structural features of impurity centers. Since some pertinent information may be derived from the methods of preparation, i t might be profitable to examine some of the procedures and the evidence derived from them. The purification of the ingredients from which the phosphor is made has long been a standard practice, and it is covered by the statement that the phosphor contains only the impurities listed and no other intentionally added impurity. This is a desirable qualification because contamination may so readily occur especially in the high temperature operations which are usually necessary in preparing efficient phosphors. Cationic impurities, more particularly of the so-called heavy metal kind, are not likely to be troublesome but anionic impurities may be quite insidious and often play a significant role in the activation process. It is only in recent years, for example, that the importance of halide
ions in the zinc sulfide phosphors has been established. The concentration of the activator ion in the ingredients of the phosphor can be closely controlled, but its exact proportion and distribution in the final phosphor is too often open to question. When no flux is used, the activator must be incorporated by some diffusion process frequently so slow' that equilibrium conditions are not established. The use of a flux may accomplish this, but there may be an unfavorable distribution of the activator between the flux and the solid phosphor. With some activators such as lead, bismuth, thallium, etc., considerable volatilization may occur in the heat treatment and the final composition is never satisfactorily determined. All of the activator which has been added is not necessarily incorporated in the host crystal. Consequently it is customary t o rely on reproducibility of results and the systematic variation of luminescence properties with the proportion of added activator t o lend some assurance about the system, but neither procedure can give an absolute measure of the concentration of the activator. The manner of inclusion of the impurity ion in the host crystal is perhaps of more importance than the foregoing. To reach a conclusion about this from the chemical preparation of the phosphor we are obliged to make some assumptions which should be recognized however reasonable they may seem. The first assumption is that the kind of solid solution formed a t very low concentrations will be the same as that a t high concentrations. When the activator has the same charge and is about the same size as the host crystal cation, we might expect a simple substitutional solid solution. It is to be supposed for example that Mn+2 could substitute for Mg+2 in MgS or for Zn+2 in ZnS since it is about the right size and MnS exists in both the NaCl and ZnS structures. But the subPt+2for Zn+2in ZnS stitution of Fe+2,C O + ~Ni+2, , could not be so certainly regarded since the structure of none of the sulfides of these elements resembles ZnS. The bonding tendencies of these ions to sulfur are quite different from that of Zn+2, and it is conceivable that drastic local distortions of the ZnS lattice could occur a t low concentrations. The second assumption is that, when a solid solution is formed with a cation of valence different from that of the host crystal cation, some kind of valence compensation must occur even a t high dilution. This could readily happen a t or near the surface by cation or anion deficiency, and it has been reasonably well established that such a cation deficiency permits the solution of trivalent rare earth ions in the strontium sulfide lattice at quite high concentrations. It is not possible to say whether this extends throughout the crystals, but it seems likely. Experimental evidence supports the idea that univalent zinc ions in zinc sulfide or
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ROLAND WARD
Vol. 57
TABLE I Ti Zr Lanthanides, Actinides
(d)
-
V Nb Ta
Cr Mo W
i
Mn
Co Rh
Re
(b )
zinc oxide are introduced by the reaction 6Zn+2+ 20-2 S-2 6Zn+' SO2 leaving an anion deficiency. Valence compensation can also be contrived by collateral substitution. This has been elegantly demonstrated by Kroger and others in the preparation of zinc sulfide phosphors in which univalent Cu, Ag or Au may be substituted using either Al+3or C1-l as collateral ions. Uncertainty as to the activator ion position in the lattice still prevails even in these carefully investigated phosphors, however, for in a-CuzS and ol-Ag2S half of the cations occupy lattice points in the diamond type lattice while the others are interstitial. Another assumption that seems reasonable is that an activator, introduced by the valence compensation method, will be located near its compensating ion or near the defect which it causes. No positive evidence of such an association is obtained