Much research has been devoted to studying aluminum oxide (Al 2 O 3) because of its excellent physical properties, such as hardness, high thermal conductivity, high temperature operation, and coloring. [1,2] This make Al 2 O 3 suitable for a wide range of technological applications as well as gemstone industry. Normally, the most stable polymorph of aluminum oxide (Al 2 O 3) is α-Al 2 O 3 named corundum, which is transparent and colorless (for pure α-Al 2 O 3). However, in nature, α-Al 2 O 3 can occur in a variety of colors depending on the presence of impurities in crystals. [3] A variety of colors are known commonly as sapphire. These precious stones are classified according to their colors, e.g., red sapphire, yellow sapphire, blue sapphire, etc. It has been reported that the presence of chromium (Cr) in α-Al 2 O 3 is a cause of red color known as Ruby. [4,5] A small amount of titanium (Ti) impurity in α-Al 2 O 3 could make its color range from colorless to pale pink depending on its growth environment. [6] This causes the energy absorption ranging between 2.21 and 2.53 eV, where two absorption maxima at 2.25 and 2.56 eV are observed. [7] Ti-doped α-Al 2 O 3 is used in tunable solid-state laser applications, which can operate in the range from red to near-IR. [8] The cause of the energy absorption in this range was described by the transition of intravalence electrons arising within the metals, and the interaction with the phonon lattice is attributed to the cause of the energy absorption in this range. [8,9] Once Ti is incorporated into α-Al 2 O 3 , it would substitute for Al's site with 3þ oxidation state. However, it has been reported that Ti can also exist in 4þ oxidation state, but the concentration of Ti IV is quite low in natural sapphire. [9] Regarding iron (Fe)-doped α-Al 2 O 3 , its color can be in the range from yellow to green crystals, and it can be used in stress sensor and optical waveguide applications. [10] Furthermore, it has been
