Nov., 1953
METALSIN
THE
ATOMIC STATEIN GLASSES
753
METALS I N THE ATOMIC STATE I N GLASSES BY W. A. WEYL School of Mineral Industries, The Pennsylvania State College, State College, Pennsylvania Receircd March 1.9, 10.55
Glasses cannot respond to “foreign” ions within their structures in the same fashion as crystalline solids. A glass can neither produce an efficient phosphor when the usual activators (Cu+, Ag+, Mn2+, etc.) are incorporated into its structure nor can its properties be changed by minor additions to the same extent as can those of metals or of semiconducting ionic crystals. The reasons for the different responses of vitreous and crystalline matter to “impurities” are explained and examples are cited. The only substances which in concentrations of less than 0.01% can produce strong light absorption for fluorescence in a glass are the metals. The interaction between glasses and metal atoms and the aggregation of metal atoms in glasses as the result of heat treatment or of irradiation are described.
I. Response of Crystals and Glasses to “Foreign” Ions
for the foreign ion to assume the charge of the major cation of the glass. Consequently, thermal Among solids a glass represents the most gener- dissociation of V206 can lead to the more stable alized array of ions. In the vitreous state the V203 or to V3+ions in glasses. Heating a mixture of Ti02 with a small concentramajor cations, e.g., the Si4+ ions, in a soda limesilica glass have the same coordination as in crystals tion of Nb20b (both oxides are white and nonof similar composition. I n the absence of long conducting) produces bluish-gray crystals which range order, however, the cations of lesser field formally can be described as containing Ti3+ strength, in our example the Na+ and the Ca2+ ions and which conduct electricity. In this case ions, can have coordination numbers which are not the foreign Nb6+ ions did not change their own as well defined as in crystals and which are better quantum state in order to fit into the host lattice, but the foreign ions forced a corresponding number described by an average value. The absence of long range order causes glasses to of normal Ti4+ ions to accept an electron and resemble liquids with respect to their behavior change into Ti3+ions. In such a crystal the couple toward additions which go into solution. It is Nb6+Ti3+ takes the place of two normal Ti4+ customary to speak of “foreign atoms” if an atom ions. Ti:t2,Nb:+Ti:+O;or ion due to its size, charge or its electron configuration does not fit into the structure of the These two types of response to foreign ions are host. At high temperature where the entropy characteristic for crystals. The addition of Nbz06 term becomes an important factor in determining to a glass containing major concentrations of Ti02 the free energy of a system, some crystals can neither produces a bluish-gray color nor electronic increase their stability by developing ‘(defects.” conductivity. A glass allows the Nb6+ ions t o Many substances, such as PbO2 or FeO, are not form their own environment so that they are not known as crystals of stoichiometric compositions. “foreign” to its structure. From the heats of formation of the iron oxides one For this reason it is not possible t o produce a would expect the compound FeO to disproportion- glass which has the intense fluorescence of some ate above 500” into magnetite and metallic iron. crystals when activated by “foreign” ions. The Instead of decomposing, the FeO increases its optical properties of an “activator” are functions stability by forming a defective structure which has of the electrical field to which it is exposed. a higher entropy because of the vacant lattice sites In aqueous solution Cu+ ions are colorless and and a certain concentration of “foreign atoms,” they do not fluoresce when irradiated with ultranamely, Fe3+ ions taking the places of regular violet light. These Cu+ ions are in positions where Fez+ ions. the electrical field is zero over a time average. It is not uncommon in high temperature chem- Under these conditions the deformation of the istry to find that a compound increases its stability electron cloud of the Cu+ ion and the probability by developing lattice vacancies or by taking into of its excitation by light have minimum values. its lattice constituents which do not fit into the A solid solution of CuCl in ZnS is a phosphor structure under normal temperature conditions. because the electron clouds of the Cu+ ions are With respect to “foreign” ions, glasses behave like deformed under the influence