PACIFIC SOUTHWEST ASSOCIATION O F CHEMISTRY TEACHERS
LEO BREWER University of California, Berkeley
WmN one thinks of the exacting material requirements of the new industries such as those that will be providing jet and rocket travel, nuclear power and similar technological advances, one wonders where one will find the compounds to meet these requirements. The conditions that must be met in these new applications are so novel that compounds of quite unusual properties will he required. The answer to the question "Where will the compounds to meet these new demands be found?" is "They will be found among the undiscovered compounds." A large number of interesting inorganic compounds remain yet undiscovered. Let us consider the principles that can guide us in our search. These compounds fall into several classes. We are dismissing as trivial the large class of those which have not been discovered merely because the component elements have not yet been mixed together. For example, gadolinium and iridium surely will form some intermetallic compound of considerable interest, but no one has yet had the time or the opportunity to mix them together to determine what kind of compounds can be formed. The undiscovered compounds to be discussed in this paper belong to systems which have been studied, often quite thoroughly, in the past. One can expect many interesting gaseous compounds to be uncovered and also, quite a number of unusual solids. 'These undiscovered solid compounds can be classified in four categories: The first class consists of compounds which are thermodynamically stable over the entire solid range of the compounds but which have eluded detection because of a very slow rate of formation from the possible starting materials. The second class consists of compounds which are metastable under all conditions but which yet can persist because of slow rates of decomposition and which can be quite valuable under a variety of conditions. The third class to be considered consists of compounds that are thermodynamically stable at low t.emperatures but which disproportionate upon heating to higher temperatures. The fourth class to be discussed consists of compounds which are thermodynamically stable at high temperatures but which disproportionate upon cooling. A simple illustratiog of classes one and two is provided by the elemental phosphorus system. Available evidence indicates that the thermodynamically stable form of solid phosphorus may he hlack phosphorus. Red phosphorus may be metastable with respect to 'An address delivered before the northern division of the Pacific Southwest Association of Chemistry Teachers a t Sacramento Stste College on October 12, 1956.
VOLUME 35, NO. 3, MARCH, 1958
black phosphorus under most conditions, and it is known that white phosphorus is very metastable with respect to either black or red phosphorus (1). When P, gas is condensed, white phosphorus is formed, even though it is the least stable form. This formation of a metastable phase in preference to a stable phase is fairly common and occurs not only upon condensation of vapors but upon transformation of ore solid to another or during crystallization from melts. This is most often due to the fact that the most stable solid modification may have a very special type of arrangement which may not be present in the starting materials. Therefore the final stable material may form only very slowly from the starting materials whereas other phases which are less stable but which have structures somewhat closer to that of the starting materials may actually form at a much more rapid rate. Thus it is not uncommon when one is carrying out a reaction to have a number of intermediate metastable phases formed before one reaches the final thermodynamically stable phase (2). The structure of the gaseous Pd molecule is so dissimilar to the structures of red and black phosphorus, which have structures somewhat similar to that of graphite where the phosphorus atoms are flattened out almost into planes, that it proves quite difficult to obtain red or black phosphorus from the gaseous P, molecules. Because of the difficulty of distorting the P4 molecules, they normally do not condense to form a solid until such a low temperature is reached, corresponding to a tremendous supersaturation with respect to red or hlack phosphorus, that the simple van der Waals' forces cause condensation to a molecular lattice of white phosphorus. Obtaining red phosphorus requires various catalysts or irradiation by gamma rays or ultraviolet light to sufficiently rearrarge phosphorus atoms so that formation of the more stable red phosphorus can occur. Black phosphorus requires not only catalysts and heat but also pressure for it^ formation. The silica system has certainly been one of the most thoroughly studied systems yielding a number of crystalline modifications (3). Yet, in just the last three or four years, three previously unknown forms have been discovered with quite unusual properties. One of these, for example, can be boiled in hydrofluorie acid solution without attack (4). Another interesting form consists of long fibers (5). I t is not known whether these compounds are metastable or stable but they were prepared by a combination of condensation reactions using high temperature and high pressure processes.