from the luminescence spectrum. Considering that this simple method is available, it is surprising how little it has been used to control the oxidation state of the activator. The most usual procedure depends on isomorphous substitution and the use of oxidizing or reducing atmospheres. There is an element of the so-called valence inductivity principle in the hope that the activator will adopt the oxidation state of one of the host crystal cations. This assumption is probably valid when the coordination tendencies of the two ions with the anion of the host crystal are the same. For example, it is to be expected that manganese would readily replace Mg+2, Zn+2, Alfa, Tif4 by adopting the appropriate valence. The coordination number, however, is also important. It has already been noted that Mn+2may have either 4 or 6 coordination with sulfide ions, but with oxygen Mn+2tends to have 6 coordination. It gives a good phosphor with MgO but not with ZnO. With oxy-anions on the other hand one might assume a somewhat greater flexibility. This could be a valid assumption where one is dealing with very large or continuous anions such as the metasilicates. It is in this situation that it seems plausible t o assume that the substitutional rules which hold for high concentrations are not followed at activator concentrations. Nevertheless it is very unlikely that ions like Mn+4, Ti+*could be substituted for Si+4under any circumstances. It has been shown, for example, that oxidizing or reducing atmospheres had essentially no influence on the luminescence of manganese activated magnesium silicates, phosphates, borates but have a pronounced effect on that of Mg2TiOc. The luminescence of certain pure inorganic crystals has been demonstrated in some cases to be due t o electronic transitions within a oomplex ion such a~ U02+a or Pt(CN)4-2 and in others (mostly rme earth compounds) to transitions within
+
Fe
+
the more or less unperturbed cation. In other cases, such as the pyrovanadates, orthomolybdates, orthotungstates the suggestion is frequently made that the luminescence is due to self-activation caused by the existence in the lattice of a low concentration of the transition metal ion in a reduced state. There is no experimental evidence in support of this idea. We might also include in this group zinc, beryllium, magnesium and thorium, zirconium, silicates and zinc borates, all of which show similar emission characteristics when in the pure state and prepared in oxidizing atmospheres. It is most reasonable to attribute the luminescence of all of these substances t o electronic transitions within a coordinated group. The behavior of this group is strongly influenced by contiguous cations but is not affected by like neighboring groups. I n all cases they appear to be discrete complexes. An extension of this category to include activated phosphors which require large concentrations of activator for maximum efficiency is reasonable, especially when the activator has been introduced under circumstances which would favor its highest oxidation state. There is another aspect of phosphor chemistry which is the primary concern of many chemists, the synthesis of new phosphors. In this exhilarating and at the same time frustrating game, relatively little assistance has come from the theoretical side. Most frequently new phosphors are discovered on the basis of the tremendous backlog of factual information coupled with the intuitive sense of the experimenter. It might be instructive t o examine in a general way some of the results of experience starting with a consideration of the established activators which are listed in Table I according to their position in the periodic chart. They fall into a few categories which are, of course, not clean cut, but some dominant characteristics are to be found within the limits set forth. Section (a) includes the most versatile of the activators. They function a t low concentrations and in a wide variety of host-crystal environmentshalides, sulfides, selenides, simple oxides and oxysalts. I n all cases they appear to behave as activators only in an oxidation state below their maximum. Following along with most of these attributes are the post transition elements Pb, Sb, Bi in section (a'). The elements in section (b) apparently function principally in oxygen- and occasionally fluoridedominated host crystals. With the exception of Ti and Cr, their effectiveness as impurity cations appears to be limited to the activation of glasses, A l a 0 8 and a few simple fluorides. Otherwise, they are restricted t o the kind of luminescence attribute
*
s
I
Nov., 1953
THE CHEMISTRY OF PHOSPHORS