of the electrical field liquid solvents. They cannot respond t o impuri- of the defective structure of the host. The fluoresties in the same manner as crystalline solids be- cence center is not a Cu+ ion surrounded by four cause the lack of long range order permits a foreign S2- ions in a symmetrical fashion as would be the ion to form its own normal environment. case if a Cu+ ion merely replaced a Zn2+ion, but it A crystalline silicate, e.g., zircon, ZrSi04, can consists of an asymmetrical group of three S2accept a small concentration of foreign ions in its ions and one C1- with the Cuf ion in the center. lattice at high temperature. Heating a mixture of Since the Cu+ ion is surrounded by anions with ZrOz, Si02 and small amounts of V206 produces a different charges, it occupies a position where the blue pigment which contains vanadium in the electrical field is no longer zero and where its elecotherwise rare quantum state V4+. Introducing tron cloud will be deformed. The same effect Vz06 into a glass also causes partial dissociation of can be produced by introducing cations with a this oxide, but an equilibrium is established pri- threefold charge, such as AI3+ ions, with the Cu+ marily between the quantum states V6+ and V3+. ions. In this case the asymmetry originates in the The lack of long range order makes it unnecessary second coordination sphere of the activator. The
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W. A. WEYL
substitution of C1- ions for S2- ions performs two functions. It assures electroneutrality and it produces a distortion of the electron cloud of the activator. Kroegerl and associates stressed the importance of the electroneutrality of the lattice for making possible the formation of a solid solution. The author2 and associates emphasized the importance of the asymmetry of the environment and the effect of the resulting electrical field upon the optical electrons of the activator. Glasses as hosts for foreign atoms resemble aqueous solutions more closely than crystals with respect to heavy metal ions as activators of fluorescence. ‘Keeping in mind this difference between the responses of crystalline and vitreous solids toward “impurities,” we cannot expect spectacular phenomena if a minor addition of an oxide is made to a glass. Glasses are neither suitable hosts for heavy metal ions to change them into efficient phosphors nor are their properties strongly affected by minor additions as is the case in metals (conductivity, ductility, etc. j . There is, however, one exception. With respect to noble metals, glasses can respond to minor concentrations of a foreign substance in the same fashion as can crystals.
11. Glasses Containing “Frozen in” Metal Vapors Some elements can be dissolved in fused salts and in glasses where their molecules or atoms play the role of a vapor which has formed a t high temperature and has been “frozen in” during cooling without having had a chance to condense and form a separate phase. Solutions of this type may exhibit the absorption and fluorescence of the vapor phase of the element. The number of elements which can form such solutions is small because it is limited to those which do not react chemically with the solvent. As far as the molecular weight of the solute is concerned, the same rules apply to the solution of an element in a glass as to the vapor phase. Except a t extremely high temperatures atomic vapors can be formed only by the noble gases and by metals. Non-metallic elements form molecules both in the vapor phase and in solution. The tendency toward formation of atoms in solution will be illustrated by the behavior of the elements sulfur, selenium and tellurium. The metallicity of these elements increases from sulfur which is an insulator to tellurium which has distinct metallic properties. Selenium forms several modifications one of which resembles sulfur, and one which resembles tellurium. The tendency of these elements to form atoms in the vapor phase or in solution increases according to their metallicity from S to Te. Even a t 2000” the vapor of sulfur contains the molecules Sz. The Sz molecule is probably responsible for the blue color of a solution of sulfur in sulfuric acid, sulfur trioxide or in certain glasses. The Sz molecule is also the color center of the blue mineral lapis (1) F. A. Kroeger and J. E. Hellingman, J . Electrochem. Soc., 93, 156 (1948). (2) J. K . Inman, A. M. Mraz and W. A. Weyl, “Solid Luminescent Material,” Cornell Symposium of the American Physical Society, John Wiley and Sons, Inc., New York, N. Y., 1948, p. 181.
