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(This page is under development, and might be completed Winter 2002).
SAPPHIRE — COLOR PRODUCED BY CHARGE TRANSFER
- Figure: Sapphire.
- Caption: The intense blue of the sapphire is caused by the addition of titanium and iron to the mineral corundum.
Like ruby, sapphire is a particularly brilliant gemstone. Unlike ruby, sapphire is blue not red. Like ruby, it is formed by adding impurity to the mineral corundum. Ruby and sapphire are among the most brilliant gemstones because corundum is the hardest crystal, second only to diamond. The hardness ensures that crystals, when polished, retain their brilliant luster. The intense red of the ruby results from adding to corundum a single impurity (chromium). The intense blue of the sapphire comes from adding to corundum two impurities (titanium and iron) and the blue color is produced in a slightly different way, by a process known as charge transfer. It turns out that many colored pigments, such as the red and yellow ochers (used to such effect throughout time, by the cave artists of the Ice Age, by Michelangelo on the Sistine Chapel ceiling and still by artists of the present day) achieve their characteristic colors through this process of charge transfer.
- Figure: Ultramarine.
- Caption: Utramarine achieves its brillian color through the process of charge transfer. Ultramarine was used by the ancient Egyptians, the French Impressionists and in the present day. Other pigments, such as red and yellow ochre are caused by charge transfer.
The most famous sapphires came from a small deposit in Kashmir, that was discovered in 1880, but this remote mountain area has since been exhausted. Many sapphires come from Australia, which has several deposits of deeply colored stones. These are more widely available in the market, but are not as valuable because of their overly dark color. Very fine sapphires are also found in Burma, but in very limited quantities. Other sapphires come from Sri Lanka, Thailand, and Cambodia. "Cornflower Blue" sapphires are most often from Sri Lanka. (http://www.gemcolor.com/gems/sap.html)
If a system is to appear colored, the system has to be able to exist in two possible states — the state it is occupying and a state of higher energy which is unoccupied (like two seats, one higher than the other, in a movie theater). The two states must separated in energy by the energy of a photon in the visible region of the spectrum. If that applies, the system can be driven from its occupied state to the unoccupied state by absorbing the "visible" photon. When that happens, the system will appear to have the color of the residual light that has not been absorbed. In ruby, the two requisite states, with the requisite energy difference, are provided by the chromium impurity; and the transition from the occupied state to the unoccupied state makes the ruby appear red.
Unlike chromium, impurities such as titanium and iron produce no intense color alone. Whatever the impurity, there will, of course, always be unoccupied states; but, for titanium and iron, the energy needed to reach those states does not match the energy of a "visible" photon. But if both titanium and iron are added together as impurities, the characteristically intense blue color of a sapphire is produced. The two impurities together act cooperatively. We can label the iron X and the titanium Y. Considere atoms of iron and titanium in adjacent sites in the crystal. We can represent this initial state as [X + Y]. Now imagine pulling an electron off X and giving it to Y. This will require energy and our new, excited state [X+ + Y-] will be a state of higher energy. Now imagine that the difference in energy between the initial state and the excited state matches the energy of a"visible" photon. If that is so, the system will be able to absorb the "visible" photon; the energy of the photon will drive the electron to jump from X to Y; and the crystal will appear colored.
[X + Y] + photon = [X+ + Y-]
An electron is transferred from X to Y; the electron carries a negative charge; and the process is described as "charge transfer" as well as electron transfer. For sapphire photons of different energies can drive the charge transfer — "red" photons, "yellow" photons, "green" photons. Only "blue" photons cannot and that is why sapphires appear blue.
How impure must the crystal be for it to appear colored? For sapphire, only 0.01% of titanium and iron are needed as the electron transfers very easily. For ruby, at least 1% of chromium is needed because electron jumps are less facile.
What happens subsequently? The process — absorbing the photon and driving the system from the initial to the excited state — is effectively instantaneous. It actually takes about a femtosecond or 10-15 second. (This time bears approximately the same relationship to a second as one second bears to the age of the universe which is 1018 seconds.) The process is instantaneous and leaves the system in an excited state. Any system which can lose energy, will always do so spontaneously after some time. Our system is no exception. It could return to its initial state by "popping out" the same "colored" photon that it originally absorbed. Actually it doesn't. Instead it gives out the energy as heat, momentarily raising, to a slight extent, the temperature of the surrounding crystal. Typically this relaxation takes 10-10 second, nearly a million times longer than the original excitation.
Rubies and sapphires are colored forms of corundum, caused by impurities undergoing, respectively, excitation and charge transfer. The analogues for beryl are emeralds and aquamarines. The two impurities that give aquamarine its characteristic greenish-blue color are the two different forms of iron, the ferrous ion Fe2+ and the ferric ion Fe3+, so that the charge-transfer corresponds to the process
[Fe2+ + Fe3+] + photon = [Fe3+ + Fe2+].
This same charge-transfer process, between the ferrous and ferric forms of iron, present as impurities in different solid environments, is a ready source of color. It makes glass bottles brown; it makes sandstone brown (familiar as the brownstone used in buildings); and, as discussed in the following section, it is the source of the blue in the pigment Prussian Blue Fe4[Fe(CN)6]. (The first Fe is the ferrous and the second the ferric.)