able to MO, groups or, in the case of platinum, t o platinous complex ions. Section (e) includes most of the elements which function best in oxide- or halide-dominated crystals. The (c’) group T1, In, Sn and possibly Hg may be the only elements of this section which function as “impurity” activators. They are usually active in their lower oxidation states. In the main, however, the elements of this section belong t o the active complex type. Some of them give luminescent solutions, and the behavior of the T1+ ion in alkali halides has been interpreted by some on the basis of a Tl+(Cl)a complex. The lanthanide and actinides. are placed in a class by themselves. They are versatile activators (like group a) but at low and high concentrations. Many of their compounds are luminescent (compare groups b and c). It should be noted that the elements Ce, Pr, Sm, Eu, T b which are very effective in sulfide dominated lattices are also those which have two stable oxidation states. Some of the elements included in this table might well be questioned. The well known killers Fe, Co, Ni in section (a), Zr, Nb, Ta, Rh in section (b) and Be, Al, Ga in section (c) have not yet produced outstanding phosphors, but there is no question about the powerful influence some of them show as co-activators, collateral ions for activators in phosphors. So far as the choice of a host crystal is concerned, the present notion seems t o be that any colorless crystal is a reasonable prospect. Consequently there is a need for some restrictive specifications, Besides the anionic preference of the indicators mentioned above there are some others which might be briefly mentioned. 1. Thermal stability-especially if the activator is to be introduced by the usual heat treatment procedure. There are very few outstanding phosphors which are not made in this way. 2. Absorption characteristics suitable for the mode of excitation to be used. 3. Cation sites appropriate to the valence and coordination tendencies of the activators and sensitizers which are to be used. Unfortunately these criteria are not particularly helpful guide posts. The colorless spinel-type compounds, the apatite-like halophosphates furnish examples of expertly exploited host crystal structures, but very frequently the attempts t o make phosphors from known phosphates, silicates and borates lead t o the discovery of hitherto unsus-
775
pected structures. Some of these structures are apparently stabilized by the activator itself and provide outstanding phosphors. Some insight into the likely proportion of activators will always be forthcoming from past experience. The (a) group should operate best at fairly low concentrations. The elements in section (b), however, apparently operate best at high concentrations but in segregated coordination groups. A suitable matrix t o accomplish this for ZrO6, NbO6 and TaO6 octahedra might yield some interesting phosphors. A similar policy should be followed with the activators in group (c). In this connection the possible effect of the cation of the host lattice cannot be overlooked. The shifts in emission peaks obtained by substitution of Ca+2by Ba+2and Srf2, the fact that BaW04 is not luminescent a t all while Caw04 and SrW04 are luminescent serve to demonstrate that the cations can be just as important as the anion in the phenomenon of luminescence. It is possible that this influence may be related to the “stability ratio” of all of the cations and anions contiguous t o the luminescence center. These observations offer nothing that is new, but it is hoped they will serve to emphasize that it is a laborious task t o make a thorough chemical study of a phosphor system that no part of it can be safely neglected. DISCUSSION
J. s. PRENER.-ThiS situation in the manganese activated MgO phosphor is somewhat complex. MgO containing a small amount of manganese and heated in a hydrogen atmosphere will give a broad red emission band under cathode ray excitation and analysis indicates the manganese to be divalent. When heated in an oxidizing atmosphere this red emission is weaker, the body color is brownish instead of white, and analysis indica.tes 2 0 4 0 % of the manganese to be in the tetravalent state. The conclusion is that the oxidized manganese is present as a separate phase. If one now puts lithium carbonate into this phosphor and heats in an oxidizing atmosphere, one gets a bright red phosphor having 6 or 7 bands in its emission spectrum under ultraviolet excitation a t liquid nitrogen temperatures. Chemical analysis, the fine structure in the emission, and determinations of the lattice constants indicate that tetravalent manganese has entered the MgO lattice and that lithium acts as a charge compensator. R. WARD.-I think this is further evidence of the influence of the matrix on the oxidation state of a dissolved ion. The manganese apparently must remain in the divalent state when dissolved in magnesium oxide. Manganese oxide under the same conditions becomes Mna04,