Vol. 57
lazuli and the synthetic counterpart, the pigment ultramarine.3 Sulfur added to fused sodium borates produces a brown glass containing Sz- ions (polysulfides) in which the value of x may be as high as seven. Addition of Bz03 or Pz06to such a brown polysulfide glass causes its color to change into blue because in the more acid medium elemental sulfur is liberated and forms Szmolecules. Recently, E(.0. Otley and W. A. Wey14found that exposure of such a blue glass to X-rays causes its color to fade. The capture of electrons by the Szmolecules changes the blue color centers into colorless anions. According to R. Auerbach,6 selenium dissolved in pyrosulfuric acid can produce either Se atoms (red) or Sez molecules (green). The green molecular solution which forms a t low temperature changes into the red atomic solution above 130”. The tendency of selenium to change from its sulfurlike modification into a more metallic modification a t higher temperature is found even in solution. Atomic selenium imparts a pink color to silicate glasses (selenium pink). Glasses which contain atomic selenium are fluorescent, they absorb green light and emit red light. Tellurium, because of its more metallic character, dissolves in pyrosulfuric acid only in atomic form. It is difficult to produce silicate glasses which contain tellurium in atomic form because of the formation of tellurides. W. D. Smiley and W. A. Weyl6 produced a bluish-red glass containing atomic tellurium by incorporating 1% tellurium under oxidizing conditions into a sodium-barium aluminophosphate.
111. On the Binding Forces between Metals and Glasses The development of the gold ruby glass has accumulated considerable empirical knowledge of the binding forces between this noble metal and the ionic glass.’ The ionization potential of gold is sufficiently high t o prevent its oxidation and the formation of a gold compound a t the melting temperature of a glass (1400-1450”). On the other hand, the vapor pressure of this metal a t this temperature is of the order of 0.1 mm. so that noticeable concentrations of gold vapor are formed during melting and are frozen in if the melt is cooled fairly rapidly. Under these conditions the glass retains gold in atomic form. Such a chilled glass is colorless and exhibits a weak fluoresceiye when exposed to ultraviolet radiation (3650 A,). Reheating the glass allows the “frozen in” gold vapor to condense. The ruby color is obtained under conditions where the condensation leads to a large number of particles of colloidal size. If the number of nuclei is too small, the particles grow too large and the glass assumes a brownish color in reflected and a faint bluish color in transmitted light. The kinetics of the aggregation of the colloidal gold (3) W. A. Weyl, ”Coloured Glasses,” Monograph published by The Society of Glass Technology, Sheffield 10, England, p. 257. (4) K. 0. Otley and W. A. Weyl, J . A p p l i e d Phya., 23,499 (1952). (5) R. Auerbach, Z. p h y s i k . Chem., 121, 337 (1926). (6) W. D. Sniiley and W. A. Weyl, Glass Science Bull., 6 , 38 (1947). See also ref. 3, up. 324-5. (7) Reference 3, pp. 331-408.