I t is clear that the understanding of the kinetics of formation of solid phases is very important for the development of methods of preparation of either the metastable phases or the stable phases which are formed only slowly from available materials. A good example of the importance of application of theories of kmetics of reactions is the experience encountered in the synthesis of diamonds. Many people have previously attempted to produce diamonds from graphite by application of pressure. Hall (6) was able to demonstrate through application of the theory of kmetics of reactions that it would he impossible t o produce diamonds directly from graphite under any conditions where diamonds would be thermodynamically stable. Recognition of this saved any further wasted effort in the attempt to produce diamonds from graphite thus permitting development of a different approach which did successfully result in the synthesis :of diamonds. GASEOUS SYSTEMS AT HIGH TEMPERATURES
What is the nature of these undiscovered compounds2 and where are they to be found? There are many undiscovered gaseous compounds as well as the four types of solid compounds listed above. As some of the solid compounds are best prepared by way of the gaseous phase, it is convenient to discuss the gaseous compounds first. There is a common belief among chemists that high temperature gaseous systems can be expected to become simpler as one increases the temperature; that is, the number of different types of gaseous species to be expected and the complexity of these species should decrease. In actual practice this is not so because one commonly studies the gaseous phase under conditions where it is in equilibrium with a solid or liquid phase. Under these conditions the number of gaseous species present in appreciable amounts increases as one increases the temperature and also, the complexity of these gaseous species usually increases. As an example one may cite the well-known dimerization in sodium vapor. At low temperature, more than 99.97T0 of sodium vapor consists of sodium atoms. However, as the temperature is increased, the proportion of dimer becomes steadily larger until a t the boiling point of sodium the vapor consists of approximately 10% dimer (7). Another illustration is that of the vapor in equilibrium with molybdenum trioxide which has recently been investigated by Berkowitz, Inghram, and Chupka (S), using the mass spectrometer. At low temperatures the main gaseous species is MoaOo,hut as the temperature is increased the proportions of Mohol2and Mo6015 steadily increase. Many other examples can he cited. A very simple thermodynamic proof can be given to demonstrate that this will be the general behavior to be expected in high temperature systems. For simplicity, let us consider that the gaseous phase contains two species, a monomeric species and a dimeric species. We normally think of two factors which determine the relative importance of the monomer and dimer species. As the pressure is increased, a t constant temperature, we expect the proportion of dimer t o increase. As the temperature is increased, a t constant pressure, we expect the proportion of dimer to decrease. What may me expect when both the temper-
ature and the pressure are increased as in the case of the saturated vapor in equilibrium with the condensed phase? A simple general thermodynamic argument can be developed t o relate the heat of dissociation of the dimer t o the heats of vaporization of the monomeric and dimeric molecules (9). Recognizing that Trouton's rule corresponds to the same entropy of vaporization for both the monomer and dimer, we can relate the relative importance of the monomer and dimer species of the saturated vapor directly t o the relative values of the heats of vaporization. From the relations AFOM = AHDnr - TAB' and AFoo = AHoo - TASD
we see that the order of the heats of vaporization becomes the order of the standard free energies of vaporization or of the equilibrium partial pressures. Whichever of these species has the smaller heat of vaporization will be the more abundant species. Consider the rather common situation where the low temperature vapor consists predominantly of the monomeric gaseous species with the dimeric species present a t low concentration. This corresponds to a higher heat of vaporization for the dimer than for the monomer. (See, for example, the accompanying figure in which the logarithm of the partial pressures of each species is plotted versus 1/T.) Since the dimer has a larger heat of vaporization, its partial pressure will rise more rapidly than that of the monomer, and as the tem-