- Vermilion, realgar, chrome yellow — representative colored pigments also produced by charge transfer
We can imagine a situationwhere the two impurities in the crystal X and Y are made so concentrated in the crystal that ultimately no crystal. If X and Y canbind to form a m olecule XY, then the solid consists of pure XY. This solid can apppear colored if an electron can transfer from the X part of the molecule to the Y part and if the photon needed to drive the transfer corresponds to a photon in the visible region of the spectrum. This is the basis for the colorof many important pigments — ranging from the red and yellow ochers that occur naturally and that have been used throughout history, to pigments such as chrome yellow, synthesized early in the nineteenth century and exploited so extensively by artists since the time of the French Impressionists. A table of these "charge-transfer" pigments follows
- Brill Table X-1, p. 204, first two columns only. New title: Pigments in which the color is produced by charge transfer. Add a line item to bottom: Prussian blue(Fe4[Fe(CN)6]) Fe3+Fe2+ =hv= Fe2+Fe3+
x
(This page is under development, and might be completed Winter 2002).
A crystal of corundum containing a few hundredths of one percent of titanium is colorless. If, instead, it contains a similar amount of iron, a very pale yellow color may be seen. If both impurities are present together, however, the result is a magnificent deep-blue color, that of blue sapphire, as seen in Plate 1. The process at work is "intervalence charge transfer," the motion of an electron from one transition-metal ion to the another produced by the absorption of light energy; this results in a temporary change in the valence state of both ions. Such a mechanism is the cause of the blue of sapphire and the dark colors of many transition metal oxides such as the black iron oxide magnetite Fe3O4 This mechanism is sometimes also called cooperative charge transfer.Consider two adjacent Al sites in corundum (see Fig. 7) occupied by Fe2+ and Ti4+ ions, as in Fig. 14. The transfer of an electron from the Fe to the Ti can now change the valence state of both atoms:
Fe2+ + Ti4+ --> Fe3+ + Ti3+
This process requires energy, as shown in Fig. 15; since the energy corresponds to the absorption of yellow light, as shown at the center of Fig. 16, the complementary color blue results. There can be adjacent pairs in directions other than that shown in Fig. 14; since the spacing between the atoms is different, so will be the energy-level spacing of Fig. 15, leading to the blue-green dichroism, seen in Fig. 16, analogous to that of ruby in Plate VI.
Blue sapphire is an example of "heteronuclear" charge transfer with two different transition-metal ions involved. In magnetite, the black iron oxide Fe3O4 or Fe2+0 . Fe23+O3, there is "homonuclear" charge transfer with two valence states of the same metal in two different sites, A and B:
FeA2+ + FeB3+ ---> FeA3+ + FeB2+.
The right-hand side of this equation represents a higher energy than the left-hand side, leading to energy levels, light absorption, and the black color. In sapphire this mechanism is also present, but there it absorbs only in the infrared, as at a in Fig. 16. This same mechanism gives the carbon-amber (beer-bottle) color in glass made with iron sulfide and charcoal, and the brilliant blue color to the pigment potassium ferric ferrocyanide, Prussian blue Fe43+[Fe2+(CN)6]3. The brown-to- red colors of many rocks, e.g., in the Painted Desert, derive from this mechanism from traces of iron.
Charge transfer can also occur between metal and ligand atoms. One example is the oxygen-to-chromium charge transfer in the yellow chromate K2CrO4 and the orange dichromate (NH4)2Cr2O7 of Plate 11; note that the formal valence of 6+ on the Cr leaves no impaired electrons and therefore rules out ligand-field colors as in the other trivalent chromium colors of Plate 11. In sapphire this mechanism is also present, but there absorption occurs only in the ultraviolet as at d in Fig. 15. A final example of charge transfer is the deep-blue gemstone lapis lazuli of Plate 1, which has the same composition as the pigment ultramarine, approximately CaNa7Al6Si6O24S3SO4. This color derives from charge transfer among the three sulfur atoms of the S3 ion.
Charge-transfer transitions are strong because they are "allowed" by quantum considerations, hence intense colors are produced by as little as 1/100 percent Fe and Ti in blue sapphire; by contrast, the "forbidden" transitions in the ligand-field- colored ruby are so weak that one to three percent Cr is required for an intense red color.
FIG. 13. Absorption and fluorescence spectrum of cresyl violet, also known as the laser dye oxazine 9, dissolved in ethanol. [After K. H. Drexhage, in F. P. Schafer, Ed., Dye Lasers, Springer Verlag, New York, 1973, p. 173.]
PLATE V11. Fisherman holding chemoluminescent "Cyalume" light sticks, which are used as fish lures.
FIG. 14. Two adjacent octahedral sites containing Fe2+ and Ti4+ in blue sapphire; compare Fig. 7.
FIG.15. Transition from the ground state to the excited state in blue sapphire.
FIG. 16. The clichroic o-ray and e-ray absorption spectra of blue sapphire. Band a is derived from Fe 2+ --> Fe 3+ charge transfer, band b from Fe 2+ --> Ti4+ charge transfer, band c from a ligand-field transition in Fe3+ , and band d from O2- ---> Fe3+ charge transfer. (After G. Lehman and H. Harder, Am. Mineral 55, 98 (1970).]