Nov., 1953
METALS
IN THE
ATOMICSTATEIN GLASSES
particles in a ruby glass was studied by Zsigmondy8 as one of the first applications of his ultramicroscope. The condensation process of the gold, i.e., the formation of nuclei and their growth into particles of colloidal size, is governed by the activation energy of the diffusion process which, in turn, depends upon the viscosity of the glass and on the strength of the binding forces between glass and gold atoms. If the forces are weak, the aggregation takes place rapidly and cannot be controlled to the extent which is essential for producing a good ruby. Normal soda lime-silica glasses do not show sufficient affinity to gold, but in an empirical way technologists have found that certain ingredients can bridge the gap between the jonic structure of a glass and the noble metal. The author has coined the word “metallophilic” in order to describe the property of an ion to serve as a bridge between glasses and metals. The ability of an ion to do so is characteristic of certain non-noble gas-like ions. Most commercial glasses contain only noble gaslike ions, namely, Si4+, Al3+, Ca2+, Mg2+, Na+, 02-ions. The behavior of a glass toward a metal is changed fundamen tally if non-noble gas-like ions are introduced. The manufacture of gold or copper ruby glAsses requires the addition of PbO or of minor amounts of Sn0 or Bi203to the glass batch. Glasses which contain noble gas-like ions only are unsuitable for this purpose, because the forces between these glasses and gold are too weak. Marboe and Weylgdeveloped a method for studying the forces acting between ionic crystals and noble metals which is based on the aggregation of noble metal atoms, adsorbed at the surface of ionic crystals. The nature of the forces acting between metallophilic cations and metal atoms can be understood on the basis of Fajans’ theory of the deformation and polarization of ions. Crystalline compounds of the general formula XO, for example, MgO or CaO, where X is a divalent noble gas-like ion, form lattices of high symmetry. I n crystalline MgO each Mg2+ is symmetrically surrounded by six oxygen ions, a configuration which is probably only slightly distorted if MgO participates in a glass structure. If, however, X is a lion-noble gaslike ion, such as Pb2+or Sn2+,the structures of the XO crystals contain asymmetrical building units. The structure of the tetragonal PbO is characterized by a prism with 0 2 -ions in each of its eight corners. Inside the prism we find the Pb2+ ion and it is significant that the Pb-0 distance: to four of the eight corners are much shorter (2.38 A.) than those to the other four corners (4.27 A.). This structure is interpreted on the basis that the four 02ions which are close to the Pb2+repel the easily polarizable shell of the Pb2+ions, in particular its outermost 6s2 electrons. The Pb2+ ion thus loses its spherical symmetry; its electron density is higher on one side than on the other.1° (8) R. Zsigmondy, “ Z u r Erkenntnis der Kolloide,” Jena, 1905. (9) E.C.Marboe and W. A. Weyl, J . Applied Phys., 20, 124 (1949). (10) K. Fajans and N. J. Kreidl, J . A m . Ceramic Soc., 81, 105 (1948)
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The essential point of this picture can be expressed as follows. The lead ion, Pb2+,in the field of the 02-ions becomes asymmetrical. The repulsion of the two outer electrons leaves only the 18 elec,trons of the 0-shell as the outer orbit. This electron configuration corresponds to that of the Pb4+. The other side of the deformed Pb2+has a higher electron density than the normal Pb2+and thus resembles the neutral P b atom. One can, therefore, describe a Pb2+ ion in an electrical field by the scheme Pb’+ = ‘/zPb‘+
+ ‘/$b
Its one side extends strong ionic forces toward the anion comparable to a Pb4+ion and the other side has the character of a metal atom.l1 Such a deformed Pb2+ or Sn2+ion bridges the gap between an ionic substance and a metal atom.
IV. Formation of Silver Atoms in Glass and their Aggregation If a small amount of AgCl is dissolved in fused KaC1, a solid solution is obtained on cooling which contains some Ag+ ions in the places of some regular Na+ ions in the crystal where they form Ag+ (C1-/6)s units. Exposure of such a solid solution to hydrogen gas at about 150” produces silver atoms according t o Ag+Cl-
+ ’/zHz
=
Ago
+ HCl
The silver ion traps an electron and changes into a silver atom and the proton enters the electron aloud of a neighboring C1- ion producing a HCI molecule. The silver atom participates in an asymmet8ricalunit Ago (C1-/6)5 (HCl), where it is exposed to and deformed by the electrical field which exists between a negatively charged chlorine ion and a neutral HC1 molecule. These silver atomsofluoresce strongly when irradiated with the 3650 A. line.12 This method of producing fluorescent Ag atoms in a solid host is also applicable to silicate glasses. Soda lime-silicate glasses containing in the order of 0.1% Ag20are colorless when melted under oxidizing condition: anddo not fluorescewhen irradiated with the 3650 A. line. Such a glass contains Ag+ ions taking the places of regular Na+ ions. Exposing the powdered glars to hydrogen a t a temperature of 100” or slightly higher changes the Ag+ ions into neutral Ag atoms and now the glass fluoresces because of the presence of atomic silver or of silver ions plus a trapped electron. Atomic silver can be formed in systems containing Ag+ ions by exposure to X-rays, cathode rays or to ultraviolet radiation. 13,l 4 A glass containing silver or gold in atomic form is metastable a t room temperature and the atoms aggregate if the activation energy for their diffusion is supplied in the form of heat or radiation. The light sensitivity of glasses containing gold or silver has been known for a long time but only re(11) W. A . Weyl, Trans. N . Y . Acad. s’ci., Series 11, 12, 245 (1950).