perature is increased, the proportion of dirner will steadily increase. This is one of the most common types of behavior that one finds in high temperatures systems. We may generalize this t o include not only monomers and dimers hut also trimers, tetwmers and various other polymers. More complex vaporization reactions such as disproportionation t o lower and higher oxidation states as well as ionization reactions may be involved similarly. Placing the main emphasis upon the translational contribution t o the entropy, or the free volume contribution ( l o ) , one can show that Trouton's rule, in a general way, can he applied t o these situations and one can derive a conclusion similar to those that we have reached for the monomer and dimer situation. A general statement of the conclusions that one can reach can be given in the following manner: If we consider a saturated vapor a t a low temperature, we will normally find that one species predominates, although there will be a large number of other species present a t smaller concentrations. Under those circumstances, the minor species a t low temperatures will have higher JOURNAL OF CHEMICAL EDUCATION
heats of vaporization than the major species and therefore as the temperature is increased the proportion of the various minor species will steadily increase. This is true whether the minor species are simpler or more complex than the major species; therefore, if one wishes to find a high temperature system in which one finds the greatest variety of molecular species and the greatest complexity of molecular species, one should go t o the highest temperature possible a t which the saturated system can still exist. The important conclusion that can be drawn from the derivation which has just been presented is that one can expect to find many new and unusual compounds in high temperature systems. These compounds will have different formulas and contain the elements in different oxidation states than one normally finds a t conventional temperatures. Because of their unusual character, these compounds will provide a severe test of any theories of bonding or structures of molecules. They also will have many important practical applications. The recognition of the formation of AlCl gas in high temperature systems has allowed the development of a process for purification of aluminum by reacting crude aluminum metal with aluminum trichloride gas to produce aluminum monochloride gas which can then reform pure aluminum upon cooling due to the disproportionation of AlCl gas a t lower temperatures (11). These gases that are stable only at high temperatures can also be used to produce many interesting new solid materials. A well-known example of the use of such high temperature molecules is the coating of lenses by silicon monoxide. Although silicon monoxide is a thermodynamically unstable solid (IB), the gaseous SiO is quite stable a t high temperatures and may be readily produced by reducing silica (13). m e n SiO gas is chilled on a surface, an amorphous SiO solid phase is formed which can persist indefinitely a t room temperature or which can be oxidized to silica by heating. As the SiO may he readily vaporized, this provides a very convenient method of coating surfaces to protect them. SOLID SYSTEMS AT HIGH TEMPERATURES
High temperature gases will undouhtedly prove t o be important starting materials for the preparation of many solid materials of classes one and two as the gaseous molecules often have structures much different from possible solid starting materials and this allows one t o form solid phases of unusual structures. Let us now consider the formation of solids of classes three and four. What are the possibilities of phases that might be stable only a t high temperatures and which would disproportionate upon cooling to room temperatures? One can consider here either a transformation from a single low temperature phase t o a single high temperature phase or the reaction of two or more low temperature phases to produce new high temperature phases. One can readily demonstrate that any such phases which are stable only a t high temperatures must form from the low temperature phases with absorption of heat and increase of entropy. I n a rough way, one could say that these high temperature phases must have weaker bonds and looser bonds in order to be stable only a t high temperatures. A well-known example of this type of behavior is found in the iron-oxygen system where the iron(I1) oxide phase is unetable a t VOLUME 35, NO. 3, MARCH, 1958
