![]() The effect of ligand fields can be ignored, as there are no unpaired electrons in the chromium (present as a Cr 6+ cation). This K 2CrO 4 contains the CrO 4 2- anion (negatively charged). An example is the oxygen-to-chromium charge transfer in yellow chromate K 2CrO 4. In this case, the charge transfer process occurs between metal and ligand atoms. The charge transfer process occurs both in whole crystals and their powdered forms, making these substances ideal pigments. Substances that display their colors even when ground very finely are good candidates for pigments. The mechanism of charge transfer between two different ionic forms of iron give aquamarine its characteristic greenish-blue color.Ĭharge transfer is also responsible for the colors of a number of pigments. However, the presence of iron impurities in beryl produces the color of aquamarines. This is attributed to ligand field effects. Beryls containing chromium impurities appear green and are known as emeralds. ![]() The surrounding environments of the two iron ions have to be different.įeA 2+ + FeB 3+ → FeA 3+ + FeB 2+ (electrons transferred from site A to site B)īlue aquamarine. A common example is the transformation that occurs in Fe. When the ions belong to the same element, homonuclear charge transfer occurs. Examples of minerals under investigation to determine the exact processes causing their colors are tanzanite, magnetite, and andalusite. Impurities of various kinds may be present at the very low concentrations required for charge transfer to take place, and other factors deriving from ligand field theory, color centers, and so on may make it difficult to isolate the specific source of a perceived color. ![]() It is sometimes difficult to prove that a specific charge transfer process is responsible for the color of a crystal. Tourmaline will appear brown when the same impurities, iron and titanium, are present, but appears yellow with the impurities manganese and titanium undergoing charge transfer. Blue kyanite is colored by the same charge transfer process as blue sapphire. Sapphires undergo "heteronuclear" charge transfer as electrons are transferred between ions of different elements. The "forbidden" transitions in the ligand field-colored ruby are much weaker. Charge-transfer transitions are strong because they are "allowed" by quantum considerations. This energy jump is comparatively easy for electrons to achieve when light shines on the crystal. While at least 1% chromium must be present in corundum before the deep red ruby color is seen, sapphire blue is apparent with the presence of only 0.01% of titanium and iron. Intervalence charge transfer is a process that produces a strong colored appearance at a low percentage of impurity. There can be adjacent pairs in different directions since the spacing between the atoms is different, so is the energy-level spacing. When this light is subtracted from incident white light, the complementary color blue results. This schematic shows the wavelength of the energy required for the transfer of electrons This schematic shows the wavelength of the energy required for the transfer of electrons from Fe 2+ to Ti 4+ corresponds to yellow light. This occurs because there is enough overlap between the outer orbitals of Fe 2+ and Ti 4+ to allow an electron to pass from one ion to another. When Fe 2+ and Ti 4+ ions occupy two adjacent sites in a corundum crystal, the transfer of an electron from the iron cation (positively charged) to the titanium cation (positively charged) can now change the valence state of both. ![]() ![]() If Fe 2+ and Ti 4+ ions are substituted for Al 3+, localized areas of charge imbalance are created. Iron can take the form Fe 2+ or Fe 3+, while titanium generally takes the form Ti 4+. The process at work is called intervalence charge transfer or cooperative charge transfer, which is the transfer of an electron from one transition-metal ion to another. If both titanium and iron impurities are present together, however, the result is a magnificent deep-blue color. If a similar amount of iron is present, a very pale yellow color may be seen. Corundum contains a repeating unit of aluminum oxide and is colorless.Ĭorundum that contains a few hundredths of one percent of titanium is colorless. ![]()
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