(12) W . A. Weyl, Sprechsaal, 7 0 , 578 (1937). (13) W. A. Weyl, J. H. Schulman, R. J. Qinther and L. W.Evans, J. Electrochem. SOC.,96,70 (1949). (14) G. E. Rindone and W. A. Weyl, J . Am. Ceramic Soc., 3 3 , 0 1 (1950).
W. A. WEYL
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cently-has the systematic work of Dalton, Stookey’s cause of the high interfacial energy of the interand Armistead in the Corning Glass Works led to a mediate state which might be called an emulsion. glass which is sufficiently sensitive to be used for In the system LizO-SiOz the energy threshold of photographic purposes. unmixing is too high for a spontaneous separation As early as 1871 Muller’6 reported that an un- of the melt into two liquids. Karkhanavala and struck gold ruby exposed to daylight developed a Wey120 postulated that such a melt contains two reddish color after several years. Badger and types of complexes which would form two liquids of Hummel” showed that on reheating metallic silver different structures if it were possible to overcome is precipitated from a glass a t a lower temperature the energy barrier. By introducing traces of colwhen the glass had been exposed previously to loidal platinum or palladium metal into the melt it ultraviolet radiation. Rindone and Weyl’* found was possible to change a homogeneous melt (10 that exposure of a silver containing glass to ultra- mole % LizO, 90 mole % SiO,) into two liquids. It violet radiation (3650 produced two antagonis- goes without saying that this catalytic effect of the platinum metals is limited to their state of extreme tic effects. The trapping of electrons by Ag+ ions produces subdivision and not shared by the bulk metal. Recently Rindone and Rhoads,21in the author’s fluorescence centers and the aggregation of the Ag atoms under the influence of the radiation destroys laboratory, observed that the introduction of fluorescence centers. As a result, the fluorescence 0.001-0.01% platinum in the form of its chloride of a silicate glass containing AgzO goes through a into fused sodium metaphosphate causes this melt maximum when it is irradiated with the 3650 A. line to crystallize. Lower concentrations of platinum are not effective because they form ions and single €or several hours. I n contrast to soda lime-silicate glasses, some atoms. Higher concentrations are not effective borates, aluminosilicates, phosphates and alu- because the resulting particles are sufficiently large minophoaphates can stabilize relatively high concen- to have developed metallic character and metallic trations of silver ions. In connection with work on platinum does not induce crystallization. Sodium silver borate glasses, Rindonelg discovered an in- metaphosphate fused in a platinum crucible forms teresting phenomenon. Silver borates containing a clear glass on cooling. up to 60 wt. yo Ag,O have been prepared. These glasses are colorless when the silver is completely in ionic form but turn brown if some of the AgzO DISCUSSIOK dissociates and forms colloidal silver. In the presence of water vapor these glasses undergo a slow H. W. LEvERE.vz.-Luminescence is essentially a Eocaldecomposition in the dark which produces a coher- ized process, and so long-range order is not always necessary ent film of metallic silver on the surface. Exposure for high efficiency. Long-range order is required to permit transmission of secondary excitants, such as exof these high silver glasses to ultraviolet radiation efficient cited electrons and excitons, but it is sometimes possible to accelerates this process so that within a few min- excite the luminescence centers directly with low-energy utes a silver mirror appears, the formation of which hotons that are not appreciably absorbed by the host. requires several months in the dark. The reaction hence, there are phosphate glasses, with manganese and other activators, and uranium glass, which are efficient when involves as a first step a “base exchaage” between excited by ultraviolet radiation. silver ions of the glass and protons of the adsorbed W. A. WEm.