room temperature with respect to iron and Fea04but becomes thermodynamically stable a t higher temperatures. Phases stable only a t high temperatures often play an unexpected role in the reaction between materials a t high temperatures. For example, in the bonding of ceramic materials t o metals, these intermediate phases may well form a t the high temperatures and aid in the sealing of the ceramic t o the metal. Yet they disproportionate upon cooling so that no evidence of their presence is found in the samples a t room temperature. More careful studies of many of the high temperature phase systems with high temperature X-ray cameras and other techniques which allow the systems t o be studied a t the high temperatures will undouhtedly reveal many such phases. Some of these phases have quite unusual formulas and properties, and one can often quench them t o lower temperatures where their rates of disproportionation can become so slow that they persist indefinitely. Not only can one use high temperatures to produce phases that are metastable under normal conditions hut the recent production of cubic boron nitride (14) illustrates the possibilities of high pressure preparations. Of particular interest has been the work of Schonherg and others in Professor Hagg's laboratory a t Uppsala where a number of quite unusual compounds have been discovered which appear to fall in the class of compounds that are unstable a t room temperature but which become stable a t higher temperatures. This work has been reviewed in detail by Brewer and Searcy (16). Some of the formulas (16) that have been found are M3'Ma10 and Mz1M4"0where M' can be Mn, Fe, Co, Ni, or Cu and M Xcan be Ti, Mo, or W. Also compounds such as MIReOx are found with X between 0 and 1 and M' = Ti, V, Cr, Mn, Fe, or Co. These oxides are unlike the oxides that we are accustomed t o and, in fact, have quite metallic properties. Their unusual properties undoubtedly will make them suitable for many interesting applications. Compounds that are stable a t low temperatures but which disproportionate a t higher temperatures also can be of considerable interest. It appears that in many instances these compounds become unstable a t a temperature a t which the rate of disproportionation or the rate of formation of the phase from the adjoining phases is very slow. As a consequence, these phases are often difficult to produce and have remained undiscovered. Let us consider the recent example that has been studied by Schonberg (17). He has demonstrated the existence of a phase MoaOwhich cannot be produced by conventional methods. If one heats a mixture of molybdenum metal and Mooz a t a high temperature, the MosO phase cannot form because it is thermodynamically unstable. If one heats a mixture of Mo and MoOz a t a low temperature where the phase is stable the rate of formation is so slow that the time required for formation would he much too long for it to he detected. For such a phase the only conditions under which one would have much hope of preparing it in a reasonable time would be in the temperature range just below its maximum temperature of existence. I n this range it is possible that the rate might be fast enough so that it could be prepared. However, we do not know where this temperature range is for yet unstudied compounds. The procedure that Schonberg used was to
heat this mirturr of Mo and Moonin a furnace and each day reduce the temperatlire by a small amount until the sample had been subjected to a temperature interval from 1000°C. down to almost room temperature. Upon examining this mixture after such a treatmrnt be did indeed find a Mo30nhase which. had been mis~edby all previous workers. There undoubtedly are many other such phases which bave not heen discovered and for the same reason. An interesting technique which can be applied t o such systems is to construct a furnace in wh.ich there is a temperature gradient. The sample is sprinkled along a tube in this furnace. Due to the gradient, the sample is subjected to thc entire range of temperature. This might include the temperature at which the compound becomes stable and can form a t a reasonable rate. After long heating in this gradicnt, the sample can then be inspected by microscopic or by X-ray means to determine whether or not a reartion has taken place, and from the position of any new phases in the tube one will know the temperature a t which the phasr had formed. SOLID PHASES WITH A RANGE OF COMPOSITION
