Sapphire crystal structure

A colorless mineral known as corundum (composed of aluminum oxide) is colorless. A red variety of corundum known as ruby, a precious stone, owes its color to impurities of chromium within the crystal structure of corundum. Blue and violet varieties of corundum are classified as sapphires, the blue being the result of iron and titanium impurities, and the violet of vanadium impurities within the corundum crystal structure. Another colorless mineral is beryl (composed of beryllium aluminum silicate) but blue aquamarine, green emerald, and pink morganite, are precious varieties of beryl including different impurities aquamarine includes iron, emerald chromium and vanadium, and morganite manganese. 

Fig. 2.1. A lens for high-resolution acoustic microscopy in reflection. The central transparent part is a single crystal of sapphire, with its c-axis accurately parallel to the axis of the cylinder. The sandwich structure at the top is the transducer, with the yellow representing an epitaxially grown layer of zinc oxide between two gold electrodes. The pink shaded areas within the sapphire represent the plane-wavefronts of an acoustic pulse they are refracted at the lens cavity so that they become spherical in the coupling fluid. A lens for use at 2 GHz would have a cavity of radius 40f[Pg.8]

Several crystal structures of the oxide are known. The most common one is a-Al203 which is made by the calcination of A1205.3H20. Natural corundum is found as semi-translucent masses with a white or greyish colour. It resembles quartz. Chemically seen, corundum is related to the much more beautiful sapphire and ruby. These have the same formula, A1203, but in their cases impurities result in the colour. 

The crystal structures of transition metal compounds and minerals have either cubic or lower symmetries. The cations may occur in regular octahedral (or tetrahedral) sites or be present in distorted coordination polyhedra in the crystal structures. When cations are located in low-symmetry coordination environments in non-cubic minerals, different absorption spectrum profiles may result when linearly polarized light is transmitted through single crystals of the anisotropic phases. Such polarization dependence of absorption bands is illustrated by the spectra ofFe2+ in gillespite (fig. 3.3) and of Fe3+in yellow sapphire (fig. 3.16). 

FIGURE 1 Crystal structure of a-sapphire. Hexagonal cell vectors are commonly used to represent the structure. 

In this paper, high quality ZnO films were deposited on sapphire substrate by MOCVD. X-ray diffraction measurement was taken to estimate the crystal structure of ZnO layer. The optical properties were investigated by transmission spectrum and photoluminescence (PL) spectrum. X-ray photoemission spectra were used to confirm the stoichiometry of ZnO film. 

Figure 6 Cosmic jewels an aggregate of tiny blue hibonite crystals from the Murray CM chondrite. This entire object is no more than 100 xm in maximum size. The color of these sapphire-like gems is real and is due to the presence of substantial titanium in the crystal structure. The color variation is partly due to the crystals being compositionaUy zoned (their margins are more titanium-rich), and also to their being pleochroic under plane polarized light the color is visible only when the crystals are in specihc orientations relative to the polarizers on the...

Alpha alumina has a rhombohedral crystal structure (a = 4.758 A and c = 12.991 A). Natural alumina is known as sapphire or ruby, depending on the types of impurities which give rise to color. The single... 

FIGURE 6.12 The sapphire crystal structure. (Top) [1120] view (bottom) [0001] view (left) atomic models (right) stacking octahedra. P, and P2 are two unoccupied octahedra. S is a triangle of more closely spaced 0 ions. Open circles in the lower left show the AB stacking of the anions. The unit cell is outlined for both projections. 

Scapolite has a complicated crystal structure and is formed by the alteration of plagioclase feldspars. Incidentally, diamond, sapphire, and ruby all fluoresce, but as with the materials above, the color depends on the UV wavelength used. A given specimen can also show different colors depending on the distribution of impurities. Trace elements can be included in synthetic gemstones to make them fluoresce and thus be easily identifiable. 

MgO is has cubic crystal structure, as shown in Fig. 2.10. MgO ceramics have excellent thermal and mechanical properties with a high melting point of 2850 °C and low density of 3.58 g cm. It has an isotropic cubic crystal strucmre, which meets one of the requirements to be transparent. PolycrystaUine infrared-transparent MgO is a potential substitute for sapphire IR windows and protectors for sensors. Due to the high sintering temperature required for full densification of MgO, fabrication of transparent MgO ceramics is stiU a challenge. Almost aU techniques discussed above have been used to prepare transparent MgO ceramics [3]. 

The highest modulus of a given substrate is obtained with a single crystal structure. Single crystal CVD-SiC whiskers (578 GPa) have a stiffen more highly ordered, structure than polycrystalline CVD-SiC fibers (190-400 GPa), and sapphire whiskers and fibers (415 GPa) are stiffer than slurry spun polycrystalline alumina fibers such as Fiber FP (380 GPa). Superimposed upon this relationship is a compositional factor. Fiber modulus and structural order generally also decrease with increasing compositional complexity, e.g., silicon carbide is intrinsically stiffer than silicon oxycarbide such as Nicalon, and slurry spun alumina fibers are stiffer than sol-gel or melt spun aluminate fibers. 

Thus, it is important to understand the unique crystal structure of GaN and its asymmetry to design a fabrication process. In other words, attention should be paid to areas where the design of the entire substrate manufacturing process flow differs signili-cantiy from that of existing crystals such as Si and sapphire. 

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