-The phosphate glasses have to contain water film. crystalline materials (opal glasses) in order to be useful for During the aggregation of metal atoms one ob- fluorescent light sources. serves an intermediate state which is no longer atomic but not yet metallic. The presence of gold, LEvEmNz.-Uranium glasses? platinum or palladium in a glass in this state of subWEYL.-The fluorescence of uranium is different from that division can induce crystallization and immisci- of the usual activators, e.g., copper or manganese. Uranium fluoresces as the (U0.J +Z group even in aqueous solutions. bility. The physical properties of alkali silicate glasses Here we deal with a localized effect of the uranyl group; but I think that Dr. Pringsheim knows more about these as a function of the composition suggest that the things than I do. presently used concept of a three-dimensional P. PRINGSHEIM.-~ think you are quite right in saying random network is an idealized one which seems to that the fluorescence of the uranyl ion is almost independent be correct for rubidium or cesium silicates only but of the nature of the surrounding medium because the cornot for those glasses which contain rations of higher responding electronic transition is well protected from perturbation, but I do not quite understand what according t o field strength. The inverted S-shaped liquidus your opinion is the reason that certain groups, or certain curve of the SiOs-Li20 system is usually inter- atoms, or certain ions do fluoresce and others do not. I preted as indicating a tendency of this system to- mean the fact that they are colored has nothing to do with it ai all. For instance, copper sulfate solution is nicely ward immiscibility. A separation of a melt into colored and does not fluoresce. Thallous chloride in aquetwo phases is quite common in siliceous melts. The ous solution shows no color and is nicely fluorescent. process itself must involve an energy barrier be-
I
w.)
(15) 8. D.Stookey, Ind. Eng. Chem., 41,856 (1949); S. D. Stookey. (J. Alexander) “Colloid Chemistry,” Vol. VII, Reinhold Publ. Corp., New York, N. Y . , 1950,p. 697. (16) W. Muller, Dinglers Polylechn. J . , 201, 117 (1871). (17) A. E. Badger and F. A. Hummel, Phys. Rea., 68,231 (1945). (18) G. E. Rindone and W. A. Weyl. J . A m . Ceramic SOC.,33, 91
(1950). (19) G. E. Rindone, J . Soc. CEass Techn., 37, 124 (1953).
WEYL.-We really have to be careful with a statement about what fluoresces and what does not fluoresce. Cadmium sulfide, for example, does not fluoresce in the visible (20) M. D. Karkhanavala and W. A. Weyl, 0 . N . R Report NO. 45 “An Experimental Study of Silicate Liquid Immiscibility,” Contraot, No. N6 onr 269, Task Order 8,N R 032-204,265 April, 1952. (21) J. L. Rhoads, M.Sc. Thesis, Division of Ceramics, The Pennsylvania State College, 1953.
v
Nov., 1953
ALKALI HALIDESCOLORED BY COLLOIDAL METAL
region in the form of large crystals. It fluoresces in the infrared. But extremely small crystals fluoresce in the viaible region. Most cadmium sulfide glasses fluoresce rather strongly. PRINGSHEIM.-YeS, 1 know that some substances fluoresce and others do not fluoresce; but I do not quite see what the
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connection between fluorescence and the symmetry of an electric field might be. WEyL.-The absorption as well as the fluorescence of ions or molecules depends strongly on the symmetry of the field in which they are located. Many substances, when brought in the asymmetrical field of a surface, for example cadmium sulfide, show strong fluorescence.