In addition to the undiscovered gaseous compo~mds and solid compounds of the four classes that have been discussed above, there is yet another important source of new materials that will prove useful in future applications. The law of definite combining proportions which must be exact for a gaseous molecule is often applied to solid compounds for which it cannot be exact except a t the absolute zero. Consider the easeous molecule NO. The smallest change that can he produced in this compound is that caused by the addition of a single atom of nitrogen or a single atom of oxygen resulting in compounds with entirely different properties, namely N20 or NO2. If we have a single crystal of sodium chloride weighing about fiO grams the formula would be Nalo?rC1,,z~ and if we add or remove one atom we see that the change in composition is quite minute. One can likewise apply thermodynamic arguments to show that the sodium chloride phaee in equilibrium with chlorine gas cannot have the same composition as the sodium chloride phase in equilibrium with sodium. The homogeneous range of compositions for a phase may be very small especially a t low temperatures, or it may he quite large as in the example of the T i 0 phaee region which has the sodium chloride crystal structure from a composition TiOP.I to TiO1+ Even when the homogeneous range for a phase is small the properties can vary appreciably across the homogeneous range. This has become particularly obvious in recent years through the study of thermicondnctors. When we recognize that a phase does have a range of composition and that many of the properties will vary with composition we see that the properties of phases that are already known can be modified to meet specific requirements. As an example, there is a cubic phase in the ceriumsulfide system (18) which has an appreciable homogeneous range that cannot be detected by X-ray examinations of ordinary accuracy as the 1at.tice constant varies only extremely minutely when the sulfur to cerium ratio is changed from 1.33 to 1.50. Even though the lattice dimensions do not change over this
range of composition, many of the other properties change quite markedly. For example, the electrical properties change from those of an insulator a t CeS,.s to those of a conductor which conducts electricity almost as well as does graphite at the low sulfur end of the homogeneous range. Likewise the thermal shock properties vary quite greatly. The high sulfur material behaves like a conventional ceramic material which must be heated slowly t o avoid cracking whereas the low sulfur material has excellent thermal shock properties and can he heated a t rapid rates. When we consider not only the large number of yet undiscovered phases which can be a t our disposal but also the considerable variation of the properties of each phase that can be obtained when we recognize the effect of variation of composition of the phase, we see that there is a tremendous reservoir of compounds available for the many specialized applications of future developments. Many of these compound have quite unique properties which will adapt them t o very special applications that cannot be met by conventional materials. 4 s we look ahead to the tremendous number of new compounds that await us, v e need not be overly concerned about how we are going to meet the requests for new materials demanded for new technological developments. LITERATURE CITED (1) BREWER, L., AND J. KANE,J . P h y ~ Chem., . 59, 105 (1955); KANE, J., Thesis, University of California Radiation Laboratory unclassified document, UCRL-2957, April, 1955.
(2) BUERGER, M. J., Crystallographic Aspeots of Phase Transformations, Chap. 6 in "Phase Transformationsin Solids," John Wiley & Sons, Inc.. edited by R. SXOLUCHOWSRI, New York, 1951. R. B., Trans. Brit. Ceram. Soe., 54,655-70 (1955). (3) SOB~MAN, (4) COES.L.. JR.. Science. 118. 13132 (1953). (5) WEIS$, A,,AND A. WEISS,'Z. anor;. U . &em. Chem., 276, 95-112 (1954). (6) HALL,H., TRACY,Proceedings of the Symposium, "High Temperature--A Tool for the Future," Berkeley, Calif., June, 1956, pp. 161-66, published by Stanford Research Institute. (7) STULL,D. R., AND G. C. SINKE,Thermodynamic Properties of the Elements, No. 18 of "Advances in Chemistry Series." American Chemical Societv. " , 1956. (8) BERKOGITZ, J., M. G. INGHRAM, W. A. CHUPKA, J. C h m . Phys., 26, 842 (1957). (9) BREWER,L., Paper 7, p. 193, "National Nuclear Energy Series," Val. 19B, edited by L. L. QUILL,McGraw-Hill Book Co., Inc., New York, 1950. DAN.JR.,AND R. J. MARCUS. J. CHEM.EDUC.. (10) MCLACHLAN. ,
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chemical ~ b s i m t s49, , 52713 (1955). L., AND F. T. GREENE, J . Phys. Chem. Solids, 2, (12) BREWER, 286-8s (1957). L., AND R. K. EDWARDS, J . Phw. Chem., 58, 351 (13) BREWER, (1954). WENTORP, R. H., JR.,J . Chem. Phw., 26,956 (1957). BREWER, I,., AND A. W. SEARCY, Ann. Reu. Phys. Chem., 7 , 259-86 (1956). SCHBNRERO, N., Aeta Chem. Scand., 8, 630-32, 932-36 (1954); Nature, 168, 558 (1951). Kuo, K., A d a Met., I , 301, 611-12 (1953). SCHONBERO, N., Acta Met., 2, 83740 (1954); 3, 14-16 (1955). SCHBNBERG, N., Ada Chem. Scand., 8,617-19 (1954). EASTMAN, E. D., LEO BREWER,L. A. BROMLEY, P. W. GILLES,AND N. L. LOFOREN, J . Am. Chem. Soe., 72, 2248-50 (1950).