ALKALI HALIDES COLORED BY COLLOIDAL METAL* BY ALLENB. SCOTT,WILLIAM A. SMITHAND MILTON A. THOMPSON Department of Chemistry, Oregon State College, Corvallis, Oregon Received March 1.2, 1965
Alkali halides colored with an excess of alkali metal develop absorption bands due to colloidal metal upon moderate heating, provided the concentration of alkali metal is above the stable concentration of F-centers a t the treatment temperature. The extinction is due almost entirely to absorption, rather than scattering, unless the particle size is greatly increased by extended treatment. The location of the band maxima in NaCl, KCI, KBr and K I can be satisfactorily calculated from the optical properties of the metal. The particle size range for the smaller particles is within the limits 10-50 A. The heat of dissociation of colloid in the four lattices is calculated. A new band in K I which has many characteristics of a colloidal band, but which absorbs in a region far from that calculated for colloidal potassium is described.
I. Introduction There has been much recent interest in the coloring of alkali halide crystals by irradiation with Xrays, bombardment by high energy particles, or by heating in the vapor of an alkali metal.’ At moderate temperatures, the color centers produced are unstable with respect to centers which can be described as “colloidal,” and a study of the nature of the colloidal centers and their equilibrium with the smaller color centers is desirable in completing the outlines of the theory of ionic solids. The occurrence of similar colloidal metal dispersions is encountered in the electrolysis of fused salts, in natural blue rock-salt, in certain colored glasses such as gold ruby glass, and in silver halides which have undergone prolonged irradiation as in the “print-out effect.” I n the alkali halides, the colloidal centers are most conveniently prepared by warming crystals containing F-centers in excess of the stable concentration and allowing the centers to coagulate. Since crystals colored by irradiation contain an equal number of F-centers and holes, a competing reaction, namely recombination of the electrons and holes, makes the formation of colloid unlikely; for this reason it is necessary to use crystals which have been additively colored by heating in an alkali metal vapor and rapidly quenched to room temperature to “freeze in” a high concentration of color centers. The colloidal centers cause strong absorption in a narrow spectral region, and, unless the particle size is greatly increased by prolonged heating, very little light scattering is observed.2 The absorption band will be referred to as the “colloidal band.” 11. Properties of Colloidal Centers (A) Absorption of Light.-The shape and location of the absorption band for small spheres of metal, of diameter much less than the wave length
* Published with the approval of the Oregon State College Monographs Committee as Research Paper 224, Department of Chemistry, School of Science. (1) N. F. Mott and R. W. Gurney, “Electronic Processes in Ionic Crystals,” Oxford, 1948. (2) R. W. Pohl, “Einfuhrung in die Optik,” Springer, Berlin, 1943.
of the incident radiation and imbedded in a medium of refractive index G,can be calculated from the equation2 n2k
36?rNvV -
Where K is the extinction coefficient defined by: (path length) K = 2.303 (optical density); N , is the number of particles per unit volume; V is the volume of a single particle, X is the wave length in air for which K is sought; n is the refractive index of the metal; and nii is the “absorption coefficient” defined by: nL = KX/4n. Pohl describes the application of (1) to the colloidal sodium band in NaC1. Since neither N , nor V is known, the maximum value of K was set equal to the empirical value; the location and shape of the band then agreed quite well with the observed location and shape. This is evidence that the centers in NaCl are very small and can be treated as spheres. It may be also considered proof that when several F-centers coagulate, an equal number of surrounding cations are discharged so that the product is in actuality a metal speck. For the alkali halides, in the spectral region of interest, n is very small compared to nk. This allows an easy calculation of the location of the band maximum by neglecting the terms in the denominator of (1) which involve n and obtaining the condition for which the denominator vanishes nk = fin,,
Since neither nk nor n, changes much with temperature, and the absorption is not influenced by lattice vibrations, neither the band maximum nor the breadth of the band should be altered by changing the temperature of observation of the band. This has been confirmed by experiment,2*8 and temperature insensitivity sometimes has been regarded as a sufficient criterion for a colloidal absorption band. We later present evidence t o show that i t may not be sufficient. (3) A.
B. Scott and W. A. Smith, Phys. Rcv.,
88, 982 (1961).
